转自：http://blog.csdn.net/jmq_0000/article/details/7536805#t136
Video for Linux Two API Specification
Revision 0.24
Michael H Schimek
Bill Dirks
Hans Verkuil
Martin Rubli
Copyright © 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Bill Dirks, Michael H. Schimek, Hans Verkuil, MartinRubli
This document is copyrighted © 1999-2008 by BillDirks, Michael H. Schimek, Hans Verkuil and Martin Rubli.
Permission is granted to copy, distribute and/or modifythis document
under the terms of the GNU Free Documentation License,Version 1.1 or any
later version published by the Free SoftwareFoundation; with no
Invariant Sections, with no Front-Cover Texts, andwith
no Back-Cover Texts. A copy of the license is included in theappendix
entitled "GNU Free Documentation License".
Programming examples can be used and distributed withoutrestrictions.
- Table of Contents
Introduction
- 1. Common API Elements
- 1.1. Opening and Closing Devices
- 1.1.1. Device Naming
- 1.1.2. Related Devices
- 1.1.3. Multiple Opens
- 1.1.4. Shared Data Streams
- 1.1.5. Functions
- 1.2. Querying Capabilities
- 1.3. Application Priority
- 1.4. Video Inputs and Outputs
- 1.5. Audio Inputs and Outputs
- 1.6. Tuners and Modulators
- 1.6.1. Tuners
- 1.6.2. Modulators
- 1.6.3. Radio Frequency
- 1.6.4. Satellite Receivers
- 1.7. Video Standards
- 1.8. User Controls
- 1.9. Extended Controls
- 1.9.1. Introduction
- 1.9.2. The Extended Control API
- 1.9.3. Enumerating Extended Controls
- 1.9.4. Creating Control Panels
- 1.9.5. MPEG Control Reference
- 1.9.6. Camera Control Reference
- 1.10. Data Formats
- 1.10.1. Data Format Negotiation
- 1.10.2. Image Format Enumeration
- 1.11. Image Cropping, Insertion and Scaling
- 1.11.1. Cropping Structures
- 1.11.2. Scaling Adjustments
- 1.11.3. Examples
- 1.12. Streaming Parameters
- 2. Image Formats
- 2.1. Standard Image Formats
- 2.2. Colorspaces
- 2.3. Indexed Format
- 2.4. RGB Formats
- Packed RGB formats -- Packed RGB formats
- V4L2_PIX_FMT_SBGGR8 ('BA81') -- Bayer RGB format
- V4L2_PIX_FMT_SBGGR16 ('BA82') -- Bayer RGB format
- 2.5. YUV Formats
- Packed YUV formats -- Packed YUV formats
- V4L2_PIX_FMT_GREY ('GREY') -- Grey-scale image
- V4L2_PIX_FMT_Y16 ('Y16 ') -- Grey-scale image
- V4L2_PIX_FMT_YUYV ('YUYV') -- Packed format with ½ horizontal chromaresolution, also known as YUV 4:2:2
- V4L2_PIX_FMT_UYVY ('UYVY') -- Variation of
V4L2_PIX_FMT_YUYVwith different order of samplesin memory - V4L2_PIX_FMT_Y41P ('Y41P') -- Format with ¼ horizontal chromaresolution, also known as YUV 4:1:1
- V4L2_PIX_FMT_YVU420 ('YV12'), V4L2_PIX_FMT_YUV420 ('YU12') -- Planar formats with ½ horizontal andvertical chroma resolution, also known as YUV 4:2:0
- V4L2_PIX_FMT_YVU410 ('YVU9'), V4L2_PIX_FMT_YUV410 ('YUV9') -- Planar formats with ¼ horizontal andvertical chroma resolution, also known as YUV 4:1:0
- V4L2_PIX_FMT_YUV422P ('422P') -- Format with ½ horizontal chroma resolution,also known as YUV 4:2:2. Planar layout as opposed to
V4L2_PIX_FMT_YUYV - V4L2_PIX_FMT_YUV411P ('411P') -- Format with ¼ horizontal chroma resolution,also known as YUV 4:1:1. Planar layout as opposed to
V4L2_PIX_FMT_Y41P - V4L2_PIX_FMT_NV12 ('NV12'), V4L2_PIX_FMT_NV21 ('NV21') -- Formats with ½ horizontal and verticalchroma resolution, also known as YUV 4:2:0. One luminance and onechrominance plane
with alternating chroma samples as opposed toV4L2_PIX_FMT_YVU420
- 2.6. Compressed Formats
- 2.7. Reserved Format Identifiers
- 3. Input/Output
- 3.1. Read/Write
- 3.2. Streaming I/O (Memory Mapping)
- 3.3. Streaming I/O (User Pointers)
- 3.4. Asynchronous I/O
- 3.5. Buffers
- 3.5.1. Timecodes
- 3.6. Field Order
- 4. Interfaces
- 4.1. Video Capture Interface
- 4.1.1. Querying Capabilities
- 4.1.2. Supplemental Functions
- 4.1.3. Image Format Negotiation
- 4.1.4. Reading Images
- 4.2. Video Overlay Interface
- 4.2.1. Querying Capabilities
- 4.2.2. Supplemental Functions
- 4.2.3. Setup
- 4.2.4. Overlay Window
- 4.2.5. Enabling Overlay
- 4.3. Video Output Interface
- 4.3.1. Querying Capabilities
- 4.3.2. Supplemental Functions
- 4.3.3. Image Format Negotiation
- 4.3.4. Writing Images
- 4.4. Video Output Overlay Interface
- 4.4.1. Querying Capabilities
- 4.4.2. Framebuffer
- 4.4.3. Overlay Window and Scaling
- 4.4.4. Enabling Overlay
- 4.5. Codec Interface
- 4.6. Effect Devices Interface
- 4.7. Raw VBI Data Interface
- 4.7.1. Querying Capabilities
- 4.7.2. Supplemental Functions
- 4.7.3. Raw VBI Format Negotiation
- 4.7.4. Reading and writing VBI images
- 4.8. Sliced VBI Data Interface
- 4.8.1. Querying Capabilities
- 4.8.2. Supplemental Functions
- 4.8.3. Sliced VBI Format Negotiation
- 4.8.4. Reading and writing sliced VBI data
- 4.9. Teletext Interface
- 4.10. Radio Interface
- 4.10.1. Querying Capabilities
- 4.10.2. Supplemental Functions
- 4.10.3. Programming
- 4.11. RDS Interface
- I. Function Reference
- V4L2 close() -- Close a V4L2 device
- V4L2 ioctl() -- Program a V4L2 device
- ioctl VIDIOC_CROPCAP -- Information about the video cropping and scaling abilities
- ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER -- Read or write hardware registers
- ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD -- Execute an encoder command
- ioctl VIDIOC_ENUMAUDIO -- Enumerate audio inputs
- ioctl VIDIOC_ENUMAUDOUT -- Enumerate audio outputs
- ioctl VIDIOC_ENUM_FMT -- Enumerate image formats
- ioctl VIDIOC_ENUM_FRAMESIZES -- Enumerate frame sizes
- ioctl VIDIOC_ENUM_FRAMEINTERVALS -- Enumerate frame intervals
- ioctl VIDIOC_ENUMINPUT -- Enumerate video inputs
- ioctl VIDIOC_ENUMOUTPUT -- Enumerate video outputs
- ioctl VIDIOC_ENUMSTD -- Enumerate supported video standards
- ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO -- Query or select the current audio input and itsattributes
- ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT -- Query or select the current audio output
- ioctl VIDIOC_G_CHIP_IDENT -- Identify the chips on a TV card
- ioctl VIDIOC_G_CROP, VIDIOC_S_CROP -- Get or set the current cropping rectangle
- ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL -- Get or set the value of a control
- ioctl VIDIOC_G_ENC_INDEX -- Get meta data about a compressed video stream
- ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,VIDIOC_TRY_EXT_CTRLS -- Get or set the value of several controls, try controlvalues
- ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters
- ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,VIDIOC_TRY_FMT -- Get or set the data format, try a format
- ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY -- Get or set tuner or modulator radiofrequency
- ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT -- Query or select the current video input
- ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP --
- ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR -- Get or set modulator attributes
- ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT -- Query or select the current video output
- ioctl VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters
- ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY -- Query or request the access priority associated with afile descriptor
- ioctl VIDIOC_G_SLICED_VBI_CAP -- Query sliced VBI capabilities
- ioctl VIDIOC_G_STD, VIDIOC_S_STD -- Query or select the video standard of the current input
- ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER -- Get or set tuner attributes
- ioctl VIDIOC_LOG_STATUS -- Log driver status information
- ioctl VIDIOC_OVERLAY -- Start or stop video overlay
- ioctl VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver
- ioctl VIDIOC_QUERYBUF -- Query the status of a buffer
- ioctl VIDIOC_QUERYCAP -- Query device capabilities
- ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items
- ioctl VIDIOC_QUERYSTD -- Sense the video standard received by the currentinput
- ioctl VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O
- ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF -- Start or stop streaming I/O
- V4L2 mmap() -- Map device memory into application address space
- V4L2 munmap() -- Unmap device memory
- V4L2 open() -- Open a V4L2 device
- V4L2 poll() -- Wait for some event on a file descriptor
- V4L2 read() -- Read from a V4L2 device
- V4L2 select() -- Synchronous I/O multiplexing
- V4L2 write() -- Write to a V4L2 device
- 5. V4L2 Driver Programming
- 6. Changes
- 6.1. Differences between V4L and V4L2
- 6.1.1. Opening and Closing Devices
- 6.1.2. Querying Capabilities
- 6.1.3. Video Sources
- 6.1.4. Tuning
- 6.1.5. Image Properties
- 6.1.6. Audio
- 6.1.7. Frame Buffer Overlay
- 6.1.8. Cropping
- 6.1.9. Reading Images, Memory Mapping
- 6.1.10. Reading Raw VBI Data
- 6.1.11. Miscellaneous
- 6.2. Changes of the V4L2 API
- 6.2.1. Early Versions
- 6.2.2. V4L2 Version 0.16 1999-01-31
- 6.2.3. V4L2 Version 0.18 1999-03-16
- 6.2.4. V4L2 Version 0.19 1999-06-05
- 6.2.5. V4L2 Version 0.20 (1999-09-10)
- 6.2.6. V4L2 Version 0.20 incremental changes
- 6.2.7. V4L2 Version 0.20 2000-11-23
- 6.2.8. V4L2 Version 0.20 2002-07-25
- 6.2.9. V4L2 in Linux 2.5.46, 2002-10
- 6.2.10. V4L2 2003-06-19
- 6.2.11. V4L2 2003-11-05
- 6.2.12. V4L2 in Linux 2.6.6, 2004-05-09
- 6.2.13. V4L2 in Linux 2.6.8
- 6.2.14. V4L2 spec erratum 2004-08-01
- 6.2.15. V4L2 in Linux 2.6.14
- 6.2.16. V4L2 in Linux 2.6.15
- 6.2.17. V4L2 spec erratum 2005-11-27
- 6.2.18. V4L2 spec erratum 2006-01-10
- 6.2.19. V4L2 spec erratum 2006-02-03
- 6.2.20. V4L2 spec erratum 2006-02-04
- 6.2.21. V4L2 in Linux 2.6.17
- 6.2.22. V4L2 spec erratum 2006-09-23 (Draft 0.15)
- 6.2.23. V4L2 in Linux 2.6.18
- 6.2.24. V4L2 in Linux 2.6.19
- 6.2.25. V4L2 spec erratum 2006-10-12 (Draft 0.17)
- 6.2.26. V4L2 in Linux 2.6.21
- 6.2.27. V4L2 in Linux 2.6.22
- 6.2.28. V4L2 in Linux 2.6.24
- 6.2.29. V4L2 in Linux 2.6.25
- 6.3. Relation of V4L2 to other Linux multimedia APIs
- 6.3.1. X Video Extension
- 6.3.2. Digital Video
- 6.3.3. Audio Interfaces
- 6.4. Experimental API Elements
- 6.5. Obsolete API Elements
- A. Video For Linux Two Header File
- B. Video Capture Example
- C. GNU Free Documentation License
- C.1. 0. PREAMBLE
- C.2. 1. APPLICABILITY AND DEFINITIONS
- C.3. 2. VERBATIM COPYING
- C.4. 3. COPYING IN QUANTITY
- C.5. 4. MODIFICATIONS
- C.6. 5. COMBINING DOCUMENTS
- C.7. 6. COLLECTIONS OF DOCUMENTS
- C.8. 7. AGGREGATION WITH INDEPENDENT WORKS
- C.9. 8. TRANSLATION
- C.10. 9. TERMINATION
- C.11. 10. FUTURE REVISIONS OF THIS LICENSE
- C.12. Addendum
- List of Types
- References
- List of Examples
- 1-1. Information about the current video input
- 1-2. Switching to the first video input
- 1-3. Information about the current audio input
- 1-4. Switching to the first audio input
- 1-5. Information about the current video standard
- 1-6. Listing the video standards supported by the currentinput
- 1-7. Selecting a new video standard
- 1-8. Enumerating all controls
- 1-9. Changing controls
- 1-10. Resetting the cropping parameters
- 1-11. Simple downscaling
- 1-12. Selecting an output area
- 1-13. Current scaling factor and pixel aspect
- 2-1. ITU-R Rec. BT.601 color conversion
- 2-1.
V4L2_PIX_FMT_BGR244 × 4 pixelimage - 2-1.
V4L2_PIX_FMT_SBGGR84 × 4pixel image - 2-1.
V4L2_PIX_FMT_SBGGR164 × 4pixel image - 2-1.
V4L2_PIX_FMT_GREY4 × 4pixel image - 2-1.
V4L2_PIX_FMT_Y164 × 4pixel image - 2-1.
V4L2_PIX_FMT_YUYV4 × 4pixel image - 2-1.
V4L2_PIX_FMT_UYVY4 × 4pixel image - 2-1.
V4L2_PIX_FMT_Y41P8 × 4pixel image - 2-1.
V4L2_PIX_FMT_YVU4204 × 4pixel image - 2-1.
V4L2_PIX_FMT_YVU4104 × 4pixel image - 2-1.
V4L2_PIX_FMT_YUV422P4 × 4pixel image - 2-1.
V4L2_PIX_FMT_YUV411P4 × 4pixel image - 2-1.
V4L2_PIX_FMT_NV124 × 4pixel image - 3-1. Mapping buffers
- 3-2. Initiating streaming I/O with user pointers
- 4-1. Finding a framebuffer device for OSD
Introduction
Video For Linux Two is the second version of the Video ForLinux API, a kernel interface for analog radio and video capture andoutput drivers.
Early drivers used ad-hoc interfaces. These were replaced inLinux 2.2
by Alan Cox' V4L API, based on the interface of the bttvdriver. In 1999
Bill Dirks started the development of V4L2 to fix someshortcomings of
V4L and to support a wider range of devices.
The APIwas revised again in 2002 prior to its inclusion in Linux
2.5/2.6, andwork continues on improvements and additions while
maintainingcompatibility with existing drivers and applications. In
2006/2007efforts began on FreeBSD drivers with a V4L2 interface.
This book documents the V4L2 API. Intended audience aredriver and application writers.
If you have questions or ideas regarding the API, pleasewrite to the Video4Linux mailing list:https://listman.redhat.com/mailman/listinfo/video4linux-list. For inquiries
aboutthe V4L2 specification contact the maintainer[email protected].
The latest version of this document and the DocBook SGMLsources are hosted at http://v4l2spec.bytesex.org,and http://linuxtv.org/downloads/video4linux/API/V4L2_API.
Chapter 1. Common API Elements
Programming a V4L2 device consists of thesesteps:
Opening the device
Changing device properties, selecting a video and audioinput, video standard, picture brightness a. o.
Negotiating a data format
Negotiating an input/output method
The actual input/output loop
Closing the device
In practice most steps are optional and can be executed out oforder.
It depends on the V4L2 device type, you can read about thedetails inChapter 4. In this chapter we will discussthe
basic concepts applicable to all devices.
1.1. Opening and Closing Devices
1.1.1. Device Naming
V4L2 drivers are implemented as kernel modules, loadedmanually by the
system administrator or automatically when a device isfirst opened. The
driver modules plug into the "videodev" kernelmodule. It provides
helper functions and a common applicationinterface
specified in this document.
Each driver thus loaded registers one or more device nodeswith major
number 81 and a minor number between 0 and 255. Assigningminor numbers
to V4L2 devices is entirely up to the system administrator,this is
primarily intended to solve conflicts between devices.[1]
The module options to select minor numbers are namedafter the device
special file with a "_nr" suffix. For example "video_nr"for/dev/video video capture devices. The number isan offset to the base minor number associated with the
device type.[2] When the driver supports multiple devices of the sametype more than one minor number can be assigned, separated by commas:
> insmod mydriver.o video_nr=0,1 radio_nr=0,1
In /etc/modules.conf this may bewritten as:
alias char-major-81-0 mydriver
alias char-major-81-1 mydriver
alias char-major-81-64 mydriver
options mydriver video_nr=0,1 radio_nr=0,1
- When an application attempts to open a devicespecial file with major number 81 and minor number 0, 1, or 64, load"mydriver" (and the "videodev" module it depends upon).
- Register the first two video capture devices withminor number 0 and 1 (base number is 0), the first two radio devicewith minor number 64 and 65 (base 64).
When no minor number is given as moduleoption the driver supplies a default. Chapter 4recommends the base minor numbers to be used for the various devicetypes. Obviously minor numbers must be unique. When the number isalready in use theoffending device will not beregistered.
By convention system administrators create variouscharacter device special files with these major and minor numbers inthe/dev directory. The names recomended for thedifferent V4L2 device types are listed inChapter 4.
The creation of character special files (withmknod) is a privileged operation anddevices cannot be opened by major and minor number. That meansapplications cannotreliable scan for loaded orinstalled drivers. The user must enter a device name, or theapplication can try the conventional device names.
Under the device filesystem (devfs) the minor numberoptions are ignored. V4L2 drivers (or by proxy the "videodev" module)automatically create the required device files in the/dev/v4l directory using the conventional devicenames above.
1.1.2. Related Devices
Devices can support several related functions. For examplevideo capturing, video overlay and VBI capturing are related becausethese functions share, amongst other, the same video input and tunerfrequency. V4L and earlier versions of V4L2 used the same device nameand minor number for video capturing and overlay, but different onesfor VBI. Experience showed this approach has several problems[3], and to make things worse the V4L videodev moduleused to prohibit multiple opens of a device.
As a remedy the present version of the V4L2 API relaxed theconcept of device types with specific names and minor numbers. Forcompatibility with old applications drivers must still register differentminor numbers to assign a default function to the device. But if relatedfunctions are supported by the driver they must be available under allregistered minor numbers. The desired function can be selected afteropening the device as described inChapter 4.
Imagine a driver supporting video capturing, videooverlay, raw VBI capturing, and FM radio reception. It registers threedevices with minor number 0, 64 and 224 (this numbering scheme isinherited from the V4L API). Regardless if/dev/video (81, 0) or/dev/vbi (81, 224) is opened the application canselect any one of the video capturing, overlay or VBI capturingfunctions. Without programming (e. g. reading from the devicewithdd orcat)/dev/video captures video images, while/dev/vbi captures raw VBI data./dev/radio (81, 64) is invariable a radio device,unrelated to the video functions. Being unrelated does not imply thedevices can be used at the same time, however. Theopen()function may very well return anEBUSY error code.
Besides video input or output the hardware may alsosupport audio sampling or playback. If so, these functions areimplemented as OSS or ALSA PCM devices and eventually OSS or ALSAaudio mixer. The V4L2 API makes no provisions yet to find theserelated devices. If you have an idea please write to the Video4Linuxmailing list: https://listman.redhat.com/mailman/listinfo/video4linux-list.
1.1.3. Multiple Opens
In general, V4L2 devices can be opened more than once.When this is supported by the driver, users can for example start a"panel" application to change controls like brightness or audiovolume, while another application captures video and audio. In other words, panelapplications are comparable to an OSS or ALSA audio mixer application.When a device supports multiple functions like capturing and overlaysimultaneously, multiple opens allow concurrentuse of the device by forked processes or specialized applications.
Multiple opens are optional, although drivers shouldpermit at least concurrent accesses without data exchange, i. e. panelapplications. This impliesopen() can return anEBUSY error code when thedevice is already in use, as well asioctl() functions initiatingdata exchange (namely theVIDIOC_S_FMT ioctl), and theread()andwrite() functions.
Mere opening a V4L2 device does not grant exclusiveaccess.[4] Initiating data exchange however assigns the rightto read or write the requested type of data, and to change relatedproperties, to this file descriptor. Applications can requestadditional access privileges using the priority mechanism described inSection 1.3.
1.1.4. Shared Data Streams
V4L2 drivers should not support multiple applicationsreading or writing the same data stream on a device by copyingbuffers, time multiplexing or similar means. This is better handled bya proxy application in user space. When the driver supports streamsharing anyway it must be implemented transparently. The V4L2 API doesnot specify how conflicts are solved.
1.2. Querying Capabilities
Because V4L2 covers a wide variety of devices not allaspects of the API are equally applicable to all types of devices.Furthermore devices of the same type have different capabilities andthis specification permits the omission of a few complicated and lessimportant parts of the API.
The VIDIOC_QUERYCAP ioctl is available to check if the kerneldevice is compatible with this specification, and to query thefunctions andI/Omethods supported by the device. Other features can be queriedby calling the respective ioctl, for exampleVIDIOC_ENUMINPUTto learn about the number, types and names of video connectors on thedevice. Although abstraction is a major objective of this API, theioctl also allows driver specific applications to reliable identifythe driver.
All V4L2 drivers must supportVIDIOC_QUERYCAP. Applications should always callthis ioctl after opening the device.
1.3. Application Priority
When multiple applications share a device it may bedesirable to assign them different priorities. Contrary to thetraditional "rm -rf /" school of thought a video recording applicationcould for example block other applications from changing videocontrols or switching the current TV channel. Another objective is topermit low priority applications working in background, which can bepreempted by user controlled applications and automatically regaincontrol of the device at a later time.
Since these features cannot be implemented entirely in userspace V4L2 defines theVIDIOC_G_PRIORITY andVIDIOC_S_PRIORITYioctls to request and query the access priority associate with a filedescriptor. Opening a device assigns a medium priority, compatiblewith earlier versions of V4L2 and drivers not supporting these ioctls.Applications requiring a different priority will usually callVIDIOC_S_PRIORITY after verifying the device withtheVIDIOC_QUERYCAP ioctl.
Ioctls changing driver properties, such as VIDIOC_S_INPUT,return an EBUSY error code after another application obtained higher priority.An event mechanism to notify applications about asynchronous propertychanges has been proposed but not added yet.
1.4. Video Inputs and Outputs
Video inputs and outputs are physical connectors of adevice. These can be for example RF connectors (antenna/cable), CVBSa.k.a. Composite Video, S-Video or RGB connectors. Only video and VBIcapture devices have inputs, output devices have outputs, at least oneeach. Radio devices have no video inputs or outputs.
To learn about the number and attributes of theavailable inputs and outputs applications can enumerate them with theVIDIOC_ENUMINPUT andVIDIOC_ENUMOUTPUT ioctl, respectively. Thestruct v4l2_input returned by theVIDIOC_ENUMINPUTioctl also contains signal status information applicable when thecurrent video input is queried.
The VIDIOC_G_INPUT and VIDIOC_G_OUTPUT ioctl return theindex of the current video input or output. To select a differentinput or output applications call theVIDIOC_S_INPUT andVIDIOC_S_OUTPUT ioctl. Drivers must implement all the input ioctlswhen the device has one or more inputs, all the output ioctls when thedevice has one or more outputs.
Example 1-1. Information about the current video input
struct v4l2_input input;
int index; if (-1 == ioctl (fd,VIDIOC_G_INPUT, &index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
} memset (&input, 0, sizeof (input));
input.index = index; if (-1 == ioctl (fd,VIDIOC_ENUMINPUT, &input)) {
perror ("VIDIOC_ENUMINPUT");
exit (EXIT_FAILURE);
} printf ("Current input: %s\n", input.name);
Example 1-2. Switching to the first video input
int index; index = 0; if (-1 == ioctl (fd,VIDIOC_S_INPUT, &index)) {
perror ("VIDIOC_S_INPUT");
exit (EXIT_FAILURE);
}
1.5. Audio Inputs and Outputs
Audio inputs and outputs are physical connectors of adevice. Video capture devices have inputs, output devices haveoutputs, zero or more each. Radio devices have no audio inputs oroutputs. They have exactly one tuner which in factis an audio source, but this API associatestuners with video inputs or outputs only, and radio devices havenone of these.[5] A connector on a TV card to loop back the receivedaudio signal to a sound card is not considered an audio output.
Audio and video inputs and outputs are associated. Selectinga video source also selects an audio source. This is most evident whenthe video and audio source is a tuner. Further audio connectors cancombine with more than one video input or output. Assumed twocomposite video inputs and two audio inputs exist, there may be up tofour valid combinations. The relation of video and audio connectorsis defined in theaudioset field of therespective struct v4l2_input or struct v4l2_output, where each bit representsthe index number, starting at zero, of one audio input or output.
To learn about the number and attributes of theavailable inputs and outputs applications can enumerate them with theVIDIOC_ENUMAUDIO andVIDIOC_ENUMAUDOUT ioctl, respectively. Thestruct v4l2_audio returned by theVIDIOC_ENUMAUDIO ioctlalso contains signal status information applicable when the currentaudio input is queried.
The VIDIOC_G_AUDIO and VIDIOC_G_AUDOUT ioctl reportthe current audio input and output, respectively. Note that, unlikeVIDIOC_G_INPUT andVIDIOC_G_OUTPUT these ioctls return a structureasVIDIOC_ENUMAUDIO andVIDIOC_ENUMAUDOUT do, not just an index.
To select an audio input and change its propertiesapplications call the VIDIOC_S_AUDIO ioctl. To select an audiooutput (which presently has no changeable properties) applicationscall theVIDIOC_S_AUDOUT ioctl.
Drivers must implement all input ioctls when the devicehas one or more inputs, all output ioctls when the device has oneor more outputs. When the device has any audio inputs or outputs thedriver must set theV4L2_CAP_AUDIO flag in thestruct v4l2_capability returned by theVIDIOC_QUERYCAP ioctl.
Example 1-3. Information about the current audio input
struct v4l2_audio audio; memset (&audio, 0, sizeof (audio)); if (-1 == ioctl (fd,VIDIOC_G_AUDIO, &audio)) {
perror ("VIDIOC_G_AUDIO");
exit (EXIT_FAILURE);
} printf ("Current input: %s\n", audio.name);
Example 1-4. Switching to the first audio input
struct v4l2_audio audio; memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */ audio.index = 0; if (-1 == ioctl (fd,VIDIOC_S_AUDIO, &audio)) {
perror ("VIDIOC_S_AUDIO");
exit (EXIT_FAILURE);
}
1.6. Tuners and Modulators
1.6.1. Tuners
Video input devices can have one or more tunersdemodulating a RF signal. Each tuner is associated with one or morevideo inputs, depending on the number of RF connectors on the tuner.Thetype field of the respectivestruct v4l2_input returned by theVIDIOC_ENUMINPUT ioctl is set toV4L2_INPUT_TYPE_TUNER and itstuner field contains the index number ofthe tuner.
Radio devices have exactly one tuner with index zero, novideo inputs.
To query and change tuner properties applications use theVIDIOC_G_TUNER andVIDIOC_S_TUNER ioctl, respectively. Thestruct v4l2_tuner returned byVIDIOC_G_TUNER alsocontains signal status information applicable when the tuner of thecurrent video input, or a radio tuner is queried. Note thatVIDIOC_S_TUNER does not switch the current tuner,when there is more than one at all. The tuner is solely determined bythe current video input. Drivers must support both ioctls and set theV4L2_CAP_TUNER flag in the struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl when the device has one ormore tuners.
1.6.2. Modulators
Video output devices can have one or more modulators, uh,modulating a video signal for radiation or connection to the antennainput of a TV set or video recorder. Each modulator is associated withone or more video outputs, depending on the number of RF connectors onthe modulator. The type field of therespective struct v4l2_output returned by theVIDIOC_ENUMOUTPUT ioctl isset toV4L2_OUTPUT_TYPE_MODULATOR and itsmodulator field contains the index numberof the modulator. This specification does not define radio outputdevices.
To query and change modulator properties applications usethe VIDIOC_G_MODULATOR and VIDIOC_S_MODULATOR ioctl. Note thatVIDIOC_S_MODULATOR does not switch the currentmodulator, when there is more than one at all. The modulator is solelydetermined by the current video output. Drivers must support bothioctls and set the V4L2_CAP_TUNER (sic) flag inthe struct v4l2_capability returned by theVIDIOC_QUERYCAP ioctl when thedevice has one or more modulators.
1.6.3. Radio Frequency
To get and set the tuner or modulator radio frequencyapplications use the VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCYioctl which both take a pointer to a struct v4l2_frequency. These ioctlsare used for TV and radio devices alike. Drivers must support bothioctls when the tuner or modulator ioctls are supported, orwhen the device is a radio device.
1.6.4. Satellite Receivers
To be discussed. See also proposals by Peter Schlaf, [email protected] on 23 Oct 2002,subject: "Re: [V4L] Re: v4l2 api".
1.7. Video Standards
Video devices typically support one or more different videostandards or variations of standards. Each video input and output maysupport another set of standards. This set is reported by thestd field of struct v4l2_input andstruct v4l2_output returned by theVIDIOC_ENUMINPUT andVIDIOC_ENUMOUTPUT ioctl, respectively.
V4L2 defines one bit for each analog video standardcurrently in use worldwide, and sets aside bits for driver definedstandards, e. g. hybrid standards to watch NTSC video tapes on PAL TVsand vice versa. Applications can use the predefined bits to select aparticular standard, although presenting the user a menu of supportedstandards is preferred. To enumerate and query the attributes of thesupported standards applications use theVIDIOC_ENUMSTD ioctl.
Many of the defined standards are actually just variationsof a few major standards. The hardware may in fact not distinguishbetween them, or do so internal and switch automatically. Thereforeenumerated standards also contain sets of one or more standardbits.
Assume a hypothetic tuner capable of demodulating B/PAL,G/PAL and I/PAL signals. The first enumerated standard is a set of Band G/PAL, switched automatically depending on the selected radiofrequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I"choice. Similar a Composite input may collapse standards, enumerating"PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".[6]
To query and select the standard used by the current videoinput or output applications call theVIDIOC_G_STD andVIDIOC_S_STD ioctl, respectively. The receivedstandard can be sensed with theVIDIOC_QUERYSTD ioctl. Note parameter of all these ioctls is a pointer to av4l2_std_id type (a standard set),not an index into the standard enumeration.[7] Drivers must implement all video standard ioctlswhen the device has one or more video inputs or outputs.
Special rules apply to USB cameras where the notion of videostandards makes little sense. More generally any capture device,output devices accordingly, which is
incapable of capturing fields or frames at the nominalrate of the video standard, or
where timestamps referto the instant the field or frame was received by the driver, not thecapture time, or
where sequence numbersrefer to the frames received by the driver, not the capturedframes.
Here the driver shall set thestd field of struct v4l2_input and struct v4l2_outputto zero, the VIDIOC_G_STD,VIDIOC_S_STD,VIDIOC_QUERYSTD andVIDIOC_ENUMSTD ioctls shall return theEINVAL error code.[8]
Example 1-5. Information about the current video standard
v4l2_std_id std_id;
struct v4l2_standard standard; if (-1 == ioctl (fd,VIDIOC_G_STD, &std_id)) {
/* Note when VIDIOC_ENUMSTD always returns EINVAL this
is no video device or it falls under the USB exception,
and VIDIOC_G_STD returning EINVAL is no error. */ perror ("VIDIOC_G_STD");
exit (EXIT_FAILURE);
} memset (&standard, 0, sizeof (standard));
standard.index = 0; while (0 == ioctl (fd,VIDIOC_ENUMSTD, &standard)) {
if (standard.id & std_id) {
printf ("Current video standard: %s\n", standard.name);
exit (EXIT_SUCCESS);
} standard.index++;
} /* EINVAL indicates the end of the enumeration, which cannot be
empty unless this device falls under the USB exception. */ if (errno == EINVAL || standard.index == 0) {
perror ("VIDIOC_ENUMSTD");
exit (EXIT_FAILURE);
}
Example 1-6. Listing the video standards supported by the currentinput
struct v4l2_input input;
struct v4l2_standard standard; memset (&input, 0, sizeof (input)); if (-1 == ioctl (fd,VIDIOC_G_INPUT, &input.index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
} if (-1 == ioctl (fd,VIDIOC_ENUMINPUT, &input)) {
perror ("VIDIOC_ENUM_INPUT");
exit (EXIT_FAILURE);
} printf ("Current input %s supports:\n", input.name); memset (&standard, 0, sizeof (standard));
standard.index = 0; while (0 == ioctl (fd,VIDIOC_ENUMSTD, &standard)) {
if (standard.id & input.std)
printf ("%s\n", standard.name); standard.index++;
} /* EINVAL indicates the end of the enumeration, which cannot be
empty unless this device falls under the USB exception. */ if (errno != EINVAL || standard.index == 0) {
perror ("VIDIOC_ENUMSTD");
exit (EXIT_FAILURE);
}
Example 1-7. Selecting a new video standard
struct v4l2_input input;
v4l2_std_id std_id; memset (&input, 0, sizeof (input)); if (-1 == ioctl (fd,VIDIOC_G_INPUT, &input.index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
} if (-1 == ioctl (fd,VIDIOC_ENUMINPUT, &input)) {
perror ("VIDIOC_ENUM_INPUT");
exit (EXIT_FAILURE);
} if (0 == (input.std & V4L2_STD_PAL_BG)) {
fprintf (stderr, "Oops. B/G PAL is not supported.\n");
exit (EXIT_FAILURE);
} /* Note this is also supposed to work when only B
or G/PAL is supported. */ std_id = V4L2_STD_PAL_BG; if (-1 == ioctl (fd,VIDIOC_S_STD, &std_id)) {
perror ("VIDIOC_S_STD");
exit (EXIT_FAILURE);
}
1.8. User Controls
Devices typically have a number of user-settable controlssuch as brightness, saturation and so on, which would be presented tothe user on a graphical user interface. But, different deviceswill have different controls available, and furthermore, the range ofpossible values, and the default value will vary from device todevice. The control ioctls provide the information and a mechanism tocreate a nice user interface for these controls that will workcorrectly with any device.
All controls are accessed using an ID value. V4L2 definesseveral IDs for specific purposes. Drivers can also implement theirown custom controls usingV4L2_CID_PRIVATE_BASEand higher values. The pre-defined control IDs have the prefixV4L2_CID_, and are listed inTable 1-1. The ID is used when querying the attributes ofa control, and when getting or setting the current value.
Generally applications should present controls to the userwithout assumptions about their purpose. Each control comes with aname string the user is supposed to understand. When the purpose isnon-intuitive the driver writer should provide a user manual, a userinterface plug-in or a driver specific panel application. PredefinedIDs were introduced to change a few controls programmatically, forexample to mute a device during a channel switch.
Drivers may enumerate different controls after switchingthe current video input or output, tuner or modulator, or audio inputor output. Different in the sense of other bounds, another default andcurrent value, step size or other menu items. A control with a certaincustom ID can also change name andtype.[9] Control values are stored globally, they do notchange when switching except to stay within the reported bounds. Theyalso do not change e. g. when the device is opened or closed, when thetuner radio frequency is changed or generally never withoutapplication request. Since V4L2 specifies no event mechanism, panelapplications intended to cooperate with other panel applications (bethey built into a larger application, as a TV viewer) may need toregularly poll control values to update their userinterface.[10]
Table 1-1. Control IDs
V4L2_CID_BASE | First predefined ID, equal toV4L2_CID_BRIGHTNESS. | |
V4L2_CID_USER_BASE | Synonym of V4L2_CID_BASE. | |
V4L2_CID_BRIGHTNESS | integer | Picture brightness, or more precisely, the blacklevel. |
V4L2_CID_CONTRAST | integer | Picture contrast or luma gain. |
V4L2_CID_SATURATION | integer | Picture color saturation or chroma gain. |
V4L2_CID_HUE | integer | Hue or color balance. |
V4L2_CID_AUDIO_VOLUME | integer | Overall audio volume. Note some drivers alsoprovide an OSS or ALSA mixer interface. |
V4L2_CID_AUDIO_BALANCE | integer | Audio stereo balance. Minimum corresponds to allthe way left, maximum to right. |
V4L2_CID_AUDIO_BASS | integer | Audio bass adjustment. |
V4L2_CID_AUDIO_TREBLE | integer | Audio treble adjustment. |
V4L2_CID_AUDIO_MUTE | boolean | Mute audio, i. e. set the volume to zero, howeverwithout affecting V4L2_CID_AUDIO_VOLUME. LikeALSA drivers, V4L2 drivers must mute at load time to avoid excessivenoise. Actually the entire device should be reset to a low powerconsumption state. |
V4L2_CID_AUDIO_LOUDNESS | boolean | Loudness mode (bass boost). |
V4L2_CID_BLACK_LEVEL | integer | Another name for brightness (not a synonym ofV4L2_CID_BRIGHTNESS). This control is deprecatedand should not be used in new drivers and applications. |
V4L2_CID_AUTO_WHITE_BALANCE | boolean | Automatic white balance (cameras). |
V4L2_CID_DO_WHITE_BALANCE | button | This is an action control. When set (the value isignored), the device will do a white balance and then hold the currentsetting. Contrast this with the booleanV4L2_CID_AUTO_WHITE_BALANCE, which, whenactivated, keeps adjusting the white balance. |
V4L2_CID_RED_BALANCE | integer | Red chroma balance. |
V4L2_CID_BLUE_BALANCE | integer | Blue chroma balance. |
V4L2_CID_GAMMA | integer | Gamma adjust. |
V4L2_CID_WHITENESS | integer | Whiteness for grey-scale devices. This is a synonymfor V4L2_CID_GAMMA. This control is deprecatedand should not be used in new drivers and applications. |
V4L2_CID_EXPOSURE | integer | Exposure (cameras). [Unit?] |
V4L2_CID_AUTOGAIN | boolean | Automatic gain/exposure control. |
V4L2_CID_GAIN | integer | Gain control. |
V4L2_CID_HFLIP | boolean | Mirror the picture horizontally. |
V4L2_CID_VFLIP | boolean | Mirror the picture vertically. |
V4L2_CID_HCENTER_DEPRECATED (formerly V4L2_CID_HCENTER) | integer | Horizontal image centering. This control isdeprecated. New drivers and applications should use theCamera class controlsV4L2_CID_PAN_ABSOLUTE,V4L2_CID_PAN_RELATIVE andV4L2_CID_PAN_RESET instead. |
V4L2_CID_VCENTER_DEPRECATED (formerly V4L2_CID_VCENTER) | integer | Vertical image centering. Centering is intended tophysically adjust cameras. For image cropping seeSection 1.11, for clippingSection 4.2. Thiscontrol is deprecated. New drivers and applications should use theCamera class controlsV4L2_CID_TILT_ABSOLUTE,V4L2_CID_TILT_RELATIVE andV4L2_CID_TILT_RESET instead. |
V4L2_CID_POWER_LINE_FREQUENCY | integer | Enables a power line frequency filter to avoidflicker. Possible values are:V4L2_CID_POWER_LINE_FREQUENCY_DISABLED (0),V4L2_CID_POWER_LINE_FREQUENCY_50HZ (1) andV4L2_CID_POWER_LINE_FREQUENCY_60HZ (2). |
V4L2_CID_HUE_AUTO | boolean | Enables automatic hue control by the device. Theeffect of setting V4L2_CID_HUE while automatichue control is enabled is undefined, drivers should ignore suchrequest. |
V4L2_CID_WHITE_BALANCE_TEMPERATURE | integer | This control specifies the white balance settingsas a color temperature in Kelvin. A driver should have a minimum of2800 (incandescent) to 6500 (daylight). For more information aboutcolor temperature seeWikipedia. |
V4L2_CID_SHARPNESS | integer | Adjusts the sharpness filters in a camera. Theminimum value disables the filters, higher values give a sharperpicture. |
V4L2_CID_BACKLIGHT_COMPENSATION | integer | Adjusts the backlight compensation in a camera. Theminimum value disables backlight compensation. |
V4L2_CID_LASTP1 | End of the predefined control IDs (currentlyV4L2_CID_BACKLIGHT_COMPENSATION + 1). | |
V4L2_CID_PRIVATE_BASE | ID of the first custom (driver specific) control.Applications depending on particular custom controls should check thedriver name and version, seeSection 1.2. |
Applications can enumerate the available controls with theVIDIOC_QUERYCTRL andVIDIOC_QUERYMENU ioctls, get and set acontrol value with theVIDIOC_G_CTRL andVIDIOC_S_CTRL ioctls.Drivers must implementVIDIOC_QUERYCTRL,VIDIOC_G_CTRL andVIDIOC_S_CTRL when the device has one or morecontrols,VIDIOC_QUERYMENU when it has one ormore menu type controls.
Example 1-8. Enumerating all controls
struct v4l2_queryctrl queryctrl;
struct v4l2_querymenu querymenu; static void
enumerate_menu (void)
{
printf (" Menu items:\n"); memset (&querymenu, 0, sizeof (querymenu));
querymenu.id = queryctrl.id; for (querymenu.index = queryctrl.minimum;
querymenu.index <= queryctrl.maximum;
querymenu.index++) {
if (0 == ioctl (fd,VIDIOC_QUERYMENU, &querymenu)) {
printf (" %s\n", querymenu.name);
} else {
perror ("VIDIOC_QUERYMENU");
exit (EXIT_FAILURE);
}
}
} memset (&queryctrl, 0, sizeof (queryctrl)); for (queryctrl.id = V4L2_CID_BASE;
queryctrl.id < V4L2_CID_LASTP1;
queryctrl.id++) {
if (0 == ioctl (fd,VIDIOC_QUERYCTRL, &queryctrl)) {
if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
continue; printf ("Control %s\n", queryctrl.name); if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
enumerate_menu ();
} else {
if (errno == EINVAL)
continue; perror ("VIDIOC_QUERYCTRL");
exit (EXIT_FAILURE);
}
} for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
queryctrl.id++) {
if (0 == ioctl (fd,VIDIOC_QUERYCTRL, &queryctrl)) {
if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
continue; printf ("Control %s\n", queryctrl.name); if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
enumerate_menu ();
} else {
if (errno == EINVAL)
break; perror ("VIDIOC_QUERYCTRL");
exit (EXIT_FAILURE);
}
}
Example 1-9. Changing controls
struct v4l2_queryctrl queryctrl;
struct v4l2_control control; memset (&queryctrl, 0, sizeof (queryctrl));
queryctrl.id = V4L2_CID_BRIGHTNESS; if (-1 == ioctl (fd,VIDIOC_QUERYCTRL, &queryctrl)) {
if (errno != EINVAL) {
perror ("VIDIOC_QUERYCTRL");
exit (EXIT_FAILURE);
} else {
printf ("V4L2_CID_BRIGHTNESS is not supported\n");
}
} else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
printf ("V4L2_CID_BRIGHTNESS is not supported\n");
} else {
memset (&control, 0, sizeof (control));
control.id = V4L2_CID_BRIGHTNESS;
control.value = queryctrl.default_value; if (-1 == ioctl (fd,VIDIOC_S_CTRL, &control)) {
perror ("VIDIOC_S_CTRL");
exit (EXIT_FAILURE);
}
} memset (&control, 0, sizeof (control));
control.id = V4L2_CID_CONTRAST; if (0 == ioctl (fd,VIDIOC_G_CTRL, &control)) {
control.value += 1; /* The driver may clamp the value or return ERANGE, ignored here */ if (-1 == ioctl (fd,VIDIOC_S_CTRL, &control)
&& errno != ERANGE) {
perror ("VIDIOC_S_CTRL");
exit (EXIT_FAILURE);
}
/* Ignore if V4L2_CID_CONTRAST is unsupported */
} else if (errno != EINVAL) {
perror ("VIDIOC_G_CTRL");
exit (EXIT_FAILURE);
} control.id = V4L2_CID_AUDIO_MUTE;
control.value = TRUE; /* silence */ /* Errors ignored */
ioctl (fd, VIDIOC_S_CTRL, &control);
1.9. Extended Controls
1.9.1. Introduction
The control mechanism as originally designed was meantto be used for user settings (brightness, saturation, etc). However,it turned out to be a very useful model for implementing morecomplicated driver APIs where each driver implements only a subset ofa larger API.
The MPEG encoding API was the driving force behinddesigning and implementing this extended control mechanism: the MPEGstandard is quite large and the currently supported hardware MPEGencoders each only implement a subset of this standard. Further more,many parameters relating to how the video is encoded into an MPEGstream are specific to the MPEG encoding chip since the MPEG standardonly defines the format of the resulting MPEG stream, not how thevideo is actually encoded into that format.
Unfortunately, the original control API lacked somefeatures needed for these new uses and so it was extended into the(not terribly originally named) extended control API.
1.9.2. The Extended Control API
Three new ioctls are available: VIDIOC_G_EXT_CTRLS,VIDIOC_S_EXT_CTRLS andVIDIOC_TRY_EXT_CTRLS. These ioctls act onarrays of controls (as opposed to theVIDIOC_G_CTRL andVIDIOC_S_CTRL ioctls that act on a single control). This is neededsince it is often required to atomically change several controls atonce.
Each of the new ioctls expects a pointer to astruct v4l2_ext_controls. This structure contains a pointer to the controlarray, a count of the number of controls in that array and a controlclass. Control classes are used to group similar controls into asingle class. For example, control classV4L2_CTRL_CLASS_USER contains all user controls(i. e. all controls that can also be set using the oldVIDIOC_S_CTRL ioctl). Control classV4L2_CTRL_CLASS_MPEG contains all controlsrelating to MPEG encoding, etc.
All controls in the control array must belong to thespecified control class. An error is returned if this is not thecase.
It is also possible to use an empty control array (count== 0) to check whether the specified control class issupported.
The control array is a struct v4l2_ext_control array. Thev4l2_ext_control structure is very similar tostruct v4l2_control, except for the fact that it also allows for 64-bitvalues and pointers to be passed (although the latter is not yet usedanywhere).
It is important to realize that due to the flexibility ofcontrols it is necessary to check whether the control you want to setactually is supported in the driver and what the valid range of valuesis. So use theVIDIOC_QUERYCTRL andVIDIOC_QUERYMENU ioctls tocheck this. Also note that it is possible that some of the menuindices in a control of typeV4L2_CTRL_TYPE_MENUmay not be supported (VIDIOC_QUERYMENU willreturn an error). A good example is the list of supported MPEG audiobitrates. Some drivers only support one or two bitrates, otherssupport a wider range.
1.9.3. Enumerating Extended Controls
The recommended way to enumerate over the extendedcontrols is by using VIDIOC_QUERYCTRL in combination with theV4L2_CTRL_FLAG_NEXT_CTRL flag:
struct v4l2_queryctrl qctrl; qctrl.id = V4L2_CTRL_FLAG_NEXT_CTRL;
while (0 == ioctl (fd,VIDIOC_QUERYCTRL, &qctrl)) {
/* ... */
qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
}
The initial control ID is set to 0 ORed with theV4L2_CTRL_FLAG_NEXT_CTRL flag. TheVIDIOC_QUERYCTRL ioctl will return the firstcontrol with a higher ID than the specified one. When no such controlsare found an error is returned.
If you want to get all controls within a specific controlclass, then you can set the initialqctrl.id value to the control class and addan extra check to break out of the loop when a control of anothercontrol class is found:
qctrl.id = V4L2_CTRL_CLASS_MPEG | V4L2_CTRL_FLAG_NEXT_CTRL;
while (0 == ioctl (fd,VIDIOC_QUERYCTRL, &qctrl)) {
if (V4L2_CTRL_ID2CLASS (qctrl.id) != V4L2_CTRL_CLASS_MPEG)
break;
/* ... */
qctrl.id |= V4L2_CTRL_FLAG_NEXT_CTRL;
}
The 32-bit qctrl.id value issubdivided into three bit ranges: the top 4 bits are reserved forflags (e. g.V4L2_CTRL_FLAG_NEXT_CTRL) and are notactually part of the ID. The remaining 28 bits form the control ID, ofwhich the most significant 12 bits define the control class and theleast significant 16 bits identify the control within the controlclass. It is guaranteed that these last 16 bits are always non-zerofor controls. The range of 0x1000 and up are reserved fordriver-specific controls. The macroV4L2_CTRL_ID2CLASS(id) returns the control classID based on a control ID.
If the driver does not support extended controls, thenVIDIOC_QUERYCTRL will fail when used incombination withV4L2_CTRL_FLAG_NEXT_CTRL. Inthat case the old method of enumerating control should be used (see1.8). But if it is supported, then it is guaranteed to enumerate overall controls, including driver-private controls.
1.9.4. Creating Control Panels
It is possible to create control panels for a graphicaluser interface where the user can select the various controls.Basically you will have to iterate over all controls using the methoddescribed above. Each control class starts with a control of typeV4L2_CTRL_TYPE_CTRL_CLASS.VIDIOC_QUERYCTRL will return the name of thiscontrol class which can be used as the title of a tab page within acontrol panel.
The flags field of struct v4l2_queryctrl also contains hints onthe behavior of the control. See theVIDIOC_QUERYCTRL documentationfor more details.
1.9.5. MPEG Control Reference
Below all controls within the MPEG control class aredescribed. First the generic controls, then controls specific forcertain hardware.
1.9.5.1. Generic MPEG Controls
Table 1-2. MPEG Control IDs
V4L2_CID_MPEG_CLASS | class | ||||||||||||||||||||||||||||
The MPEG classdescriptor. Calling VIDIOC_QUERYCTRL for this control will return adescription of this control class. This description can be used as thecaption of a Tab page in a GUI, for example. | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_TYPE | enum | ||||||||||||||||||||||||||||
| The MPEG-1, -2 or -4output stream type. One cannot assume anything here. Each hardwareMPEG encoder tends to support different subsets of the available MPEGstream types. The currently defined stream types are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_PMT | integer | ||||||||||||||||||||||||||||
| Program Map TablePacket ID for the MPEG transport stream (default 16) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_AUDIO | integer | ||||||||||||||||||||||||||||
| Audio Packet ID forthe MPEG transport stream (default 256) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_VIDEO | integer | ||||||||||||||||||||||||||||
| Video Packet ID forthe MPEG transport stream (default 260) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PID_PCR | integer | ||||||||||||||||||||||||||||
| Packet ID for theMPEG transport stream carrying PCR fields (default 259) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PES_ID_AUDIO | integer | ||||||||||||||||||||||||||||
| Audio ID for MPEGPES | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_PES_ID_VIDEO | integer | ||||||||||||||||||||||||||||
| Video ID for MPEGPES | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_STREAM_VBI_FMT | enum | ||||||||||||||||||||||||||||
| Some cards can embedVBI data (e. g. Closed Caption, Teletext) into the MPEG stream. Thiscontrol selects whether VBI data should be embedded, and if so, whatembedding method should be used. The list of possible VBI formatsdepends on the driver. The currently defined VBI format typesare: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ | enum | ||||||||||||||||||||||||||||
| MPEG Audio samplingfrequency. Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_ENCODING | enum | ||||||||||||||||||||||||||||
| MPEG Audio encoding.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_L1_BITRATE | enum | ||||||||||||||||||||||||||||
| Layer I bitrate.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_L2_BITRATE | enum | ||||||||||||||||||||||||||||
| Layer II bitrate.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_L3_BITRATE | enum | ||||||||||||||||||||||||||||
| Layer III bitrate.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_MODE | enum | ||||||||||||||||||||||||||||
| MPEG Audio mode.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_MODE_EXTENSION | enum | ||||||||||||||||||||||||||||
| Joint Stereoaudio mode extension. In Layer I and II they indicate which subbandsare in intensity stereo. All other subbands are coded in stereo. LayerIII is not (yet) supported. Possible valuesare: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_EMPHASIS | enum | ||||||||||||||||||||||||||||
| Audio Emphasis.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_CRC | enum | ||||||||||||||||||||||||||||
| CRC method. Possiblevalues are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_AUDIO_MUTE | bool | ||||||||||||||||||||||||||||
| Mutes the audio whencapturing. This is not done by muting audio hardware, which can stillproduce a slight hiss, but in the encoder itself, guaranteeing a fixedand reproducable audio bitstream. 0 = unmuted, 1 = muted. | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_ENCODING | enum | ||||||||||||||||||||||||||||
| MPEG Video encodingmethod. Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_ASPECT | enum | ||||||||||||||||||||||||||||
| Video aspect.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_B_FRAMES | integer | ||||||||||||||||||||||||||||
| Number of B-Frames(default 2) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_GOP_SIZE | integer | ||||||||||||||||||||||||||||
| GOP size (default12) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_GOP_CLOSURE | bool | ||||||||||||||||||||||||||||
| GOP closure (default1) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_PULLDOWN | bool | ||||||||||||||||||||||||||||
| Enable 3:2 pulldown(default 0) | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_BITRATE_MODE | enum | ||||||||||||||||||||||||||||
| Video bitrate mode.Possible values are: | |||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_BITRATE | integer | ||||||||||||||||||||||||||||
| Video bitrate in bitsper second. | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_BITRATE_PEAK | integer | ||||||||||||||||||||||||||||
| Peak video bitrate inbits per second. Must be larger or equal to the average video bitrate.It is ignored if the video bitrate mode is set to constantbitrate. | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION | integer | ||||||||||||||||||||||||||||
| For every capturedframe, skip this many subsequent frames (default 0). | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_MUTE | bool | ||||||||||||||||||||||||||||
| "Mutes" the video to afixed color when capturing. This is useful for testing, to produce afixed video bitstream. 0 = unmuted, 1 = muted. | |||||||||||||||||||||||||||||
V4L2_CID_MPEG_VIDEO_MUTE_YUV | integer | ||||||||||||||||||||||||||||
| Sets the "mute" colorof the video. The supplied 32-bit integer is interpreted as follows (bit0 = least significant bit): | |||||||||||||||||||||||||||||
|
1.9.5.2. CX2341x MPEG Controls
The following MPEG class controls deal with MPEGencoding settings that are specific to the Conexant CX23415 andCX23416 MPEG encoding chips.
Table 1-3. CX2341x Control IDs
V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE | enum | ||||||||||
Sets the SpatialFilter mode (default MANUAL). Possible valuesare: | |||||||||||
| |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER | integer (0-15) | ||||||||||
| The setting for theSpatial Filter. 0 = off, 15 = maximum. (Default is 0.) | |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE | enum | ||||||||||
Select the algorithmto use for the Luma Spatial Filter (default1D_HOR). Possible values: | |||||||||||
| |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE | enum | ||||||||||
Select the algorithmfor the Chroma Spatial Filter (default 1D_HOR).Possible values are: | |||||||||||
| |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE | enum | ||||||||||
Sets the TemporalFilter mode (default MANUAL). Possible valuesare: | |||||||||||
| |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER | integer (0-31) | ||||||||||
| The setting for theTemporal Filter. 0 = off, 31 = maximum. (Default is 8 for full-scalecapturing and 0 for scaled capturing.) | |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE | enum | ||||||||||
Median Filter Type(default OFF). Possible values are: | |||||||||||
| |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_BOTTOM | integer (0-255) | ||||||||||
| Threshold above whichthe luminance median filter is enabled (default 0) | |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_TOP | integer (0-255) | ||||||||||
| Threshold below whichthe luminance median filter is enabled (default 255) | |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_BOTTOM | integer (0-255) | ||||||||||
| Threshold above whichthe chroma median filter is enabled (default 0) | |||||||||||
V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_TOP | integer (0-255) | ||||||||||
| Threshold below whichthe chroma median filter is enabled (default 255) | |||||||||||
V4L2_CID_MPEG_CX2341X_STREAM_INSERT_NAV_PACKETS | bool | ||||||||||
| The CX2341X MPEG encodercan insert one empty MPEG-2 PES packet into the stream between everyfour video frames. The packet size is 2048 bytes, including thepacket_start_code_prefix and stream_id fields. The stream_id is 0xBF(private stream 2). The payload consists of 0x00 bytes, to be filledin by the application. 0 = do not insert, 1 = insert packets. |
1.9.6. Camera Control Reference
The Camera class includes controls for mechanical (orequivalent digital) features of a device such as controllable lensesor sensors.
Table 1-4. Camera Control IDs
V4L2_CID_CAMERA_CLASS | class | ||||||||
The Camera classdescriptor. Calling VIDIOC_QUERYCTRL for this control will return adescription of this control class. | |||||||||
V4L2_CID_EXPOSURE_AUTO | integer | ||||||||
| Enables automaticadjustments of the exposure time and/or iris aperture. The effect ofmanual changes of the exposure time or iris aperture while thesefeatures are enabled is undefined, drivers should ignore suchrequests. Possible values are: | |||||||||
| |||||||||
V4L2_CID_EXPOSURE_ABSOLUTE | integer | ||||||||
| Determines the exposuretime of the camera sensor. The exposure time is limited by the frameinterval. Drivers should interpret the values as 100 µs units,where the value 1 stands for 1/10000th of a second, 10000 for 1 secondand 100000 for 10 seconds. | |||||||||
V4L2_CID_EXPOSURE_AUTO_PRIORITY | boolean | ||||||||
WhenV4L2_CID_EXPOSURE_AUTO is set toAUTO orSHUTTER_PRIORITY,this control determines if the device may dynamically vary the framerate. By default this feature is disabled (0) and the frame rate mustremain constant. | |||||||||
V4L2_CID_PAN_RELATIVE | integer | ||||||||
| This control turns thecamera horizontally by the specified amount. The unit is undefined. Apositive value moves the camera to the right (clockwise when viewedfrom above), a negative value to the left. A value of zero does notcause motion. | |||||||||
V4L2_CID_TILT_RELATIVE | integer | ||||||||
| This control turns thecamera vertically by the specified amount. The unit is undefined. Apositive value moves the camera up, a negative value down. A value ofzero does not cause motion. | |||||||||
V4L2_CID_PAN_RESET | boolean | ||||||||
When this control is setto TRUE (1), the camera moves horizontally to thedefault position. | |||||||||
V4L2_CID_TILT_RESET | boolean | ||||||||
When this control is setto TRUE (1), the camera moves vertically to thedefault position. | |||||||||
V4L2_CID_PAN_ABSOLUTE | integer | ||||||||
| This controlturns the camera horizontally to the specified position. Positivevalues move the camera to the right (clockwise when viewed from above),negative values to the left. Drivers should interpret the values as arcseconds, with valid values between -180 * 3600 and +180 * 3600inclusive. | |||||||||
V4L2_CID_TILT_ABSOLUTE | integer | ||||||||
| This controlturns the camera vertically to the specified position. Positive valuesmove the camera up, negative values down. Drivers should interpret thevalues as arc seconds, with valid values between -180 * 3600 and +180* 3600 inclusive. | |||||||||
V4L2_CID_FOCUS_ABSOLUTE | integer | ||||||||
| This control sets thefocal point of the camera to the specified position. The unit isundefined. Positive values set the focus closer to the camera,negative values towards infinity. | |||||||||
V4L2_CID_FOCUS_RELATIVE | integer | ||||||||
| This control moves thefocal point of the camera by the specified amount. The unit isundefined. Positive values move the focus closer to the camera,negative values towards infinity. | |||||||||
V4L2_CID_FOCUS_AUTO | boolean | ||||||||
| Enables automatic focusadjustments. The effect of manual focus adjustments while this featureis enabled is undefined, drivers should ignore such requests. | |||||||||
1.10. Data Formats
1.10.1. Data Format Negotiation
Different devices exchange different kinds of data withapplications, for example video images, raw or sliced VBI data, RDSdatagrams. Even within one kind many different formats are possible,in particular an abundance of image formats. Although drivers mustprovide a default and the selection persists across closing andreopening a device, applications should always negotiate a data formatbefore engaging in data exchange. Negotiation means the applicationasks for a particular format and the driver selects and reports thebest the hardware can do to satisfy the request. Of courseapplications can also just query the current selection.
A single mechanism exists to negotiate all data formatsusing the aggregate struct v4l2_format and theVIDIOC_G_FMT andVIDIOC_S_FMT ioctls. Additionally theVIDIOC_TRY_FMT ioctl can beused to examine what the hardwarecould do,without actually selecting a new data format. The data formatssupported by the V4L2 API are covered in the respective device sectioninChapter 4. For a closer look at image formats seeChapter 2.
The VIDIOC_S_FMT ioctl is a majorturning-point in the initialization sequence. Prior to this pointmultiple panel applications can access the same device concurrently toselect the current input, change controls or modify other properties.The first VIDIOC_S_FMT assigns a logical stream(video data, VBI data etc.) exclusively to one file descriptor.
Exclusive means no other application, more precisely noother file descriptor, can grab this stream or change deviceproperties inconsistent with the negotiated parameters. A videostandard change for example, when the new standard uses a differentnumber of scan lines, can invalidate the selected image format.Therefore only the file descriptor owning the stream can makeinvalidating changes. Accordingly multiple file descriptors whichgrabbed different logical streams prevent each other from interferingwith their settings. When for example video overlay is about to startor already in progress, simultaneous video capturing may be restrictedto the same cropping and image size.
When applications omit theVIDIOC_S_FMT ioctl its locking side effects areimplied by the next step, the selection of an I/O method with theVIDIOC_REQBUFS ioctl or implicit with the first read() orwrite() call.
Generally only one logical stream can be assigned to afile descriptor, the exception being drivers permitting simultaneousvideo capturing and overlay using the same file descriptor forcompatibility with V4L and earlier versions of V4L2. Switching thelogical stream or returning into "panel mode" is possible by closingand reopening the device. Driversmay support aswitch usingVIDIOC_S_FMT.
All drivers exchanging data withapplications must support the VIDIOC_G_FMT andVIDIOC_S_FMT ioctl. Implementation of theVIDIOC_TRY_FMT is highly recommended butoptional.
1.10.2. Image Format Enumeration
Apart of the generic format negotiation functionsa special ioctl to enumerate all image formats supported by videocapture, overlay or output devices is available.[11]
The VIDIOC_ENUM_FMT ioctl must be supportedby all drivers exchanging image data with applications.
1.11. Image Cropping, Insertion and Scaling
Some video capture devices can sample a subsection of thepicture and shrink or enlarge it to an image of arbitrary size. Wecall these abilities cropping and scaling. Some video output devicescan scale an image up or down and insert it at an arbitrary scan lineand horizontal offset into a video signal.
Applications can use the following API to select an area inthe video signal, query the default area and the hardware limits.Despite their name, theVIDIOC_CROPCAP,VIDIOC_G_CROPandVIDIOC_S_CROP ioctls apply to input as well as outputdevices.
Scaling requires a source and a target. On a video captureor overlay device the source is the video signal, and the croppingioctls determine the area actually sampled. The target are imagesread by the application or overlaid onto the graphics screen. Theirsize (and position for an overlay) is negotiated with theVIDIOC_G_FMT andVIDIOC_S_FMT ioctls.
On a video output device the source are the images passed inby the application, and their size is again negotiated with theVIDIOC_G/S_FMT ioctls, or may be encoded in acompressed video stream. The target is the video signal, and thecropping ioctls determine the area where the images areinserted.
Source and target rectangles are defined even if the devicedoes not support scaling or theVIDIOC_G/S_CROPioctls. Their size (and position where applicable) will be fixed inthis case.All capture and output device must support theVIDIOC_CROPCAP ioctl such that applications candetermine if scaling takes place.
1.11.1. Cropping Structures
Figure 1-1. Image Cropping, Insertion and Scaling
For capture devices the coordinates of the top leftcorner, width and height of the area which can be sampled is given bythebounds substructure of thestruct v4l2_cropcap returned by theVIDIOC_CROPCAPioctl. To support a wide range of hardware this specification does notdefine an origin or units. However by convention drivers shouldhorizontally count unscaled samples relative to 0H (the leading edgeof the horizontal sync pulse, see Figure 4-1).Vertically ITU-R linenumbers of the first field (Figure 4-2,Figure 4-3), multiplied by two if the driver can capture bothfields.
The top left corner, width and height of the sourcerectangle, that is the area actually sampled, is given by struct v4l2_cropusing the same coordinate system as struct v4l2_cropcap. Applications canuse the VIDIOC_G_CROP andVIDIOC_S_CROP ioctls to get and set thisrectangle. It must lie completely within the capture boundaries andthe driver may further adjust the requested size and/or positionaccording to hardware limitations.
Each capture device has a default source rectangle, givenby the defrect substructure ofstruct v4l2_cropcap. The center of this rectangle shall align with thecenter of the active picture area of the video signal, and cover whatthe driver writer considers the complete picture. Drivers shall resetthe source rectangle to the default when the driver is first loaded,but not later.
For output devices these structures and ioctls are usedaccordingly, defining thetarget rectangle wherethe images will be inserted into the video signal.
1.11.2. Scaling Adjustments
Video hardware can have various cropping, insertion andscaling limitations. It may only scale up or down, support onlydiscrete scaling factors, or have different scaling abilities inhorizontal and vertical direction. Also it may not support scaling atall. At the same time the struct v4l2_crop rectangle may have to bealigned, and both the source and target rectangles may have arbitraryupper and lower size limits. In particular the maximumwidth and heightin struct v4l2_crop may be smaller than thestruct v4l2_cropcap.bounds area. Therefore, asusual, drivers are expected to adjust the requested parameters andreturn the actual values selected.
Applications can change the source or the target rectanglefirst, as they may prefer a particular image size or a certain area inthe video signal. If the driver has to adjust both to satisfy hardwarelimitations, the last requested rectangle shall take priority, and thedriver should preferably adjust the opposite one. The VIDIOC_TRY_FMTioctl however shall not change the driver state and therefore onlyadjust the requested rectangle.
Suppose scaling on a video capture device is restricted toa factor 1:1 or 2:1 in either direction and the target image size mustbe a multiple of 16 × 16 pixels. The source croppingrectangle is set to defaults, which are also the upper limit in thisexample, of 640 × 400 pixels at offset 0, 0. Anapplication requests an image size of 300 × 225pixels, assuming video will be scaled down from the "full picture"accordingly. The driver sets the image size to the closest possiblevalues 304 × 224, then chooses the cropping rectangleclosest to the requested size, that is 608 × 224(224 × 2:1 would exceed the limit 400). The offset0, 0 is still valid, thus unmodified. Given the default croppingrectangle reported byVIDIOC_CROPCAP theapplication can easily propose another offset to center the croppingrectangle.
Now the application may insist on covering an area using apicture aspect ratio closer to the original request, so it asks for acropping rectangle of 608 × 456 pixels. The presentscaling factors limit cropping to 640 × 384, so thedriver returns the cropping size 608 × 384 and adjuststhe image size to closest possible 304 × 192.
1.11.3. Examples
Source and target rectangles shall remain unchanged acrossclosing and reopening a device, such that piping data into or out of adevice will work without special preparations. More advancedapplications should ensure the parameters are suitable before startingI/O.
Example 1-10. Resetting the cropping parameters
(A video capture device is assumed; changeV4L2_BUF_TYPE_VIDEO_CAPTURE for otherdevices.)
struct v4l2_cropcap cropcap;
struct v4l2_crop crop; memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd,VIDIOC_CROPCAP, &cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
} memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect; /* Ignore if cropping is not supported (EINVAL). */ if (-1 == ioctl (fd,VIDIOC_S_CROP, &crop)
&& errno != EINVAL) {
perror ("VIDIOC_S_CROP");
exit (EXIT_FAILURE);
}
Example 1-11. Simple downscaling
(A video capture device is assumed.)
struct v4l2_cropcap cropcap;
struct v4l2_format format; reset_cropping_parameters (); /* Scale down to 1/4 size of full picture. */ memset (&format, 0, sizeof (format)); /* defaults */ format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; format.fmt.pix.width = cropcap.defrect.width >> 1;
format.fmt.pix.height = cropcap.defrect.height >> 1;
format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV; if (-1 == ioctl (fd,VIDIOC_S_FMT, &format)) {
perror ("VIDIOC_S_FORMAT");
exit (EXIT_FAILURE);
} /* We could check the actual image size now, the actual scaling factor
or if the driver can scale at all. */
Example 1-12. Selecting an output area
struct v4l2_cropcap cropcap;
struct v4l2_crop crop; memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_OUTPUT; if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
} memset (&crop, 0, sizeof (crop)); crop.type = V4L2_BUF_TYPE_VIDEO_OUTPUT;
crop.c = cropcap.defrect; /* Scale the width and height to 50 % of their original size
and center the output. */ crop.c.width /= 2;
crop.c.height /= 2;
crop.c.left += crop.c.width / 2;
crop.c.top += crop.c.height / 2; /* Ignore if cropping is not supported (EINVAL). */ if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
&& errno != EINVAL) {
perror ("VIDIOC_S_CROP");
exit (EXIT_FAILURE);
}
Example 1-13. Current scaling factor and pixel aspect
(A video capture device is assumed.)
struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
struct v4l2_format format;
double hscale, vscale;
double aspect;
int dwidth, dheight; memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd,VIDIOC_CROPCAP, &cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
} memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd,VIDIOC_G_CROP, &crop)) {
if (errno != EINVAL) {
perror ("VIDIOC_G_CROP");
exit (EXIT_FAILURE);
} /* Cropping not supported. */
crop.c = cropcap.defrect;
} memset (&format, 0, sizeof (format));
format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == ioctl (fd,VIDIOC_G_FMT, &format)) {
perror ("VIDIOC_G_FMT");
exit (EXIT_FAILURE);
} /* The scaling applied by the driver. */ hscale = format.fmt.pix.width / (double) crop.c.width;
vscale = format.fmt.pix.height / (double) crop.c.height; aspect = cropcap.pixelaspect.numerator /
(double) cropcap.pixelaspect.denominator;
aspect = aspect * hscale / vscale; /* Devices following ITU-R BT.601 do not capture
square pixels. For playback on a computer monitor
we should scale the images to this size. */ dwidth = format.fmt.pix.width / aspect;
dheight = format.fmt.pix.height;
1.12. Streaming Parameters
Streaming parameters are intended to optimize the videocapture process as well as I/O. Presently applications can request ahigh quality capture mode with theVIDIOC_S_PARM ioctl.
The current video standard determines a nominal number offrames per second. If less than this number of frames is to becaptured or output, applications can request frame skipping orduplicating on the driver side. This is especially useful when usingtheread() orwrite(), which are not augmented by timestampsor sequence counters, and to avoid unneccessary data copying.
Finally these ioctls can be used to determine the number ofbuffers used internally by a driver in read/write mode. Forimplications see the section discussing theread()function.
To get and set the streaming parameters applications callthe VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They takea pointer to a struct v4l2_streamparm, which contains a union holdingseparate parameters for input and output devices.
These ioctls are optional, drivers need not implementthem. If so, they return theEINVAL error code.
Chapter 2. Image Formats
The V4L2 API was primarily designed for devices exchangingimage data with applications. Thev4l2_pix_format structure defines the formatand layout of an image in memory. Image formats are negotiated withtheVIDIOC_S_FMT ioctl. (The explanations here focus on videocapturing and output, for overlay frame buffer formats see alsoVIDIOC_G_FBUF.)
Table 2-1. struct v4l2_pix_format
| __u32 | width | Image width in pixels. |
| __u32 | height | Image height in pixels. |
Applications set these fields torequest an image size, drivers return the closest possible values. Incase of planar formats thewidth andheight applies to the largest plane. Toavoid ambiguities drivers must return values rounded up to a multipleof the scale factor of any smaller planes. For example when the imageformat is YUV 4:2:0,width andheight must be multiples of two. | ||
| __u32 | pixelformat | The pixel format or type of compression, set by theapplication. This is a little endianfour character code. V4L2 definesstandard RGB formats inTable 2-1, YUV formats inSection 2.5, and reserved codes inTable 2-8 |
| enum v4l2_field | field | Video images are typically interlaced. Applicationscan request to capture or output only the top or bottom field, or bothfields interlaced or sequentially stored in one buffer or alternatingin separate buffers. Drivers return the actual field order selected.For details see Section 3.6. |
| __u32 | bytesperline | Distance in bytes between the leftmost pixels in twoadjacent lines. |
Both applications and driverscan set this field to request padding bytes at the end of each line.Drivers however may ignore the value requested by the application,returning Video hardware may access padding bytes,therefore they must reside in accessible memory. Consider cases wherepadding bytes after the last line of an image cross a system pageboundary. Input devices may write padding bytes, the value isundefined. Output devices ignore the contents of paddingbytes. When the image format is planar the | ||
| __u32 | sizeimage | Size in bytes of the buffer to hold a complete image,set by the driver. Usually this isbytesperline timesheight. When the image consists of variablelength compressed data this is the maximum number of bytes required tohold an image. |
| enum v4l2_colorspace | colorspace | This information supplements thepixelformat and must be set by the driver,seeSection 2.2. |
| __u32 | priv | Reserved for custom (driver defined) additionalinformation about formats. When not used drivers and applications mustset this field to zero. |
2.1. Standard Image Formats
In order to exchange images between drivers andapplications, it is necessary to have standard image data formatswhich both sides will interpret the same way. V4L2 includes severalsuch formats, and this section is intended to be an unambiguousspecification of the standard image data formats in V4L2.
V4L2 drivers are not limited to these formats, however.Driver-specific formats are possible. In that case the application maydepend on a codec to convert images to one of the standard formatswhen needed. But the data can still be stored and retrieved in theproprietary format. For example, a device may support a proprietarycompressed format. Applications can still capture and save the data inthe compressed format, saving much disk space, and later use a codecto convert the images to the X Windows screen format when the video isto be displayed.
Even so, ultimately, some standard formats are needed, sothe V4L2 specification would not be complete without well-definedstandard formats.
The V4L2 standard formats are mainly uncompressed formats. Thepixels are always arranged in memory from left to right, and from topto bottom. The first byte of data in the image buffer is always forthe leftmost pixel of the topmost row. Following that is the pixelimmediately to its right, and so on until the end of the top row ofpixels. Following the rightmost pixel of the row there may be zero ormore bytes of padding to guarantee that each row of pixel data has acertain alignment. Following the pad bytes, if any, is data for theleftmost pixel of the second row from the top, and so on. The last rowhas just as many pad bytes after it as the other rows.
In V4L2 each format has an identifier which looks likePIX_FMT_XXX, defined in thevideodev.h header file. These identifiersrepresentfour character codeswhich are also listed below, however they are not the same as thoseused in the Windows world.
2.2. Colorspaces
[intro]
- Gamma Correction
[to do]
E' = f(R)
E' = f(G)
E' = f(B)
- Construction of luminance and color-differencesignals
[to do]
E' =Coeff E'+ Coeff E'+ Coeff E'
(E' - E') = E'- Coeff E'- Coeff E'- Coeff E'
(E' - E') = E'- Coeff E'- Coeff E'- Coeff E'
- Re-normalized color-difference signals
The color-difference signals are scaled back to unityrange [-0.5;+0.5]:
K = 0.5 / (1 - Coeff)
K = 0.5 / (1 - Coeff)
P =K (E' - E') = 0.5 (Coeff / Coeff) E'+ 0.5 (Coeff / Coeff) E'+ 0.5 E'
P =K (E' - E') = 0.5 E'+ 0.5 (Coeff / Coeff) E'+ 0.5 (Coeff / Coeff) E'
- Quantization
[to do]
Y' = (Lum. Levels - 1) · E' + Lum. Offset
C = (Chrom. Levels - 1)· P + Chrom. Offset
C = (Chrom. Levels - 1)· P + Chrom. Offset
Rounding to the nearest integer and clamping to the range[0;255] finally yields the digital color components Y'CbCrstored in YUV images.
Example 2-1. ITU-R Rec. BT.601 color conversion
Forward Transformation
int ER, EG, EB; /* gamma corrected RGB input [0;255] */
int Y1, Cb, Cr; /* output [0;255] */ double r, g, b; /* temporaries */
double y1, pb, pr; int
clamp (double x)
{
int r = x; /* round to nearest */ if (r < 0) return 0;
else if (r > 255) return 255;
else return r;
} r = ER / 255.0;
g = EG / 255.0;
b = EB / 255.0; y1 = 0.299 * r + 0.587 * g + 0.114 * b;
pb = -0.169 * r - 0.331 * g + 0.5 * b;
pr = 0.5 * r - 0.419 * g - 0.081 * b; Y1 = clamp (219 * y1 + 16);
Cb = clamp (224 * pb + 128);
Cr = clamp (224 * pr + 128); /* or shorter */ y1 = 0.299 * ER + 0.587 * EG + 0.114 * EB; Y1 = clamp ( (219 / 255.0) * y1 + 16);
Cb = clamp (((224 / 255.0) / (2 - 2 * 0.114)) * (EB - y1) + 128);
Cr = clamp (((224 / 255.0) / (2 - 2 * 0.299)) * (ER - y1) + 128);
Inverse Transformation
int Y1, Cb, Cr; /* gamma pre-corrected input [0;255] */
int ER, EG, EB; /* output [0;255] */ double r, g, b; /* temporaries */
double y1, pb, pr; int
clamp (double x)
{
int r = x; /* round to nearest */ if (r < 0) return 0;
else if (r > 255) return 255;
else return r;
} y1 = (255 / 219.0) * (Y1 - 16);
pb = (255 / 224.0) * (Cb - 128);
pr = (255 / 224.0) * (Cr - 128); r = 1.0 * y1 + 0 * pb + 1.402 * pr;
g = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
b = 1.0 * y1 + 1.772 * pb + 0 * pr; ER = clamp (r * 255); /* [ok? one should prob. limit y1,pb,pr] */
EG = clamp (g * 255);
EB = clamp (b * 255);
Table 2-2. enum v4l2_colorspace
V4L2_COLORSPACE_SMPTE170M | 1 | NTSC/PAL according to , | x = 0.630, y = 0.340 | x = 0.310, y = 0.595 | x = 0.155, y = 0.070 | x = 0.3127, y = 0.3290, Illuminant D | E' = 4.5 I for I ≤0.018,1.099 I - 0.099 for 0.018 < I | 0.299 E'+ 0.587 E'+ 0.114 E' | 219 E' + 16 | 224 P + 128 |
V4L2_COLORSPACE_SMPTE240M | 2 | 1125-Line (US) HDTV, see | x = 0.630, y = 0.340 | x = 0.310, y = 0.595 | x = 0.155, y = 0.070 | x = 0.3127, y = 0.3290, Illuminant D | E' = 4 I for I ≤0.0228,1.1115 I - 0.1115 for 0.0228 < I | 0.212 E'+ 0.701 E'+ 0.087 E' | 219 E' + 16 | 224 P + 128 |
V4L2_COLORSPACE_REC709 | 3 | HDTV and modern devices, see | x = 0.640, y = 0.330 | x = 0.300, y = 0.600 | x = 0.150, y = 0.060 | x = 0.3127, y = 0.3290, Illuminant D | E' = 4.5 I for I ≤0.018,1.099 I - 0.099 for 0.018 < I | 0.2125 E' + 0.7154 E'+ 0.0721 E' | 219 E' + 16 | 224 P + 128 |
V4L2_COLORSPACE_BT878 | 4 | Broken Bt878 extents[b], | ? | ? | ? | ? | ? | 0.299 E'+ 0.587 E'+ 0.114 E' | 237 E' + 16 | 224 P + 128 (probably) |
V4L2_COLORSPACE_470_SYSTEM_M | 5 | M/NTSC[c] according to, | x = 0.67, y = 0.33 | x = 0.21, y = 0.71 | x = 0.14, y = 0.08 | x = 0.310, y = 0.316, Illuminant C | ? | 0.299 E'+ 0.587 E'+ 0.114 E' | 219 E' + 16 | 224 P + 128 |
V4L2_COLORSPACE_470_SYSTEM_BG | 6 | 625-line PAL and SECAM systems according to , | x = 0.64, y = 0.33 | x = 0.29, y = 0.60 | x = 0.15, y = 0.06 | x = 0.313, y = 0.329,Illuminant D | ? | 0.299 E'+ 0.587 E'+ 0.114 E' | 219 E' + 16 | 224 P + 128 |
V4L2_COLORSPACE_JPEG | 7 | JPEG Y'CbCr, see , | ? | ? | ? | ? | ? | 0.299 E'+ 0.587 E'+ 0.114 E' | 256 E' + 16[d] | 256 P + 128 |
V4L2_COLORSPACE_SRGB | 8 | [?] | x = 0.640, y = 0.330 | x = 0.300, y = 0.600 | x = 0.150, y = 0.060 | x = 0.3127, y = 0.3290, Illuminant D | E' = 4.5 I for I ≤0.018,1.099 I - 0.099 for 0.018 < I | n/a | ||
| Notes: a. The coordinates of the color primaries aregiven in the CIE system (1931) b. The ubiquitous Bt878 video capture chipquantizes E' to 238 levels, yielding a rangeof Y' = 16 … 253, unlike Rec. 601 Y' = 16 …235. This is not a typo in the Bt878 documentation, it has beenimplemented in silicon. The chroma extents are unclear. c. No identifier exists for M/PAL which usesthe chromaticities of M/NTSC, the remaining parameters are equal to B andG/PAL. d. Note JFIF quantizesY'PP in range [0;+1] and[-0.5;+0.5] to257 levels, however Y'CbCr signalsare still clamped to [0;255]. |
2.3. Indexed Format
In this format each pixel is represented by an 8 bit indexinto a 256 entry ARGB palette. It is intended forVideo Output Overlays only. There are no ioctls toaccess the palette, this
must be done with ioctls of the Linux framebuffer API.
Table 2-3. Indexed Image Format
V4L2_PIX_FMT_PAL8 | 'PAL8' | i | i | i | i | i | i | i | i |
2.4. RGB Formats
- Table of Contents
- Packed RGB formats -- Packed RGB formats
- V4L2_PIX_FMT_SBGGR8 ('BA81') -- Bayer RGB format
- V4L2_PIX_FMT_SBGGR16 ('BA82') -- Bayer RGB format
Packed RGB formats
Name
Packed RGB formats -- Packed RGB formats
Description
These formats are designed to match the pixel formats oftypical PC
graphics frame buffers. They occupy 8, 16, 24 or 32 bitsper pixel. These
are all packed-pixel formats, meaning all the datafor a pixel lie next
to each other in memory.
When one of these formats is used, drivers shall report thecolorspace
V4L2_COLORSPACE_SRGB
Table 2-1. Packed RGB Image Formats
V4L2_PIX_FMT_RGB332 | 'RGB1' | b | b | g | g | g | r | r | r | ||||||||||||||||||||||||||||
V4L2_PIX_FMT_RGB444 | 'R444' | g | g | g | g | b | b | b | b | a | a | a | a | r | r | r | r | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555 | 'RGBO' | g | g | g | r | r | r | r | r | a | b | b | b | b | b | g | g | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565 | 'RGBP' | g | g | g | r | r | r | r | r | b | b | b | b | b | g | g | g | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555X | 'RGBQ' | a | b | b | b | b | b | g | g | g | g | g | r | r | r | r | r | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565X | 'RGBR' | b | b | b | b | b | g | g | g | g | g | g | r | r | r | r | r | ||||||||||||||||||||
V4L2_PIX_FMT_BGR24 | 'BGR3' | b | b | b | b | b | b | b | b | g | g | g | g | g | g | g | g | r | r | r | r | r | r | r | r | ||||||||||||
V4L2_PIX_FMT_RGB24 | 'RGB3' | r | r | r | r | r | r | r | r | g | g | g | g | g | g | g | g | b | b | b | b | b | b | b | b | ||||||||||||
V4L2_PIX_FMT_BGR32 | 'BGR4' | b | b | b | b | b | b | b | b | g | g | g | g | g | g | g | g | r | r | r | r | r | r | r | r | a | a | a | a | a | a | a | a | ||||
V4L2_PIX_FMT_RGB32 | 'RGB4' | r | r | r | r | r | r | r | r | g | g | g | g | g | g | g | g | b | b | b | b | b | b | b | b | a | a | a | a | a | a | a | a |
Bit 7 is the most significant bit. The value of a = alphabits is
undefined when reading from the driver, ignored when writingto the
driver, except when alpha blending has been negotiated for aVideo
Overlay or Video Output Overlay.
Example 2-1. V4L2_PIX_FMT_BGR24 4 × 4 pixelimage
Byte Order. Each cell is one byte.
| start + 0: | B | G | R | B | G | R | B | G | R | B | G | R |
| start + 12: | B | G | R | B | G | R | B | G | R | B | G | R |
| start + 24: | B | G | R | B | G | R | B | G | R | B | G | R |
| start + 36: | B | G | R | B | G | R | B | G | R | B | G | R |
Some RGB formats above are uncommon and were probablydefined in error. Drivers may interpret them as inTable 2-2.
Table 2-2. Packed RGB Image Formats (corrected)
V4L2_PIX_FMT_RGB332 | 'RGB1' | r | r | r | g | g | g | b | b | ||||||||||||||||||||||||||||
V4L2_PIX_FMT_RGB444 | 'R444' | g | g | g | g | b | b | b | b | a | a | a | a | r | r | r | r | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555 | 'RGBO' | g | g | g | b | b | b | b | b | a | r | r | r | r | r | g | g | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565 | 'RGBP' | g | g | g | b | b | b | b | b | r | r | r | r | r | g | g | g | ||||||||||||||||||||
V4L2_PIX_FMT_RGB555X | 'RGBQ' | a | r | r | r | r | r | g | g | g | g | g | b | b | b | b | b | ||||||||||||||||||||
V4L2_PIX_FMT_RGB565X | 'RGBR' | r | r | r | r | r | g | g | g | g | g | g | b | b | b | b | b | ||||||||||||||||||||
V4L2_PIX_FMT_BGR24 | 'BGR3' | b | b | b | b | b | b | b | b | g | g | g | g | g | g | g | g | r | r | r | r | r | r | r | r | ||||||||||||
V4L2_PIX_FMT_RGB24 | 'RGB3' | r | r | r | r | r | r | r | r | g | g | g | g | g | g | g | g | b | b | b | b | b | b | b | b | ||||||||||||
V4L2_PIX_FMT_BGR32 | 'BGR4' | b | b | b | b | b | b | b | b | g | g | g | g | g | g | g | g | r | r | r | r | r | r | r | r | a | a | a | a | a | a | a | a | ||||
V4L2_PIX_FMT_RGB32 | 'RGB4' | a | a | a | a | a | a | a | a | r | r | r | r | r | r | r | r | g | g | g | g | g | g | g | g | b | b | b | b | b | b | b | b |
A test utility to determine which RGB formats a driveractually supports is available from the LinuxTV v4l-dvb repository.Seehttp://linuxtv.org/repo/ for access instructions.
V4L2_PIX_FMT_SBGGR8 ('BA81')
Name
V4L2_PIX_FMT_SBGGR8 -- Bayer RGB format
Description
This is commonly the native format of digital cameras,reflecting the
arrangement of sensors on the CCD device. Only one red,green or blue
value is given for each pixel. Missing components mustbe interpolated
from neighbouring pixels. From left to right the
firstrow consists of a blue and green value, the second row of a green
andred value. This scheme repeats to the right and down for every
twocolumns and rows.
Example 2-1. V4L2_PIX_FMT_SBGGR8 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | B | G | B | G |
| start + 4: | G | R | G | R |
| start + 8: | B | G | B | G |
| start + 12: | G | R | G | R |
V4L2_PIX_FMT_SBGGR16 ('BA82')
Name
V4L2_PIX_FMT_SBGGR16 -- Bayer RGB format
Description
This format is similar to V4L2_PIX_FMT_SBGGR8, except each pixel
hasa depth of 16 bits. The least significant byte is stored at
lowermemory addresses (little-endian). Note the actual sampling
precisionmay be lower than 16 bits, for example 10 bits per
pixel with valuesin range 0 to 1023.
Example 2-1. V4L2_PIX_FMT_SBGGR16 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | B | B | G | G | B | B | G | G |
| start + 8: | G | G | R | R | G | G | R | R |
| start + 16: | B | B | G | G | B | B | G | G |
| start + 24: | G | G | R | R | G | G | R | R |
2.5. YUV Formats
- Table of Contents
- Packed YUV formats -- Packed YUV formats
- V4L2_PIX_FMT_GREY ('GREY') -- Grey-scale image
- V4L2_PIX_FMT_Y16 ('Y16 ') -- Grey-scale image
- V4L2_PIX_FMT_YUYV ('YUYV') -- Packed format with ½ horizontal chromaresolution, also known as YUV 4:2:2
- V4L2_PIX_FMT_UYVY ('UYVY') -- Variation of
V4L2_PIX_FMT_YUYVwith different order of samplesin memory - V4L2_PIX_FMT_Y41P ('Y41P') -- Format with ¼ horizontal chromaresolution, also known as YUV 4:1:1
- V4L2_PIX_FMT_YVU420 ('YV12'), V4L2_PIX_FMT_YUV420 ('YU12') -- Planar formats with ½ horizontal andvertical chroma resolution, also known as YUV 4:2:0
- V4L2_PIX_FMT_YVU410 ('YVU9'), V4L2_PIX_FMT_YUV410 ('YUV9') -- Planar formats with ¼ horizontal andvertical chroma resolution, also known as YUV 4:1:0
- V4L2_PIX_FMT_YUV422P ('422P') -- Format with ½ horizontal chroma resolution,also known as YUV 4:2:2. Planar layout as opposed to
V4L2_PIX_FMT_YUYV - V4L2_PIX_FMT_YUV411P ('411P') -- Format with ¼ horizontal chroma resolution,also known as YUV 4:1:1. Planar layout as opposed to
V4L2_PIX_FMT_Y41P - V4L2_PIX_FMT_NV12 ('NV12'), V4L2_PIX_FMT_NV21 ('NV21') -- Formats with ½ horizontal and verticalchroma resolution, also known as YUV 4:2:0. One luminance and onechrominance plane
with alternating chroma samples as opposed toV4L2_PIX_FMT_YVU420
YUV is the format native to TV broadcast and composite videosignals.
It separates the brightness information (Y) from the colorinformation (U
and V or Cb and Cr). The color information consists ofred and bluecolor
difference signals, this waythe green component can be reconstructed by subtracting from thebrightness component. SeeSection 2.2
for conversionexamples. YUV was chosen
because early television would only transmitbrightness information. To
add color in a way compatible with existingreceivers a new signal
carrier was added to transmit the colordifference signals. Secondary in
the YUV format the U and V componentsusually have
lower resolution than the Y component. This is an analogvideo
compression technique taking advantage of a property of thehuman visual
system, being more sensitive to brightnessinformation.
Packed YUV formats
Name
Packed YUV formats -- Packed YUV formats
Description
Similar to the packed RGB formats these formats storethe Y, Cb and Cr component of each pixel in one 16 or 32 bitword.
Table 2-1. Packed YUV Image Formats
V4L2_PIX_FMT_YUV444 | 'Y444' | Cb | Cb | Cb | Cb | Cr | Cr | Cr | Cr | a | a | a | a | Y' | Y' | Y' | Y' | ||||||||||||||||||||
V4L2_PIX_FMT_YUV555 | 'YUVO' | Cb | Cb | Cb | Cr | Cr | Cr | Cr | Cr | a | Y' | Y' | Y' | Y' | Y' | Cb | Cb | ||||||||||||||||||||
V4L2_PIX_FMT_YUV565 | 'YUVP' | Cb | Cb | Cb | Cr | Cr | Cr | Cr | Cr | Y' | Y' | Y' | Y' | Y' | Cb | Cb | Cb | ||||||||||||||||||||
V4L2_PIX_FMT_YUV32 | 'YUV4' | a | a | a | a | a | a | a | a | Y' | Y' | Y' | Y' | Y' | Y' | Y' | Y' | Cb | Cb | Cb | Cb | Cb | Cb | Cb | Cb | Cr | Cr | Cr | Cr | Cr | Cr | Cr | Cr |
Bit 7 is the most significant bit. The value of a = alphabits is
undefined when reading from the driver, ignored when writingto the
driver, except when alpha blending has been negotiated for aVideo
Overlay or Video Output Overlay.
V4L2_PIX_FMT_GREY ('GREY')
Name
V4L2_PIX_FMT_GREY -- Grey-scale image
Description
This is a grey-scale image. It is really a degenerateY'CbCr format which simply contains no Cb or Cr data.
Example 2-1. V4L2_PIX_FMT_GREY 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Y' | Y' | Y' |
| start + 4: | Y' | Y' | Y' | Y' |
| start + 8: | Y' | Y' | Y' | Y' |
| start + 12: | Y' | Y' | Y' | Y' |
V4L2_PIX_FMT_Y16 ('Y16 ')
Name
V4L2_PIX_FMT_Y16 -- Grey-scale image
Description
This is a grey-scale image with a depth of 16 bits perpixel. The
least significant byte is stored at lower memory
addresses(little-endian). Note the actual sampling precision may be
lower than16 bits, for example 10 bits per pixel with values in range 0
to1023.
Example 2-1. V4L2_PIX_FMT_Y16 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Y' | Y' | Y' | Y' | Y' | Y' | Y' |
| start + 8: | Y' | Y' | Y' | Y' | Y' | Y' | Y' | Y' |
| start + 16: | Y' | Y' | Y' | Y' | Y' | Y' | Y' | Y' |
| start + 24: | Y' | Y' | Y' | Y' | Y' | Y' | Y' | Y' |
V4L2_PIX_FMT_YUYV ('YUYV')
Name
V4L2_PIX_FMT_YUYV -- Packed format with ½ horizontal chromaresolution, also known as YUV 4:2:2
Description
In this format each four bytes is two pixels. Each fourbytes is two
Y's, a Cb and a Cr. Each Y goes to one of the pixels, andthe Cb and Cr
belong to both pixels. As you can see, the Cr and Cbcomponents have half
the horizontal resolution of the Y component.V4L2_PIX_FMT_YUYVis
known in the Windowsenvironment as YUY2.
Example 2-1. V4L2_PIX_FMT_YUYV 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Cb | Y' | Cr | Y' | Cb | Y' | Cr |
| start + 8: | Y' | Cb | Y' | Cr | Y' | Cb | Y' | Cr |
| start + 16: | Y' | Cb | Y' | Cr | Y' | Cb | Y' | Cr |
| start + 24: | Y' | Cb | Y' | Cr | Y' | Cb | Y' | Cr |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
V4L2_PIX_FMT_UYVY ('UYVY')
Name
V4L2_PIX_FMT_UYVY -- Variation ofV4L2_PIX_FMT_YUYV with different order of samplesin memory
Description
In this format each four bytes is two pixels. Each fourbytes is two
Y's, a Cb and a Cr. Each Y goes to one of the pixels, andthe Cb and Cr
belong to both pixels. As you can see, the Cr and Cbcomponents have half
the horizontal resolution of the Ycomponent.
Example 2-1. V4L2_PIX_FMT_UYVY 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' |
| start + 8: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' |
| start + 16: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' |
| start + 24: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
V4L2_PIX_FMT_Y41P ('Y41P')
Name
V4L2_PIX_FMT_Y41P -- Format with ¼ horizontal chromaresolution, also known as YUV 4:1:1
Description
In this format each 12 bytes is eight pixels. In thetwelve bytes are
two CbCr pairs and eight Y's. The first CbCr pairgoes with the first
four Y's, and the second CbCr pair goes with theother four Y's. The Cb
and Cr components have one fourth thehorizontal
resolution of the Y component.
Do not confuse this format with V4L2_PIX_FMT_YUV411P. Y41P is derived from "YUV 4:1:1packed", whileYUV411P stands for "YUV 4:1:1planar".
Example 2-1. V4L2_PIX_FMT_Y41P 8 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' | Y' | Y' | Y' | Y' |
| start + 12: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' | Y' | Y' | Y' | Y' |
| start + 24: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' | Y' | Y' | Y' | Y' |
| start + 36: | Cb | Y' | Cr | Y' | Cb | Y' | Cr | Y' | Y' | Y' | Y' | Y' |
Color Sample Location.
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | ||||||||
| 0 | Y | Y | C | Y | Y | Y | Y | C | Y | Y | |||||
| 1 | Y | Y | C | Y | Y | Y | Y | C | Y | Y | |||||
| 2 | Y | Y | C | Y | Y | Y | Y | C | Y | Y | |||||
| 3 | Y | Y | C | Y | Y | Y | Y | C | Y | Y |
V4L2_PIX_FMT_YVU420 ('YV12'), V4L2_PIX_FMT_YUV420 ('YU12')
Name
V4L2_PIX_FMT_YVU420, V4L2_PIX_FMT_YUV420 -- Planar formats with ½ horizontal andvertical chroma resolution, also known as YUV 4:2:0
Description
These are planar formats, as opposed to a packed format.The three
components are separated into three sub- images or planes.The Y plane is
first. The Y plane has one byte per pixel. ForV4L2_PIX_FMT_YVU420,
the Cr plane immediatelyfollows
the Y plane in memory. The Cr plane is half the width and halfthe
height of the Y plane (and of the image). Each Cr belongs to fourpixels,
a two-by-two square of the image. For example,Cr belongs to Y',Y', Y',
andY'. Following the Cr plane is the Cb plane,just like the Cr plane.V4L2_PIX_FMT_YUV420 isthe same except the Cb plane comes first, then the Cr plane.
If the Y plane has pad bytes after each row, then the Crand Cb planes
have half as many pad bytes after their rows. In otherwords, two Cx
rows (including padding) is exactly as long as one Y row(including
padding).
Example 2-1. V4L2_PIX_FMT_YVU420 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Y' | Y' | Y' |
| start + 4: | Y' | Y' | Y' | Y' |
| start + 8: | Y' | Y' | Y' | Y' |
| start + 12: | Y' | Y' | Y' | Y' |
| start + 16: | Cr | Cr | ||
| start + 18: | Cr | Cr | ||
| start + 20: | Cb | Cb | ||
| start + 22: | Cb | Cb |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | Y | Y | |||
| C | C | ||||||
| 1 | Y | Y | Y | Y | |||
| 2 | Y | Y | Y | Y | |||
| C | C | ||||||
| 3 | Y | Y | Y | Y |
V4L2_PIX_FMT_YVU410 ('YVU9'), V4L2_PIX_FMT_YUV410 ('YUV9')
Name
V4L2_PIX_FMT_YVU410, V4L2_PIX_FMT_YUV410 -- Planar formats with ¼ horizontal andvertical chroma resolution, also known as YUV 4:1:0
Description
These are planar formats, as opposed to a packed format.The three
components are separated into three sub-images or planes.The Y plane is
first. The Y plane has one byte per pixel. ForV4L2_PIX_FMT_YVU410,
the Cr plane immediatelyfollows
the Y plane in memory. The Cr plane is ¼ the width and¼ the height of
the Y plane (and of the image). Each Cr belongsto 16 pixels, a
four-by-four square of the image. Following the Crplane is the Cb plane,
just like the Cr plane.V4L2_PIX_FMT_YUV410
is the same, except the Cbplane comes first, then the Cr plane.
If the Y plane has pad bytes after each row, then the Crand Cb planes
have ¼ as many pad bytes after their rows. Inother words, four Cx rows
(including padding) are exactly as long asone Y row (including padding).
Example 2-1. V4L2_PIX_FMT_YVU410 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Y' | Y' | Y' |
| start + 4: | Y' | Y' | Y' | Y' |
| start + 8: | Y' | Y' | Y' | Y' |
| start + 12: | Y' | Y' | Y' | Y' |
| start + 16: | Cr | |||
| start + 17: | Cb |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | Y | Y | |||
| 1 | Y | Y | Y | Y | |||
| C | |||||||
| 2 | Y | Y | Y | Y | |||
| 3 | Y | Y | Y | Y |
V4L2_PIX_FMT_YUV422P ('422P')
Name
V4L2_PIX_FMT_YUV422P -- Format with ½ horizontal chroma resolution,also known as YUV 4:2:2. Planar layout as opposed toV4L2_PIX_FMT_YUYV
Description
This format is not commonly used. This is a planarversion of the YUYV
format. The three components are separated intothree sub-images or
planes. The Y plane is first. The Y plane has onebyte per pixel. The Cb
plane immediately follows the Y plane inmemory.
The Cb plane is half the width of the Y plane (and of theimage). Each
Cb belongs to two pixels. For example,Cb belongs to Y',Y'. Following the Cb plane is the Cr plane,just like the Cb plane.
If the Y plane has pad bytes after each row, then the Crand Cb planes
have half as many pad bytes after their rows. In otherwords, two Cx
rows (including padding) is exactly as long as one Y row(including
padding).
Example 2-1. V4L2_PIX_FMT_YUV422P 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Y' | Y' | Y' |
| start + 4: | Y' | Y' | Y' | Y' |
| start + 8: | Y' | Y' | Y' | Y' |
| start + 12: | Y' | Y' | Y' | Y' |
| start + 16: | Cb | Cb | ||
| start + 18: | Cb | Cb | ||
| start + 20: | Cb | Cb | ||
| start + 22: | Cb | Cb | ||
| start + 24: | Cr | Cr | ||
| start + 26: | Cr | Cr | ||
| start + 28: | Cr | Cr | ||
| start + 30: | Cr | Cr |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | C | Y | Y | C | Y | |
| 1 | Y | C | Y | Y | C | Y | |
| 2 | Y | C | Y | Y | C | Y | |
| 3 | Y | C | Y | Y | C | Y |
V4L2_PIX_FMT_YUV411P ('411P')
Name
V4L2_PIX_FMT_YUV411P -- Format with ¼ horizontal chroma resolution,also known as YUV 4:1:1. Planar layout as opposed toV4L2_PIX_FMT_Y41P
Description
This format is not commonly used. This is a planarformat similar to
the 4:2:2 planar format except with half as manychroma. The three
components are separated into three sub-images orplanes. The Y plane is
first. The Y plane has one byte per pixel. TheCb
plane immediately follows the Y plane in memory. The Cb plane is¼ the
width of the Y plane (and of the image). Each Cb belongsto 4 pixels all
on the same row. For example,Cb belongs to Y',Y', Y' andY'.
Following the Cb plane is the Cr plane,just like the Cb plane.
If the Y plane has pad bytes after each row, then the Crand Cb planes
have ¼ as many pad bytes after their rows. Inother words, four C x rows
(including padding) is exactly as long asone Y row (including padding).
Example 2-1. V4L2_PIX_FMT_YUV411P 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Y' | Y' | Y' |
| start + 4: | Y' | Y' | Y' | Y' |
| start + 8: | Y' | Y' | Y' | Y' |
| start + 12: | Y' | Y' | Y' | Y' |
| start + 16: | Cb | |||
| start + 17: | Cb | |||
| start + 18: | Cb | |||
| start + 19: | Cb | |||
| start + 20: | Cr | |||
| start + 21: | Cr | |||
| start + 22: | Cr | |||
| start + 23: | Cr |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | C | Y | Y | ||
| 1 | Y | Y | C | Y | Y | ||
| 2 | Y | Y | C | Y | Y | ||
| 3 | Y | Y | C | Y | Y |
V4L2_PIX_FMT_NV12 ('NV12'), V4L2_PIX_FMT_NV21 ('NV21')
Name
V4L2_PIX_FMT_NV12, V4L2_PIX_FMT_NV21 -- Formats
with ½ horizontal and verticalchroma resolution, also known as YUV
4:2:0. One luminance and onechrominance plane with alternating chroma
samples as opposed
toV4L2_PIX_FMT_YVU420
Description
These are two-plane versions of the YUV 4:2:0 format.The three
components are separated into two sub-images or planes. TheY plane is
first. The Y plane has one byte per pixel. ForV4L2_PIX_FMT_NV12,
a combined CbCr planeimmediately
follows the Y plane in memory. The CbCr plane is the samewidth, in
bytes, as the Y plane (and of the image), but is half astall in pixels.
Each CbCr pair belongs to four pixels. For example,Cb/Cr belongs toY', Y',Y',
Y'.V4L2_PIX_FMT_NV21 is the same except the Cb andCr bytes are swapped, the CrCb plane starts with a Cr byte.
If the Y plane has pad bytes after each row, then theCbCr plane has as many pad bytes after its rows.
Example 2-1. V4L2_PIX_FMT_NV12 4 × 4pixel image
Byte Order. Each cell is one byte.
| start + 0: | Y' | Y' | Y' | Y' |
| start + 4: | Y' | Y' | Y' | Y' |
| start + 8: | Y' | Y' | Y' | Y' |
| start + 12: | Y' | Y' | Y' | Y' |
| start + 16: | Cb | Cr | Cb | Cr |
| start + 20: | Cb | Cr | Cb | Cr |
Color Sample Location.
| 0 | 1 | 2 | 3 | ||||
| 0 | Y | Y | Y | Y | |||
| C | C | ||||||
| 1 | Y | Y | Y | Y | |||
| 2 | Y | Y | Y | Y | |||
| C | C | ||||||
| 3 | Y | Y | Y | Y |
2.6. Compressed Formats
Table 2-7. Compressed Image Formats
V4L2_PIX_FMT_JPEG | 'JPEG' | TBD. See also VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP. |
V4L2_PIX_FMT_MPEG | 'MPEG' | MPEG stream. The actual format is determined byextended control |
2.7. Reserved Format Identifiers
These formats are not defined by this specification, theyare just
listed for reference and to avoid naming conflicts. If youwant to
register your own format, send an e-mail to the V4L mailinglisthttps://listman.redhat.com/mailman/listinfo/video4linux-list
for inclusion in thevideodev.hfile. If you
want to share your format with other developers add alink to your
documentation and send a copy to the maintainer of thisdocument, Michael
Schimek<[email protected]>,
forinclusion in this section. If you think your format should be
listedin a standard format section please make a proposal on the V4L
mailinglist.
Table 2-8. Reserved Image Formats
V4L2_PIX_FMT_DV | 'dvsd' | unknown |
V4L2_PIX_FMT_ET61X251 | 'E625' | Compressed format of the ET61X251 driver. |
V4L2_PIX_FMT_HI240 | 'HI24' | 8 bit RGB format used by the BTTV driver,http://bytesex.org/bttv/ |
V4L2_PIX_FMT_HM12 | 'HM12' | YUV 4:2:0 format used by theIVTV driver, http://www.ivtvdriver.org/ The format is documented in thekernel sources in the file |
V4L2_PIX_FMT_MJPEG | 'MJPG' | Compressed format used by the Zoran driver |
V4L2_PIX_FMT_PWC1 | 'PWC1' | Compressed format of the PWC driver. |
V4L2_PIX_FMT_PWC2 | 'PWC2' | Compressed format of the PWC driver. |
V4L2_PIX_FMT_SN9C10X | 'S910' | Compressed format of the SN9C102 driver. |
V4L2_PIX_FMT_WNVA | 'WNVA' | Used by the Winnov Videum driver, http://www.thedirks.org/winnov/ |
V4L2_PIX_FMT_YYUV | 'YYUV' | unknown |
Chapter 3. Input/Output
The V4L2 API defines several different methods to read from orwrite
to a device. All drivers exchanging data with applications mustsupport
at least one of them.
The classic I/O method using the read()and
write()
device. When the driver does not support thismethod attempts to read or
write will fail at any time.
Other methods must be negotiated. To select the streaming I/Omethod with memory mapped or user buffers applications call theVIDIOC_REQBUFS ioctl.
The asynchronous I/O method is not definedyet.
Video overlay can be considered another I/O method, althoughthe
application does not directly receive the image data. It isselected by
initiating video overlay with theVIDIOC_S_FMT
ioctl.For more information seeSection 4.2.
Generally exactly one I/O method, including overlay, isassociated
with each file descriptor. The only exceptions areapplications not
exchanging data with a driver ("panel applications",seeSection
1.1) and drivers permitting simultaneous video capturingand overlay
using the same file descriptor, for compatibility with V4Land earlier
versions of V4L2.
VIDIOC_S_FMT andVIDIOC_REQBUFS
would permit this to some degree,but for simplicity drivers need not
support switching the I/O method(after first switching away from
read/write) other than by closingand
reopening the device.
The following sections describe the various I/O methods inmore detail.
3.1. Read/Write
Input and output devices support theread() and
write()V4L2_CAP_READWRITE flag inthecapabilities field of struct v4l2_capabilityreturned
by theVIDIOC_QUERYCAP ioctl is set.
Drivers may need the CPU to copy the data, but they may alsosupport
DMA to or from user memory, so this I/O method is notnecessarily less
efficient than other methods merely exchanging bufferpointers. It is
considered inferior though because no meta-informationlike
frame counters or timestamps are passed. This information isnecessary
to recognize frame dropping and to synchronize with otherdata streams.
However this is also the simplest I/O method, requiringlittle or no
setup to exchange data. It permits command line
stuntslike this (the vidctrl tool isfictitious):
> vidctrl /dev/video --input=0 --format=YUYV --size=352x288
> dd if=/dev/video of=myimage.422 bs=202752 count=1
To read from the device applications use theread() function, to write thewrite() function.Drivers must implement one I/O method if theyexchange data with applications, but it need not be this.[12] When reading or writing is supported, the drivermust also support the select() and poll()function.[13]
3.2. Streaming I/O (Memory Mapping)
Input and output devices support this I/O method when theV4L2_CAP_STREAMING flag in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl is set. There are twostreaming methods, to determine if the memory mapping flavor issupported applications must call theVIDIOC_REQBUFS ioctl.
Streaming is an I/O method where only pointers to buffersare exchanged between application and driver, the data itself is notcopied. Memory mapping is primarily intended to map buffers in devicememory into the application's address space. Device memory can be forexample the video memory on a graphics card with a video captureadd-on. However, being the most efficient I/O method available for along time, many other drivers support streaming as well, allocatingbuffers in DMA-able main memory.
A driver can support many sets of buffers. Each set isidentified by a unique buffer type value. The sets are independent andeach set can hold a different type of data. To access different setsat the same time different file descriptors must be used.[14]
To allocate device buffers applications call theVIDIOC_REQBUFS ioctl with the desired number of buffers and buffertype, for exampleV4L2_BUF_TYPE_VIDEO_CAPTURE.This ioctl can also be used to change the number of buffers or to freethe allocated memory, provided none of the buffers are stillmapped.
Before applications can access the buffers they must mapthem into their address space with themmap() function. Thelocation of the buffers in device memory can be determined with theVIDIOC_QUERYBUF ioctl. Them.offset andlength returned in a struct v4l2_buffer arepassed as sixth and second parameter to themmap() function. The offset and length valuesmust not be modified. Remember the buffers are allocated in physicalmemory, as opposed to virtual memory which can be swapped out to disk.Applications should free the buffers as soon as possible with themunmap() function.
Example 3-1. Mapping buffers
struct v4l2_requestbuffers reqbuf;
struct {
void *start;
size_t length;
} *buffers;
unsigned int i; memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_MMAP;
reqbuf.count = 20; if (-1 == ioctl (fd,VIDIOC_REQBUFS, &reqbuf)) {
if (errno == EINVAL)
printf ("Video capturing or mmap-streaming is not supported\n");
else
perror ("VIDIOC_REQBUFS"); exit (EXIT_FAILURE);
} /* We want at least five buffers. */ if (reqbuf.count < 5) {
/* You may need to free the buffers here. */
printf ("Not enough buffer memory\n");
exit (EXIT_FAILURE);
} buffers = calloc (reqbuf.count, sizeof (*buffers));
assert (buffers != NULL); for (i = 0; i < reqbuf.count; i++) {
struct v4l2_buffer buffer; memset (&buffer, 0, sizeof (buffer));
buffer.type = reqbuf.type;
buffer.memory = V4L2_MEMORY_MMAP;
buffer.index = i; if (-1 == ioctl (fd,VIDIOC_QUERYBUF, &buffer)) {
perror ("VIDIOC_QUERYBUF");
exit (EXIT_FAILURE);
} buffers[i].length = buffer.length; /* remember for munmap() */ buffers[i].start = mmap (NULL, buffer.length,
PROT_READ | PROT_WRITE, /* recommended */
MAP_SHARED, /* recommended */
fd, buffer.m.offset); if (MAP_FAILED == buffers[i].start) {
/* If you do not exit here you should unmap() and free()
the buffers mapped so far. */
perror ("mmap");
exit (EXIT_FAILURE);
}
} /* Cleanup. */ for (i = 0; i < reqbuf.count; i++)
munmap (buffers[i].start, buffers[i].length);
Conceptually streaming drivers maintain two buffer queues, an incomingand an outgoing queue. They separate the synchronous capture or outputoperation locked to a video clock from the application which issubject to random disk or network delays and preemption byother processes, thereby reducing the probability of data loss.The queues are organized as FIFOs, buffers will beoutput in the order enqueued in the incoming FIFO, and werecaptured in the order dequeued from the outgoing FIFO.
The driver may require a minimum number of buffers enqueuedat all times to function, apart of this no limit exists on the numberof buffers applications can enqueue in advance, or dequeue andprocess. They can also enqueue in a different order than buffers havebeen dequeued, and the driver can fill enqueuedempty buffers in any order.[15] The index number of a buffer (struct v4l2_bufferindex) plays no role here, it onlyidentifies the buffer.
Initially all mapped buffers are in dequeued state,inaccessible by the driver. For capturing applications it is customaryto first enqueue all mapped buffers, then to start capturing and enterthe read loop. Here the application waits until a filled buffer can bedequeued, and re-enqueues the buffer when the data is no longerneeded. Output applications fill and enqueue buffers, when enoughbuffers are stacked up the output is started withVIDIOC_STREAMON. In the write loop, whenthe application runs out of free buffers, it must wait until an emptybuffer can be dequeued and reused.
To enqueue and dequeue a buffer applications use theVIDIOC_QBUF andVIDIOC_DQBUF ioctl. The status of a buffer beingmapped, enqueued, full or empty can be determined at any time using theVIDIOC_QUERYBUF ioctl. Two methods exist to suspend execution of theapplication until one or more buffers can be dequeued. By defaultVIDIOC_DQBUF blocks when no buffer is in theoutgoing queue. When theO_NONBLOCK flag wasgiven to theopen() function,VIDIOC_DQBUFreturns immediately with anEAGAIN error code when no buffer is available. Theselect() orpoll() function are always available.
To start and stop capturing or output applications call theVIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl. NoteVIDIOC_STREAMOFF removes all buffers from bothqueues as a side effect. Since there is no notion of doing anything"now" on a multitasking system, if an application needs to synchronizewith another event it should examine the struct v4l2_buffertimestamp of captured buffers, or set thefield before enqueuing buffers for output.
Drivers implementing memory mapping I/O mustsupport the VIDIOC_REQBUFS,VIDIOC_QUERYBUF,VIDIOC_QBUF,VIDIOC_DQBUF,VIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl, themmap(),munmap(),select() andpoll()function.[16]
[capture example]
3.3. Streaming I/O (User Pointers)
Input and output devices support this I/O method when theV4L2_CAP_STREAMING flag in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl is set. If the particular userpointer method (not only memory mapping) is supported must bedetermined by calling theVIDIOC_REQBUFS ioctl.
This I/O method combines advantages of the read/write andmemory mapping methods. Buffers are allocated by the applicationitself, and can reside for example in virtual or shared memory. Onlypointers to data are exchanged, these pointers and meta-informationare passed in struct v4l2_buffer. The driver must be switchedinto user pointer I/O mode by calling theVIDIOC_REQBUFS with thedesired buffer type. No buffers are allocated beforehands,consequently they are not indexed and cannot be queried like mappedbuffers with theVIDIOC_QUERYBUF ioctl.
Example 3-2. Initiating streaming I/O with user pointers
struct v4l2_requestbuffers reqbuf; memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_USERPTR; if (ioctl (fd,VIDIOC_REQBUFS, &reqbuf) == -1) {
if (errno == EINVAL)
printf ("Video capturing or user pointer streaming is not supported\n");
else
perror ("VIDIOC_REQBUFS"); exit (EXIT_FAILURE);
}
Buffer addresses and sizes are passed on the fly with theVIDIOC_QBUF ioctl. Although buffers are commonly cycled,applications can pass different addresses and sizes at eachVIDIOC_QBUF call. If required by the hardware thedriver swaps memory pages within physical memory to create acontinuous area of memory. This happens transparently to theapplication in the virtual memory subsystem of the kernel. When bufferpages have been swapped out to disk they are brought back and finallylocked in physical memory for DMA.[17]
Filled or displayed buffers are dequeued with theVIDIOC_DQBUF ioctl. The driver can unlock the memory pages at anytime between the completion of the DMA and this ioctl. The memory isalso unlocked when VIDIOC_STREAMOFF is called, VIDIOC_REQBUFS, orwhen the device is closed. Applications must take care not to freebuffers without dequeuing. For once, the buffers remain locked untilfurther, wasting physical memory. Second the driver will not benotified when the memory is returned to the application's free listand subsequently reused for other purposes, possibly completing therequested DMA and overwriting valuable data.
For capturing applications it is customary to enqueue anumber of empty buffers, to start capturing and enter the read loop.Here the application waits until a filled buffer can be dequeued, andre-enqueues the buffer when the data is no longer needed. Outputapplications fill and enqueue buffers, when enough buffers are stackedup output is started. In the write loop, when the applicationruns out of free buffers it must wait until an empty buffer can bedequeued and reused. Two methods exist to suspend execution of theapplication until one or more buffers can be dequeued. By defaultVIDIOC_DQBUF blocks when no buffer is in theoutgoing queue. When theO_NONBLOCK flag wasgiven to theopen() function,VIDIOC_DQBUFreturns immediately with anEAGAIN error code when no buffer is available. Theselect() orpoll() function are always available.
To start and stop capturing or output applications call theVIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl. NoteVIDIOC_STREAMOFF removes all buffers from bothqueues and unlocks all buffers as a side effect. Since there is nonotion of doing anything "now" on a multitasking system, if anapplication needs to synchronize with another event it should examinethe struct v4l2_buffertimestamp of capturedbuffers, or set the field before enqueuing buffers for output.
Drivers implementing user pointer I/O mustsupport the VIDIOC_REQBUFS,VIDIOC_QBUF,VIDIOC_DQBUF,VIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl, theselect() andpoll() function.[18]
3.4. Asynchronous I/O
This method is not defined yet.
3.5. Buffers
A buffer contains data exchanged by application anddriver using one of the Streaming I/O methods. Only pointers tobuffers are exchanged, the data itself is not copied. These pointers,together with meta-information like timestamps or field parity, arestored in a struct v4l2_buffer, argument tothe VIDIOC_QUERYBUF, VIDIOC_QBUF and VIDIOC_DQBUF ioctl.
Nominally timestamps refer to the first data byte transmitted.In practice however the wide range of hardware covered by the V4L2 APIlimits timestamp accuracy. Often an interrupt routine willsample the system clock shortly after the field or frame was storedcompletely in memory. So applications must expect a constantdifference up to one field or frame period plus a small (few scanlines) random error. The delay and error can be muchlarger due to compression or transmission over an external bus whenthe frames are not properly stamped by the sender. This is frequentlythe case with USB cameras. Here timestamps refer to the instant thefield or frame was received by the driver, not the capture time. Thesedevices identify by not enumerating any video standards, seeSection 1.7.
Similar limitations apply to output timestamps. Typicallythe video hardware locks to a clock controlling the video timing, thehorizontal and vertical synchronization pulses. At some point in theline sequence, possibly the vertical blanking, an interrupt routinesamples the system clock, compares against the timestamp and programsthe hardware to repeat the previous field or frame, or to display thebuffer contents.
Apart of limitations of the video device and naturalinaccuracies of all clocks, it should be noted system time itself isnot perfectly stable. It can be affected by power saving cycles,warped to insert leap seconds, or even turned back or forth by thesystem administrator affecting long term measurements. [19]
Table 3-1. struct v4l2_buffer
| __u32 | index | Number of the buffer, set by the application. Thisfield is only used for memory mapping I/Oand can range from zero to the number of buffers allocatedwith theVIDIOC_REQBUFS ioctl (struct v4l2_requestbufferscount) minus one. | |
| enum v4l2_buf_type | type | Type of the buffer, same as struct v4l2_formattype or struct v4l2_requestbufferstype, set by the application. | |
| __u32 | bytesused | The number of bytes occupied by the data in thebuffer. It depends on the negotiated data format and may change witheach buffer for compressed variable size data like JPEG images.Drivers must set this field whentyperefers to an input stream, applications when an output stream. | |
| __u32 | flags | Flags set by the application or driver, see Table 3-3. | |
| enum v4l2_field | field | Indicates the field order of the image in thebuffer, see Table 3-8. This field is not used whenthe buffer contains VBI data. Drivers must set it whentype refers to an input stream,applications when an output stream. | |
| struct timeval | timestamp | For input streams this is thesystem time (as returned by the | |
| struct v4l2_timecode | timecode | When type isV4L2_BUF_TYPE_VIDEO_CAPTURE and theV4L2_BUF_FLAG_TIMECODE flag is set inflags, this structure contains a frametimecode. In V4L2_FIELD_ALTERNATEmode the top and bottom field contain the same timecode.Timecodes are intended to help video editing and are typically recorded onvideo tapes, but also embedded in compressed formats like MPEG. Thisfield is independent of the timestamp andsequence fields. | |
| __u32 | sequence | Set by the driver, counting the frames in thesequence. | |
In V4L2_FIELD_ALTERNATE mode the top andbottom field have the same sequence number. The count starts at zeroand includes dropped or repeated frames. A dropped frame was receivedby an input device but could not be stored due to lack of free bufferspace. A repeated frame was displayed again by an output devicebecause the application did not pass new data intime. Note this may count the frames receivede.g. over USB, without taking into account the frames dropped by theremote hardware due to limited compression throughput or busbandwidth. These devices identify by not enumerating any videostandards, seeSection 1.7. | |||
| enum v4l2_memory | memory | This field must be set by applications and/or driversin accordance with the selected I/O method. | |
| union | m | ||
| __u32 | offset | When memory isV4L2_MEMORY_MMAP this is the offset of the bufferfrom the start of the device memory. The value is returned by thedriver and apart of serving as parameter to themmap() functionnot useful for applications. SeeSection 3.2 for details. | |
| unsigned long | userptr | When memory isV4L2_MEMORY_USERPTR this is a pointer to thebuffer (casted to unsigned long type) in virtual memory, set by theapplication. SeeSection 3.3 for details. | |
| __u32 | length | Size of the buffer (not the payload) in bytes. | |
| __u32 | input | Some video capture drivers support rapid andsynchronous video input changes, a function useful for example invideo surveillance applications. For this purpose applications set theV4L2_BUF_FLAG_INPUT flag, and this field to thenumber of a video input as in struct v4l2_input fieldindex. | |
| __u32 | reserved | A place holder for future extensions and custom(driver defined) buffer typesV4L2_BUF_TYPE_PRIVATE and higher. |
Table 3-2. enum v4l2_buf_type
V4L2_BUF_TYPE_VIDEO_CAPTURE | 1 | Buffer of a video capture stream, see Section 4.1. |
V4L2_BUF_TYPE_VIDEO_OUTPUT | 2 | Buffer of a video output stream, see Section 4.3. |
V4L2_BUF_TYPE_VIDEO_OVERLAY | 3 | Buffer for video overlay, see Section 4.2. |
V4L2_BUF_TYPE_VBI_CAPTURE | 4 | Buffer of a raw VBI capture stream, see Section 4.7. |
V4L2_BUF_TYPE_VBI_OUTPUT | 5 | Buffer of a raw VBI output stream, see Section 4.7. |
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE | 6 | Buffer of a sliced VBI capture stream, see Section 4.8. |
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT | 7 | Buffer of a sliced VBI output stream, see Section 4.8. |
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY | 8 | Buffer for video output overlay (OSD), see Section 4.4. Status: Experimental. |
V4L2_BUF_TYPE_PRIVATE | 0x80 | This and higher values are reserved for custom(driver defined) buffer types. |
Table 3-3. Buffer Flags
V4L2_BUF_FLAG_MAPPED | 0x0001 | The buffer resides in device memory and has been mappedinto the application's address space, seeSection 3.2 for details.Drivers set or clear this flag when theVIDIOC_QUERYBUF,VIDIOC_QBUF orVIDIOC_DQBUF ioctl is called. Set by the driver. |
V4L2_BUF_FLAG_QUEUED | 0x0002 | Internally drivers maintain two buffer queues, anincoming and outgoing queue. When this flag is set, the buffer iscurrently on the incoming queue. It automatically moves to theoutgoing queue after the buffer has been filled (capture devices) ordisplayed (output devices). Drivers set or clear this flag when theVIDIOC_QUERYBUF ioctl is called. After(successful) calling theVIDIOC_QBUFioctl it isalways set and afterVIDIOC_DQBUF alwayscleared. |
V4L2_BUF_FLAG_DONE | 0x0004 | When this flag is set, the buffer is currently onthe outgoing queue, ready to be dequeued from the driver. Drivers setor clear this flag when theVIDIOC_QUERYBUF ioctlis called. After calling theVIDIOC_QBUF orVIDIOC_DQBUF it is always cleared. Of course abuffer cannot be on both queues at the same time, theV4L2_BUF_FLAG_QUEUED andV4L2_BUF_FLAG_DONE flag are mutually exclusive.They can be both cleared however, then the buffer is in "dequeued"state, in the application domain to say so. |
V4L2_BUF_FLAG_KEYFRAME | 0x0008 | Drivers set or clear this flag when calling theVIDIOC_DQBUF ioctl. It may be set by videocapture devices when the buffer contains a compressed image which is akey frame (or field), i. e. can be decompressed on its own. |
V4L2_BUF_FLAG_PFRAME | 0x0010 | Similar to V4L2_BUF_FLAG_KEYFRAMEthis flags predicted frames or fields which contain only differences to aprevious key frame. |
V4L2_BUF_FLAG_BFRAME | 0x0020 | Similar to V4L2_BUF_FLAG_PFRAME this is a bidirectional predicted frame or field. [ooc tbd] |
V4L2_BUF_FLAG_TIMECODE | 0x0100 | The timecode field is valid.Drivers set or clear this flag when theVIDIOC_DQBUFioctl is called. |
V4L2_BUF_FLAG_INPUT | 0x0200 | The input field is valid.Applications set or clear this flag before calling theVIDIOC_QBUF ioctl. |
Table 3-4. enum v4l2_memory
V4L2_MEMORY_MMAP | 1 | The buffer is used for memorymapping I/O. |
V4L2_MEMORY_USERPTR | 2 | The buffer is used for userpointer I/O. |
V4L2_MEMORY_OVERLAY | 3 | [to do] |
3.5.1. Timecodes
The v4l2_timecode structure isdesigned to hold a or similar timecode.(structtimeval timestamps are stored instruct v4l2_buffer fieldtimestamp.)
Table 3-5. struct v4l2_timecode
| __u32 | type | Frame rate the timecodes are based on, see Table 3-6. |
| __u32 | flags | Timecode flags, see Table 3-7. |
| __u8 | frames | Frame count, 0 ... 23/24/29/49/59, depending on the type of timecode. |
| __u8 | seconds | Seconds count, 0 ... 59. This is a binary, not BCD number. |
| __u8 | minutes | Minutes count, 0 ... 59. This is a binary, not BCD number. |
| __u8 | hours | Hours count, 0 ... 29. This is a binary, not BCD number. |
| __u8 | userbits[4] | The "user group" bits from the timecode. |
Table 3-6. Timecode Types
V4L2_TC_TYPE_24FPS | 1 | 24 frames per second, i. e. film. |
V4L2_TC_TYPE_25FPS | 2 | 25 frames per second, i. e. PAL or SECAM video. |
V4L2_TC_TYPE_30FPS | 3 | 30 frames per second, i. e. NTSC video. |
V4L2_TC_TYPE_50FPS | 4 | |
V4L2_TC_TYPE_60FPS | 5 |
Table 3-7. Timecode Flags
V4L2_TC_FLAG_DROPFRAME | 0x0001 | Indicates "drop frame" semantics for counting framesin 29.97 fps material. When set, frame numbers 0 and 1 at the start ofeach minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from thecount. |
V4L2_TC_FLAG_COLORFRAME | 0x0002 | The "color frame" flag. |
V4L2_TC_USERBITS_field | 0x000C | Field mask for the "binary group flags". |
V4L2_TC_USERBITS_USERDEFINED | 0x0000 | Unspecified format. |
V4L2_TC_USERBITS_8BITCHARS | 0x0008 | 8-bit ISO characters. |
3.6. Field Order
We have to distinguish between progressive and interlacedvideo. Progressive video transmits all lines of a video imagesequentially. Interlaced video divides an image into two fields,containing only the odd and even lines of the image, respectively.Alternating the so called odd and even field are transmitted, and dueto a small delay between fields a cathode ray TV displays the linesinterleaved, yielding the original frame. This curious technique wasinvented because at refresh rates similar to film the image wouldfade out too quickly. Transmitting fields reduces the flicker withoutthe necessity of doubling the frame rate and with it the bandwidthrequired for each channel.
It is important to understand a video camera does not exposeone frame at a time, merely transmitting the frames separated intofields. The fields are in fact captured at two different instances intime. An object on screen may well move between one field and thenext. For applications analysing motion it is of paramount importanceto recognize which field of a frame is older, thetemporalorder.
When the driver provides or accepts images field by fieldrather than interleaved, it is also important applications understandhow the fields combine to frames. We distinguish between top andbottom fields, thespatial order: The first lineof the top field is the first line of an interlaced frame, the firstline of the bottom field is the second line of that frame.
However because fields were captured one after the other,arguing whether a frame commences with the top or bottom field ispointless. Any two successive top and bottom, or bottom and top fieldsyield a valid frame. Only when the source was progressive to beginwith, e. g. when transferring film to video, two fields may come fromthe same frame, creating a natural order.
Counter to intuition the top field is not necessarily theolder field. Whether the older field contains the top or bottom linesis a convention determined by the video standard. Hence thedistinction between temporal and spatial order of fields. The diagramsbelow should make this clearer.
All video capture and output devices must report the currentfield order. Some drivers may permit the selection of a differentorder, to this end applications initialize thefield field of struct v4l2_pix_format beforecalling the VIDIOC_S_FMT ioctl. If this is not desired it shouldhave the valueV4L2_FIELD_ANY (0).
Table 3-8. enum v4l2_field
V4L2_FIELD_ANY | 0 | Applications request this field order when anyone of the V4L2_FIELD_NONE,V4L2_FIELD_TOP,V4L2_FIELD_BOTTOM, orV4L2_FIELD_INTERLACED formats is acceptable.Drivers choose depending on hardware capabilities or e. g. therequested image size, and return the actual field order. struct v4l2_bufferfield can never beV4L2_FIELD_ANY. |
V4L2_FIELD_NONE | 1 | Images are in progressive format, not interlaced.The driver may also indicate this order when it cannot distinguishbetweenV4L2_FIELD_TOP andV4L2_FIELD_BOTTOM. |
V4L2_FIELD_TOP | 2 | Images consist of the top field only. |
V4L2_FIELD_BOTTOM | 3 | Images consist of the bottom field only.Applications may wish to prevent a device from capturing interlacedimages because they will have "comb" or "feathering" artefacts aroundmoving objects. |
V4L2_FIELD_INTERLACED | 4 | Images contain both fields, interleaved line byline. The temporal order of the fields (whether the top or bottomfield is first transmitted) depends on the current video standard.M/NTSC transmits the bottom field first, all other standards the topfield first. |
V4L2_FIELD_SEQ_TB | 5 | Images contain both fields, the top field linesare stored first in memory, immediately followed by the bottom fieldlines. Fields are always stored in temporal order, the older one firstin memory. Image sizes refer to the frame, not fields. |
V4L2_FIELD_SEQ_BT | 6 | Images contain both fields, the bottom fieldlines are stored first in memory, immediately followed by the topfield lines. Fields are always stored in temporal order, the older onefirst in memory. Image sizes refer to the frame, not fields. |
V4L2_FIELD_ALTERNATE | 7 | The two fields of a frame are passed in separatebuffers, in temporal order, i. e. the older one first. To indicate the fieldparity (whether the current field is a top or bottom field) the driveror application, depending on data direction, must set struct v4l2_bufferfield toV4L2_FIELD_TOP orV4L2_FIELD_BOTTOM. Any two successive fields pairto build a frame. If fields are successive, without any dropped fieldsbetween them (fields can drop individually), can be determined fromthe struct v4l2_buffersequence field. Imagesizes refer to the frame, not fields. This format cannot be selectedwhen using the read/write I/O method. |
V4L2_FIELD_INTERLACED_TB | 8 | Images contain both fields, interleaved line byline, top field first. The top field is transmitted first. |
V4L2_FIELD_INTERLACED_BT | 9 | Images contain both fields, interleaved line byline, top field first. The bottom field is transmitted first. |
Figure 3-1. Field Order, Top Field First Transmitted
Figure 3-2. Field Order, Bottom Field First Transmitted
Chapter 4. Interfaces
4.1. Video Capture Interface
Video capture devices sample an analog video signal and storethe digitized images in memory. Today nearly all devices can captureat full 25 or 30 frames/second. With this interface applications cancontrol the capture process and move images from the driver into userspace.
Conventionally V4L2 video capture devices are accessed throughcharacter device special files named/dev/videoand/dev/video0 to/dev/video63 with major number 81 and minornumbers 0 to 63./dev/video is typically asymbolic link to the preferred video device. Note the same devicefiles are used for video output devices.
4.1.1. Querying Capabilities
Devices supporting the video capture interface set theV4L2_CAP_VIDEO_CAPTURE flag in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl. As secondary device functionsthey may also support thevideo overlay(V4L2_CAP_VIDEO_OVERLAY) and theraw VBI capture(V4L2_CAP_VBI_CAPTURE) interface. At least one ofthe read/write or streaming I/O methods must be supported. Tuners andaudio inputs are optional.
4.1.2. Supplemental Functions
Video capture devices shall support audio input, tuner, controls,cropping and scaling andstreaming parameter ioctls as needed.Thevideo input andvideo standard ioctls must be supported byall video capture devices.
4.1.3. Image Format Negotiation
The result of a capture operation is determined bycropping and image format parameters. The former select an area of thevideo picture to capture, the latter how images are stored in memory,i. e. in RGB or YUV format, the number of bits per pixel or width andheight. Together they also define how images are scaled in theprocess.
As usual these parameters are not resetatopen() time to permit Unix tool chains, programming a deviceand then reading from it as if it was a plain file. Well written V4L2applications ensure they really get what they want, including croppingand scaling.
Cropping initialization at minimum requires to reset theparameters to defaults. An example is given inSection 1.11.
To query the current image format applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_CAPTURE and call theVIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fillthe struct v4l2_pix_formatpix member of thefmt union.
To request different parameters applications set thetype field of a struct v4l2_format as above andinitialize all fields of the struct v4l2_pix_formatvbi member of thefmt union, or better just modify theresults ofVIDIOC_G_FMT, and call theVIDIOC_S_FMT ioctl with a pointer to this structure. Drivers mayadjust the parameters and finally return the actual parameters asVIDIOC_G_FMT does.
Like VIDIOC_S_FMT theVIDIOC_TRY_FMT ioctl can be used to learn about hardware limitationswithout disabling I/O or possibly time consuming hardwarepreparations.
The contents of struct v4l2_pix_format are discussed inChapter 2. See also the specification of theVIDIOC_G_FMT,VIDIOC_S_FMTandVIDIOC_TRY_FMT ioctls for details. Videocapture devices must implement both theVIDIOC_G_FMT andVIDIOC_S_FMT ioctl, even ifVIDIOC_S_FMT ignores all requests and alwaysreturns default parameters asVIDIOC_G_FMT does.VIDIOC_TRY_FMT is optional.
4.1.4. Reading Images
A video capture device may support the read() function and/or streaming (memory mapping oruser pointer) I/O. SeeChapter 3 for details.
4.2. Video Overlay Interface
Also known as Framebuffer Overlay or Previewing
Video overlay devices have the ability to genlock (TV-)videointo the (VGA-)video signal of a graphics card, or to store capturedimages directly in video memory of a graphics card, typically withclipping. This can be considerable more efficient than capturingimages and displaying them by other means. In the old days when onlynuclear power plants needed cooling towers this used to be the onlyway to put live video into a window.
Video overlay devices are accessed through the same characterspecial files as video capture devices.Note the default function of a /dev/video deviceis video capturing. The overlay function is only available aftercalling theVIDIOC_S_FMT ioctl.
The driver may support simultaneous overlay and capturingusing the read/write and streaming I/O methods. If so, operation atthe nominal frame rate of the video standard is not guaranteed. Framesmay be directed away from overlay to capture, or one field may be usedfor overlay and the other for capture if the capture parameters permitthis.
Applications should use different file descriptors forcapturing and overlay. This must be supported by all drivers capableof simultaneous capturing and overlay. Optionally these drivers mayalso permit capturing and overlay with a single file descriptor forcompatibility with V4L and earlier versions of V4L2.[20]
4.2.1. Querying Capabilities
Devices supporting the video overlay interface set theV4L2_CAP_VIDEO_OVERLAY flag in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl. The overlay I/O method specifiedbelow must be supported. Tuners and audio inputs are optional.
4.2.2. Supplemental Functions
Video overlay devices shall support audio input, tuner, controls,cropping and scaling andstreaming parameter ioctls as needed.Thevideo input andvideo standard ioctls must be supported byall video overlay devices.
4.2.3. Setup
Before overlay can commence applications must program thedriver with frame buffer parameters, namely the address and size ofthe frame buffer and the image format, for example RGB 5:6:5. TheVIDIOC_G_FBUF and VIDIOC_S_FBUF ioctls are available to getand set these parameters, respectively. TheVIDIOC_S_FBUF ioctl is privileged because itallows to set up DMA into physical memory, bypassing the memoryprotection mechanisms of the kernel. Only the superuser can change theframe buffer address and size. Users are not supposed to run TVapplications as root or with SUID bit set. A small helper applicationwith suitable privileges should query the graphics system and programthe V4L2 driver at the appropriate time.
Some devices add the video overlay to the output signalof the graphics card. In this case the frame buffer is not modified bythe video device, and the frame buffer address and pixel format arenot needed by the driver. TheVIDIOC_S_FBUF ioctlis not privileged. An application can check for this type of device bycalling theVIDIOC_G_FBUF ioctl.
A driver may support any (or none) of five clipping/blendingmethods:
Chroma-keying displays the overlaid image only wherepixels in the primary graphics surface assume a certain color.
A bitmap can be specified where each bit correspondsto a pixel in the overlaid image. When the bit is set, thecorresponding video pixel is displayed, otherwise a pixel of thegraphics surface.
A list of clipping rectangles can be specified. Inthese regions no video is displayed, so thegraphics surface can be seen here.
The framebuffer has an alpha channel that can be usedto clip or blend the framebuffer with the video.
A global alpha value can be specified to blend theframebuffer contents with video images.
When simultaneous capturing and overlay is supported andthe hardware prohibits different image and frame buffer formats, theformat requested first takes precedence. The attempt to capture(VIDIOC_S_FMT) or overlay (VIDIOC_S_FBUF) may fail with anEBUSY error code or return accordingly modified parameters..
4.2.4. Overlay Window
The overlaid image is determined by cropping and overlaywindow parameters. The former select an area of the video picture tocapture, the latter how images are overlaid and clipped. Croppinginitialization at minimum requires to reset the parameters todefaults. An example is given in Section 1.11.
The overlay window is described by a struct v4l2_window. Itdefines the size of the image, its position over the graphics surfaceand the clipping to be applied. To get the current parametersapplications set the type field of astruct v4l2_format toV4L2_BUF_TYPE_VIDEO_OVERLAY andcall theVIDIOC_G_FMT ioctl. The driver fills thev4l2_window substructure namedwin. It is not possible to retrieve apreviously programmed clipping list or bitmap.
To program the overlay window applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OVERLAY, initialize thewin substructure and call theVIDIOC_S_FMT ioctl. The driver adjusts the parameters againsthardware limits and returns the actual parameters asVIDIOC_G_FMT does. LikeVIDIOC_S_FMT, theVIDIOC_TRY_FMT ioctl can beused to learn about driver capabilities without actually changingdriver state. UnlikeVIDIOC_S_FMT this also worksafter the overlay has been enabled.
The scaling factor of the overlaid image is implied by thewidth and height given in struct v4l2_window and the size of the croppingrectangle. For more information seeSection 1.11.
When simultaneous capturing and overlay is supported andthe hardware prohibits different image and window sizes, the sizerequested first takes precedence. The attempt to capture or overlay aswell (VIDIOC_S_FMT) may fail with an EBUSY error code or return accordinglymodified parameters.
Table 4-1. struct v4l2_window
| struct v4l2_rect | w | Size and position of the window relative to thetop, left corner of the frame buffer defined withVIDIOC_S_FBUF. Thewindow can extend the frame buffer width and height, thex andycoordinates can be negative, and it can lie completely outside theframe buffer. The driver clips the window accordingly, or if that isnot possible, modifies its size and/or position. |
| enum v4l2_field | field | Applications set this field to determine whichvideo field shall be overlaid, typically one ofV4L2_FIELD_ANY (0),V4L2_FIELD_TOP,V4L2_FIELD_BOTTOM orV4L2_FIELD_INTERLACED. Drivers may have to choosea different field order and return the actual setting here. |
| __u32 | chromakey | When chroma-keying has been negotiated withVIDIOC_S_FBUF applications set this field to the desired pixel valuefor the chroma key. The format is the same as the pixel format of theframebuffer (struct v4l2_framebufferfmt.pixelformat field), with bytes in hostorder. E. g. forV4L2_PIX_FMT_BGR24 the value should be 0xRRGGBB on a little endian, 0xBBGGRR on a bigendian host. |
| struct v4l2_clip * | clips | When chroma-keying has notbeen negotiated andVIDIOC_G_FBUF indicated this capability,applications can set this field to point to an array ofclipping rectangles. |
Like the window coordinatesw, clipping rectangles are defined relativeto the top, left corner of the frame buffer. However clippingrectangles must not extend the frame buffer width and height, and theymust not overlap. If possible applications should merge adjacentrectangles. Whether this must create x-y or y-x bands, or the order ofrectangles, is not defined. When clip lists are not supported thedriver ignores this field. Its contents after callingVIDIOC_S_FMTare undefined. | ||
| __u32 | clipcount | When the application set theclips field, this field must contain thenumber of clipping rectangles in the list. When clip lists are notsupported the driver ignores this field, its contents after callingVIDIOC_S_FMT are undefined. When clip lists aresupported but no clipping is desired this field must be set tozero. |
| void * | bitmap | When chroma-keying hasnot been negotiated andVIDIOC_G_FBUF indicatedthis capability, applications can set this field to point to aclipping bit mask. |
It must be of the same sizeas the window, ((__u8 *) where When a clippingbit mask is not supported the driver ignores this field, its contentsafter calling Applications need not create aclip list or bit mask. When they pass both, or despite negotiatingchroma-keying, the results are undefined. Regardless of the chosenmethod, the clipping abilities of the hardware may be limited inquantity or quality. The results when these limits are exceeded areundefined. | ||
| __u8 | global_alpha | The global alpha value used to blend theframebuffer with video images, if global alpha blending has beennegotiated ( Notethis field was added in Linux 2.6.23, extending the structure. Howeverthe VIDIOC_G/S/TRY_FMT ioctls,which take a pointer to a v4l2_format parent structure with paddingbytes at the end, are not affected. |
| Notes: a. Should we require w.width to be a multiple of eight?b. When the image is written into frame buffermemory it will be undesirable if the driver clips out less pixelsthan expected, because the application and graphics system are notaware these regions need to be refreshed. The driver should clip outmore pixels or not write the image at all. |
Table 4-2. struct v4l2_clip[21]
| struct v4l2_rect | c | Coordinates of the clipping rectangle, relative tothe top, left corner of the frame buffer. Only window pixelsoutside all clipping rectangles aredisplayed. |
| struct v4l2_clip * | next | Pointer to the next clipping rectangle, NULL whenthis is the last rectangle. Drivers ignore this field, it cannot beused to pass a linked list of clipping rectangles. |
Table 4-3. struct v4l2_rect
| __s32 | left | Horizontal offset of the top, left corner of therectangle, in pixels. |
| __s32 | top | Vertical offset of the top, left corner of therectangle, in pixels. Offsets increase to the right and down. |
| __s32 | width | Width of the rectangle, in pixels. |
| __s32 | height | Height of the rectangle, in pixels. Width andheight cannot be negative, the fields are signed for hystericalreasons. |
4.2.5. Enabling Overlay
To start or stop the frame buffer overlay applications callthe VIDIOC_OVERLAY ioctl.
4.3. Video Output Interface
Video output devices encode stills or image sequences asanalog video
signal. With this interface applications cancontrol the encoding process
and move images from user space tothe driver.
Conventionally V4L2 video output devices are accessed throughcharacter device special files named/dev/videoand/dev/video0 to/dev/video63 with major number 81 and minornumbers 0
to 63./dev/video is typically asymbolic link to the preferred video device. Note the same devicefiles are used for video capture devices.
4.3.1. Querying Capabilities
Devices supporting the video output interface set theV4L2_CAP_VIDEO_OUTPUT flag in thecapabilities field of struct v4l2_capabilityreturned
by the VIDIOC_QUERYCAP ioctl. As secondary device functionsthey may also support theraw VBIoutput (V4L2_CAP_VBI_OUTPUT) interface.
Atleast one of the read/write or streaming I/O methods must besupported. Modulators and audio outputs are optional.
4.3.2. Supplemental Functions
Video output devices shall support audio output, modulator, controls,cropping and scaling andstreaming parameter ioctls as needed.Thevideo
output andvideo standard ioctls must be supported byall video output devices.
4.3.3. Image Format Negotiation
The output is determined by cropping and image formatparameters. The
former select an area of the video picture where theimage will appear,
the latter how images are stored in memory, i. e. inRGB or YUV format,
the number of bits per pixel or width and height.Together
they also define how images are scaled in the process.
As usual these parameters are not resetatopen()
time to permit Unix tool chains, programming
a deviceand then writing to it as if it was a plain file. Well written
V4L2applications ensure they really get what they want, including
croppingand scaling.
Cropping initialization at minimum requires to reset theparameters to defaults. An example is given inSection 1.11.
To query the current image format applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OUTPUT
and call theVIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fillthe struct v4l2_pix_formatpix
member of thefmt union.
To request different parameters applications set thetype field of a struct v4l2_format as above andinitialize all fields of the struct v4l2_pix_formatvbi
member of thefmt union, or better just modify theresults ofVIDIOC_G_FMT, and call theVIDIOC_S_FMT
ioctl with a pointer to this structure. Drivers mayadjust the parameters and finally return the actual parameters asVIDIOC_G_FMT does.
Like VIDIOC_S_FMT theVIDIOC_TRY_FMT ioctl can be used to learn about hardware limitationswithout disabling I/O or
possibly time consuming hardwarepreparations.
The contents of struct v4l2_pix_format are discussed inChapter 2. See also the specification of
theVIDIOC_G_FMT,VIDIOC_S_FMTandVIDIOC_TRY_FMT ioctls for details. Videooutput devices must implement both theVIDIOC_G_FMT andVIDIOC_S_FMT
ioctl, even ifVIDIOC_S_FMT ignores all requests and alwaysreturns default parameters asVIDIOC_G_FMT does.VIDIOC_TRY_FMT is optional.
4.3.4. Writing Images
A video output device may support the write() function and/or streaming (memory mapping oruser pointer) I/O. SeeChapter
3 for details.
4.4. Video Output Overlay Interface
Also known as On-Screen Display (OSD)
Some video output devices can overlay a framebuffer image ontothe
outgoing video signal. Applications can set up such an overlayusing this
interface, which borrows structures and ioctls of theVideo
Overlay interface.
The OSD function is accessible through the same characterspecial file as the Video Output function.Note the default function of such a
/dev/video deviceis video capturing or output. The OSD function is only available aftercalling theVIDIOC_S_FMT ioctl.
4.4.1. Querying Capabilities
Devices supporting the Video OutputOverlay interface set theV4L2_CAP_VIDEO_OUTPUT_OVERLAY flag in thecapabilities field of struct v4l2_capabilityreturned
by the VIDIOC_QUERYCAP ioctl.
4.4.2. Framebuffer
Contrary to the Video Overlayinterface
the framebuffer is normally implemented on the TV card andnot the
graphics card. On Linux it is accessible as a framebufferdevice (/dev/fbN). Given a V4L2 device,applications
can find the corresponding framebuffer device by callingthe VIDIOC_G_FBUF ioctl. It returns, amongst other information, thephysical address of the framebuffer in thebase field of struct v4l2_framebuffer.
Theframebuffer device ioctl FBIOGET_FSCREENINFOreturns the same address in thesmem_startfield of structfb_fix_screeninfo. TheFBIOGET_FSCREENINFO
ioctl and structfb_fix_screeninfo are defined in thelinux/fb.h header file.
The width and height of the framebuffer depends on thecurrent video
standard. A V4L2 driver may reject attempts to changethe video standard
(or any other ioctl which would imply a framebuffersize change) with anEBUSY error
code until all applications closed theframebuffer device.
Example 4-1. Finding a framebuffer device for OSD
#include <linux/fb.h> struct v4l2_framebuffer fbuf;
unsigned int i;
int fb_fd; if (-1 == ioctl (fd, VIDIOC_G_FBUF, &fbuf)) {
perror ("VIDIOC_G_FBUF");
exit (EXIT_FAILURE);
} for (i = 0; i < 30; ++i) {
char dev_name[16];
struct fb_fix_screeninfo si; snprintf (dev_name, sizeof (dev_name), "/dev/fb%u", i); fb_fd = open (dev_name, O_RDWR);
if (-1 == fb_fd) {
switch (errno) {
case ENOENT: /* no such file */
case ENXIO: /* no driver */
continue; default:
perror ("open");
exit (EXIT_FAILURE);
}
} if (0 == ioctl (fb_fd, FBIOGET_FSCREENINFO, &si)) {
if (si.smem_start == (unsigned long) fbuf.base)
break;
} else {
/* Apparently not a framebuffer device. */
} close (fb_fd);
fb_fd = -1;
} /* fb_fd is the file descriptor of the framebuffer device
for the video output overlay, or -1 if no device was found. */
4.4.3. Overlay Window and Scaling
The overlay is controlled by source and target rectangles.The source rectangle selects a subsection of the framebuffer image tobe overlaid, the target rectangle an area in the outgoing video signalwhere the image will appear. Drivers may or may not support scaling,and arbitrary sizes and positions of these rectangles. Further driversmay support any (or none) of the clipping/blending methods defined fortheVideo Overlay interface.
A struct v4l2_window defines the size of the source rectangle,its position in the framebuffer and the clipping/blending method to beused for the overlay. To get the current parameters applications setthe type field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY and call theVIDIOC_G_FMT ioctl. The driver fills thev4l2_window substructure namedwin. It is not possible to retrieve apreviously programmed clipping list or bitmap.
To program the source rectangle applications set thetype field of a struct v4l2_format toV4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY, initializethe win substructure and call theVIDIOC_S_FMT ioctl. The driver adjusts the parameters againsthardware limits and returns the actual parameters asVIDIOC_G_FMT does. LikeVIDIOC_S_FMT, theVIDIOC_TRY_FMT ioctl can beused to learn about driver capabilities without actually changingdriver state. UnlikeVIDIOC_S_FMT this also worksafter the overlay has been enabled.
A struct v4l2_crop defines the size and position of the targetrectangle. The scaling factor of the overlay is implied by the widthand height given in struct v4l2_window and struct v4l2_crop. The cropping APIapplies toVideo Output andVideoOutput Overlay devices in the same way as toVideo Capture andVideoOverlay devices, merely reversing the direction of thedata flow. For more information seeSection 1.11.
4.4.4. Enabling Overlay
There is no V4L2 ioctl to enable or disable the overlay,however the framebuffer interface of the driver may support theFBIOBLANK ioctl.
4.5. Codec Interface
A V4L2 codec can compress, decompress, transform, or otherwiseconvert video data from one format into another format, in memory.Applications send data to be converted to the driver through awrite() call, and receive the converted data through aread() call. For efficiency a driver may also support streamingI/O.
[to do]
4.6. Effect Devices Interface
A V4L2 video effect device can do image effects, filtering, orcombine two or more images or image streams. For example videotransitions or wipes. Applications send data to be processed andreceive the result data either withread() andwrite()functions, or through the streaming I/O mechanism.
[to do]
4.7. Raw VBI Data Interface
VBI is an abbreviation of Vertical Blanking Interval, a gapin the sequence of lines of an analog video signal. During VBIno picture information is transmitted, allowing some time while theelectron beam of a cathode ray tube TV returns to the top of thescreen. Using an oscilloscope you will find here the verticalsynchronization pulses and short data packages ASKmodulated[22]onto the video signal. These are transmissions of services such asTeletext or Closed Caption.
Subject of this interface type is raw VBI data, as sampled offa video signal, or to be added to a signal for output.The data format is similar to uncompressed video images, a number oflines times a number of samples per line, we call this a VBI image.
Conventionally V4L2 VBI devices are accessed through characterdevice special files named/dev/vbi and/dev/vbi0 to/dev/vbi31 withmajor number 81 and minor numbers 224 to 255./dev/vbi is typically a symbolic link to thepreferred VBI device. This convention applies to both input and outputdevices.
To address the problems of finding related video and VBIdevices VBI capturing and output is also available as device functionunder/dev/video. To capture or output raw VBIdata with these devices applications must call theVIDIOC_S_FMTioctl. Accessed as/dev/vbi, raw VBI capturingor output is the default device function.
4.7.1. Querying Capabilities
Devices supporting the raw VBI capturing or output API setthe V4L2_CAP_VBI_CAPTURE orV4L2_CAP_VBI_OUTPUT flags, respectively, in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl. At least one of theread/write, streaming or asynchronous I/O methods must besupported. VBI devices may or may not have a tuner or modulator.
4.7.2. Supplemental Functions
VBI devices shall support videoinput or output, tuner ormodulator, and controls ioctlsas needed. The video standard ioctls provideinformation vital to program a VBI device, therefore must besupported.
4.7.3. Raw VBI Format Negotiation
Raw VBI sampling abilities can vary, in particular thesampling frequency. To properly interpret the data V4L2 specifies anioctl to query the sampling parameters. Moreover, to allow for someflexibility applications can also suggest different parameters.
As usual these parameters are notreset atopen() time to permit Unix tool chains, programming adevice and then reading from it as if it was a plain file. Wellwritten V4L2 applications should always ensure they really get whatthey want, requesting reasonable parameters and then checking if theactual parameters are suitable.
To query the current raw VBI capture parametersapplications set the type field of astruct v4l2_format toV4L2_BUF_TYPE_VBI_CAPTURE orV4L2_BUF_TYPE_VBI_OUTPUT, and call theVIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fillthe struct v4l2_vbi_formatvbi member of thefmt union.
To request different parameters applications set thetype field of a struct v4l2_format as above andinitialize all fields of the struct v4l2_vbi_formatvbi member of thefmt union, or better just modify theresults ofVIDIOC_G_FMT, and call theVIDIOC_S_FMT ioctl with a pointer to this structure. Drivers returnanEINVAL error code only when the given parameters are ambiguous, otherwisethey modify the parameters according to the hardware capabilites andreturn the actual parameters. When the driver allocates resources atthis point, it may return an EBUSY error code to indicate the returnedparameters are valid but the required resources are currently notavailable. That may happen for instance when the video and VBI areasto capture would overlap, or when the driver supports multiple opensand another process already requested VBI capturing or output. Anyway,applications must expect other resource allocation points which mayreturnEBUSY, at theVIDIOC_STREAMON ioctland the first read(), write() and select() call.
VBI devices must implement both theVIDIOC_G_FMT andVIDIOC_S_FMT ioctl, even ifVIDIOC_S_FMT ignores all requests and alwaysreturns default parameters asVIDIOC_G_FMT does.VIDIOC_TRY_FMT is optional.
Table 4-4. struct v4l2_vbi_format
| __u32 | sampling_rate | Samples per second, i. e. unit 1 Hz. |
| __u32 | offset | Horizontal offset of the VBI image,relative to the leading edge of the line synchronization pulse andcounted in samples: The first sample in the VBI image will be located |
| __u32 | samples_per_line | |
| __u32 | sample_format | Defines the sample format as in Chapter 2, a four-character-code. Usually this is |
| __u32 | start[2] | This is the scanning system line numberassociated with the first line of the VBI image, of the first and thesecond field respectively. SeeFigure 4-2 andFigure 4-3 for valid values. VBI input drivers canreturn start values 0 if the hardware cannot reliable identifyscanning lines, VBI acquisition may not require thisinformation. |
| __u32 | count[2] | The number of lines in the first and secondfield image, respectively. |
Drivers should be asflexibility as possible. For example, it may be possible to extend ormove the VBI capture window down to the picture area, implementing a'full field mode' to capture data service transmissions embedded inthe picture. An application can set the first or second Both To initialize the | ||
| __u32 | flags | See Table 4-5 below. Currentlyonly drivers set flags, applications must set this field tozero. |
| __u32 | reserved[2] | This array is reserved for future extensions.Drivers and applications must set it to zero. |
| Notes: a. A few devices may be unable tosample VBI data at all but can extend the video capture window to theVBI region. |
Table 4-5. Raw VBI Format Flags
V4L2_VBI_UNSYNC | 0x0001 | This flag indicates hardware which does notproperly distinguish |
V4L2_VBI_INTERLACED | 0x0002 | By default the two field images will be passedsequentially; all lines of the first field followed by all lines ofthe second field (compareSection 3.6 V4L2_FIELD_SEQ_TBand V4L2_FIELD_SEQ_BT, whether the top orbottomfield is first in memory depends on the video standard). When thisflag is set, the two fields are interlaced (cf. V4L2_FIELD_INTERLACED).The first line of thefirst field followed by the first line of the second field, then thetwo second lines, and so on. Such a layout may be necessary when thehardware has been programmed to capture or output interlaced videoimages and is unable to separate the fields for VBI capturing atthe same time. For simplicity setting this flag implies that both count values are equal and non-zero. |
| Notes: a. Most VBI services transmit on both fields, butsome have different semantics depending on the field number. Thesecannot be reliable decoded or encoded when V4L2_VBI_UNSYNC is set. |
Figure 4-1. Line synchronization
Figure 4-2. ITU-R 525 line numbering (M/NTSC and M/PAL)
(1) For the purpose of this specification field 2starts in line 264 and not 263.5 because half line capturing is notsupported.
Figure 4-3. ITU-R 625 line numbering
(1) For the purpose of this specification field 2starts in line 314 and not 313.5 because half line capturing is notsupported.
Remember the VBI image format depends on the selectedvideo standard,
therefore the application must choose a new standard orquery the current
standard first. Attempts to read or write data aheadof format
negotiation, or after switching the video standard
which mayinvalidate the negotiated VBI parameters, should be refused by
thedriver. A format change during active I/O is not permitted.
4.7.4. Reading and writing VBI images
To assure synchronization with the field number and
easierimplementation, the smallest unit of data passed at a time is
oneframe, consisting of two fields of VBI images immediately following
inmemory.
The total size of a frame computes as follows:
(count[0] +count[1]) *
samples_per_line* sample size in bytes
The sample size is most likely always one byte,applications must check the sample_formatfield though, to function properly with other drivers.
A VBI device may support read/write and/or streaming (memory mapping oruser pointer) I/O. The latter bears thepossibility of synchronizing video andVBI data by using buffer timestamps.
Remember the VIDIOC_STREAMON ioctl and the first read(),write() and select() call can be resource allocation points returninganEBUSY error code if the required hardware resources are temporarilyunavailable, for example the device is already in use by anotherprocess.
4.8. Sliced VBI Data Interface
VBI stands for Vertical Blanking Interval, a gap in thesequence of lines of an analog video signal. During VBI no pictureinformation is transmitted, allowing some time while the electron beamof a cathode ray tube TV returns to the top of the screen.
Sliced VBI devices use hardware to demodulate data transmittedin the VBI. V4L2 drivers shallnot do this bysoftware, see also theraw VBIinterface. The data is passed as short packets of fixed size,covering one scan line each. The number of packets per video frame isvariable.
Sliced VBI capture and output devices are accessed through thesame character special files as raw VBI devices. When a driversupports both interfaces, the default function of a/dev/vbi device israw VBIcapturing or output, and the sliced VBI function is only availableafter calling theVIDIOC_S_FMT ioctl as defined below. Likewise a/dev/video device may support the sliced VBI API,however the default function here is video capturing or output.Different file descriptors must be used to pass raw and sliced VBIdata simultaneously, if this is supported by the driver.
4.8.1. Querying Capabilities
Devices supporting the sliced VBI capturing or output APIset the V4L2_CAP_SLICED_VBI_CAPTURE orV4L2_CAP_SLICED_VBI_OUTPUT flag respectively, inthecapabilities field of struct v4l2_capabilityreturned by theVIDIOC_QUERYCAP ioctl. At least one of theread/write, streaming or asynchronousI/Omethods must be supported. Sliced VBI devices may have a tuneror modulator.
4.8.2. Supplemental Functions
Sliced VBI devices shall support videoinput or output and tuner ormodulator ioctls if they have these capabilities, and they maysupportcontrol ioctls. Thevideo standard ioctls provide informationvital to program a sliced VBI device, therefore must besupported.
4.8.3. Sliced VBI Format Negotiation
To find out which data services are supported by thehardware applications can call theVIDIOC_G_SLICED_VBI_CAP ioctl.All drivers implementing the sliced VBI interface must support thisioctl. The results may differ from those of theVIDIOC_S_FMT ioctlwhen the number of VBI lines the hardware can capture or output perframe, or the number of services it can identify on a given line arelimited. For example on PAL line 16 the hardware may be able to lookfor a VPS or Teletext signal, but not both at the same time.
To determine the currently selected services applicationsset the type field of struct v4l2_format to V4L2_BUF_TYPE_SLICED_VBI_CAPTURE orV4L2_BUF_TYPE_SLICED_VBI_OUTPUT, and theVIDIOC_G_FMTioctl fills thefmt.sliced member, astruct v4l2_sliced_vbi_format.
Applications can request different parameters byinitializing or modifying the fmt.slicedmember and calling the VIDIOC_S_FMT ioctl with a pointer to thev4l2_format structure.
The sliced VBI API is more complicated than the raw VBI APIbecause the hardware must be told which VBI service to expect on eachscan line. Not all services may be supported by the hardware on alllines (this is especially true for VBI output where Teletext is oftenunsupported and other services can only be inserted in one specificline). In many cases, however, it is sufficient to just set theservice_set field to the required servicesand let the driver fill theservice_linesarray according to hardware capabilities. Only if more precise controlis needed should the programmer set theservice_lines array explicitly.
The VIDIOC_S_FMT ioctl returns anEINVAL error code only when thegiven parameters are ambiguous, otherwise it modifies the parametersaccording to hardware capabilities. When the driver allocatesresources at this point, it may return anEBUSY error code if the requiredresources are temporarily unavailable. Other resource allocationpoints which may returnEBUSY can be theVIDIOC_STREAMON ioctl and the firstread(),write() andselect() call.
Table 4-6. structv4l2_sliced_vbi_format
| __u32 | service_set | If On return the driver sets this field to the union of allelements of the returned | ||
| __u16 | service_lines[2][24] | Applications initialize thisarray with sets of data services the driver shall look for or inserton the respective scan line. Subject to hardware capabilities driversreturn the requested set, a subset, which may be just a singleservice, or an empty set. When the hardware cannot handle multipleservices on the same line the driver shall choose one. No assumptionscan be made on which service the driver chooses. Dataservices are defined in Table 4-7. Array indicesmap to ITU-R line numbers (see also Figure 4-2 and Figure 4-3) as follows: | ||
| Element | 525 line systems | 625 line systems | ||
service_lines[0][1] | 1 | 1 | ||
service_lines[0][23] | 23 | 23 | ||
service_lines[1][1] | 264 | 314 | ||
service_lines[1][23] | 286 | 336 | ||
Drivers must setservice_lines[0][0] andservice_lines[1][0] to zero. | ||||
| __u32 | io_size | Maximum number of bytes passed byone read() or write() call, and the buffer size in bytes fortheVIDIOC_QBUF andVIDIOC_DQBUF ioctl. Drivers set this field tothe size of struct v4l2_sliced_vbi_data times the number of non-zeroelements in the returnedservice_linesarray (that is the number of lines potentially carrying data). | ||
| __u32 | reserved[2] | This array is reserved for futureextensions. Applications and drivers must set it to zero. | ||
| Notes: a. According to ETS 300 706 lines 6-22 of thefirst field and lines 5-22 of the second field may carry Teletextdata. |
Table 4-7. Sliced VBI services
Drivers may return an EINVAL error code when applications attempt toread or write data without prior format negotiation, after switchingthe video standard (which may invalidate the negotiated VBIparameters) and after switching the video input (which may change thevideo standard as a side effect). The VIDIOC_S_FMT ioctl may returnan EBUSY error code when applications attempt to change the format while i/o isin progress (between aVIDIOC_STREAMON andVIDIOC_STREAMOFF call,and after the firstread() orwrite() call).
4.8.4. Reading and writing sliced VBI data
A single read() or write() call must pass all databelonging to one video frame. That is an array ofv4l2_sliced_vbi_data structures with one ormore elements and a total size not exceedingio_size bytes. Likewise in streaming I/Omode one buffer of io_size bytes mustcontain data of one video frame. Theid ofunusedv4l2_sliced_vbi_data elements must bezero.
Table 4-8. structv4l2_sliced_vbi_data
| __u32 | id | A flag from Table 2identifying the type of data in this packet. Only a single bit must beset. When theid of a captured packet iszero, the packet is empty and the contents of other fields areundefined. Applications shall ignore empty packets. When theid of a packet for output is zero thecontents of thedata field are undefinedand the driver must no longer insert data on the requestedfield andline. |
| __u32 | field | The video field number this data has been capturedfrom, or shall be inserted at.0 for the firstfield,1 for the second field. |
| __u32 | line | The field (as opposed to frame) line number thisdata has been captured from, or shall be inserted at. SeeFigure 4-2 andFigure 4-3 for validvalues. Sliced VBI capture devices can set the line number of allpackets to0 if the hardware cannot reliablyidentify scan lines. The field number must always be valid. |
| __u32 | reserved | This field is reserved for future extensions.Applications and drivers must set it to zero. |
| __u8 | data[48] | The packet payload. See Table 2 for the contents and number ofbytes passed for each data type. The contents of padding bytes at theend of this array are undefined, drivers and applications shall ignorethem. |
Packets are always passed in ascending line number order,without duplicate line numbers. Thewrite() function and theVIDIOC_QBUF ioctl must return an EINVAL error code when applications violatethis rule. They must also return anEINVAL error code when applications pass anincorrect field or line number, or a combination offield,line andid which has not been negotiated with theVIDIOC_G_FMT orVIDIOC_S_FMT ioctl. When the line numbers areunknown the driver must pass the packets in transmitted order. Thedriver can insert empty packets withid setto zero anywhere in the packet array.
To assure synchronization and to distinguish from framedropping, when a captured frame does not carry any of the requesteddata services drivers must pass one or more empty packets. When anapplication fails to pass VBI data in time for output, the drivermust output the last VPS and WSS packet again, and disable the outputof Closed Caption and Teletext data, or output data which is ignoredby Closed Caption and Teletext decoders.
A sliced VBI device may support read/write and/or streaming (memory mapping and/oruserpointer) I/O. The latter bears the possibility of synchronizingvideo and VBI data by using buffer timestamps.
4.9. Teletext Interface
This interface aims at devices receiving and demodulatingTeletext data [,], evaluating theTeletext packages and storing formatted pages in cache memory. Suchdevices are usually implemented as microcontrollers with serialinterface (IC) and can be found on olderTV cards, dedicated Teletext decoding cards and home-brew devicesconnected to the PC parallel port.
The Teletext API was designed by Martin Buck. It is defined inthe kernel header filelinux/videotext.h, thespecification is available fromhttp://home.pages.de/~videotext/. (Videotext is the name ofthe German public television Teletext service.) Conventional characterdevice file names are/dev/vtx and/dev/vttuner, with device number 83, 0 and 83, 16respectively. A similar interface exists for the Philips SAA5249Teletext decoder [specification?] with character device file names/dev/tlkN, device number 102, N.
Eventually the Teletext API was integrated into the V4L APIwith character device file names/dev/vtx0 to/dev/vtx31, device major number 81, minor numbers192 to 223. For reference the V4L Teletext API specification isreproduced here in full: "Teletext interfaces talk the existing VTXAPI." Teletext devices with major number 83 and 102 will be removed inLinux 2.6.
There are no plans to replace the Teletext API or to integrateit into V4L2. Please write to the Video4Linux mailing list:https://listman.redhat.com/mailman/listinfo/video4linux-list when the need arises.
4.10. Radio Interface
This interface is intended for AM and FM (analog) radioreceivers.
Conventionally V4L2 radio devices are accessed throughcharacter device special files named/dev/radioand/dev/radio0 to/dev/radio63 with major number 81 and minornumbers 64 to 127.
4.10.1. Querying Capabilities
Devices supporting the radio interface set theV4L2_CAP_RADIO andV4L2_CAP_TUNER flag in thecapabilities field of struct v4l2_capabilityreturned by the VIDIOC_QUERYCAP ioctl. Other combinations ofcapability flags are reserved for future extensions.
4.10.2. Supplemental Functions
Radio devices can support controls, and must support the tuner ioctls.
They do not support the video input or output, audio inputor output, video standard, cropping and scaling, compression andstreaming parameter, or overlay ioctls. All other ioctls and I/Omethods are reserved for future extensions.
4.10.3. Programming
Radio devices may have a couple audio controls (as discussedin Section 1.8) such as a volume control, possibly customcontrols. Further all radio devices have one tuner (these arediscussed inSection 1.6) with index number zero to selectthe radio frequency and to determine if a monaural or FM stereoprogram is received. Drivers switch automatically between AM and FMdepending on the selected frequency. TheVIDIOC_G_TUNER ioctlreports the supported frequency range.
4.11. RDS Interface
The Radio Data System transmits supplementaryinformation in binary format, for example the station name or travelinformation, on a inaudible audio subcarrier of a radio program. Thisinterface aims at devices capable of receiving and decoding RDSinformation.
The V4L API defines its RDS API as follows.
From radio devices supporting it, RDS data can be readwith the read() function. The data is packed in groups of three,as follows:
First Octet Least Significant Byte of RDS Block
Second Octet Most Significant Byte of RDS Block
Third Octet Bit 7: Error bit. Indicates that anuncorrectable error occurred during reception of this block. Bit 6:Corrected bit. Indicates that an error was corrected for this datablock. Bits 5-3: Received Offset. Indicates the offset received by thesync system. Bits 2-0: Offset Name. Indicates the offset applied tothis data.
It was argued the RDS API should beextended before integration into V4L2, no new API has been devised yet.Please write to the Video4Linux mailing list for discussion:https://listman.redhat.com/mailman/listinfo/video4linux-list. Meanwhile no V4L2 driver should set theV4L2_CAP_RDS_CAPTURE capability flag.
I. Function Reference
- Table of Contents
- V4L2 close() -- Close a V4L2 device
- V4L2 ioctl() -- Program a V4L2 device
- ioctl VIDIOC_CROPCAP -- Information about the video cropping and scaling abilities
- ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER -- Read or write hardware registers
- ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD -- Execute an encoder command
- ioctl VIDIOC_ENUMAUDIO -- Enumerate audio inputs
- ioctl VIDIOC_ENUMAUDOUT -- Enumerate audio outputs
- ioctl VIDIOC_ENUM_FMT -- Enumerate image formats
- ioctl VIDIOC_ENUM_FRAMESIZES -- Enumerate frame sizes
- ioctl VIDIOC_ENUM_FRAMEINTERVALS -- Enumerate frame intervals
- ioctl VIDIOC_ENUMINPUT -- Enumerate video inputs
- ioctl VIDIOC_ENUMOUTPUT -- Enumerate video outputs
- ioctl VIDIOC_ENUMSTD -- Enumerate supported video standards
- ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO -- Query or select the current audio input and itsattributes
- ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT -- Query or select the current audio output
- ioctl VIDIOC_G_CHIP_IDENT -- Identify the chips on a TV card
- ioctl VIDIOC_G_CROP, VIDIOC_S_CROP -- Get or set the current cropping rectangle
- ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL -- Get or set the value of a control
- ioctl VIDIOC_G_ENC_INDEX -- Get meta data about a compressed video stream
- ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,VIDIOC_TRY_EXT_CTRLS -- Get or set the value of several controls, try controlvalues
- ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters
- ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,VIDIOC_TRY_FMT -- Get or set the data format, try a format
- ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY -- Get or set tuner or modulator radiofrequency
- ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT -- Query or select the current video input
- ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP --
- ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR -- Get or set modulator attributes
- ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT -- Query or select the current video output
- ioctl VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters
- ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY -- Query or request the access priority associated with afile descriptor
- ioctl VIDIOC_G_SLICED_VBI_CAP -- Query sliced VBI capabilities
- ioctl VIDIOC_G_STD, VIDIOC_S_STD -- Query or select the video standard of the current input
- ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER -- Get or set tuner attributes
- ioctl VIDIOC_LOG_STATUS -- Log driver status information
- ioctl VIDIOC_OVERLAY -- Start or stop video overlay
- ioctl VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver
- ioctl VIDIOC_QUERYBUF -- Query the status of a buffer
- ioctl VIDIOC_QUERYCAP -- Query device capabilities
- ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items
- ioctl VIDIOC_QUERYSTD -- Sense the video standard received by the currentinput
- ioctl VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O
- ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF -- Start or stop streaming I/O
- V4L2 mmap() -- Map device memory into application address space
- V4L2 munmap() -- Unmap device memory
- V4L2 open() -- Open a V4L2 device
- V4L2 poll() -- Wait for some event on a file descriptor
- V4L2 read() -- Read from a V4L2 device
- V4L2 select() -- Synchronous I/O multiplexing
- V4L2 write() -- Write to a V4L2 device
V4L2 close()
Name
v4l2-close -- Close a V4L2 device
Synopsis
#include <unistd.h>
int close(int fd);
Arguments
fdFile descriptor returned by
open().
Description
Closes the device. Any I/O in progress is terminated andresources associated with the file descriptor are freed. However dataformat parameters, current input or output, control values or otherproperties remain unchanged.
Return Value
The function returns 0 onsuccess, -1 on failure and theerrno is set appropriately. Possible errorcodes:
- EBADF
fdis not a valid open filedescriptor.
V4L2 ioctl()
Name
v4l2-ioctl -- Program a V4L2 device
Synopsis
#include <sys/ioctl.h>
int ioctl(int fd, int request, void *argp);
Arguments
fdFile descriptor returned by
open().requestV4L2 ioctl request code as defined in the videodev.h header file, for exampleVIDIOC_QUERYCAP.
argpPointer to a function parameter, usually a structure.
Description
The ioctl() function is used to programV4L2 devices. The argumentfd must be an openfile descriptor. An ioctlrequest has encodedin it whether the argument is an input, output or read/writeparameter, and the size of the argumentargp inbytes. Macros and defines specifying V4L2 ioctl requests are locatedin thevideodev.h header file.Applications should use their own copy, not include the version in thekernel sources on the system they compile on. All V4L2 ioctl requests,their respective function and parameters are specified inReference I,Function Reference.
Return Value
On success the ioctl() function returns0 and does not reset theerrno variable. On failure-1 is returned, when the ioctl takes anoutput or read/write parameter it remains unmodified, and theerrno variable is set appropriately. See below forpossible error codes. Generic errors likeEBADForEFAULT are not listed in the sectionsdiscussing individual ioctl requests.
Note ioctls may return undefined error codes. Since errorsmay have side effects such as a driver reset applications shouldabort on unexpected errors.
- EBADF
fdis not a valid open filedescriptor.- EBUSY
The property cannot be changed right now. Typicallythis error code is returned when I/O is in progress or the driversupports multiple opens and another process locked the property.
- EFAULT
argpreferences an inaccessiblememory area.- ENOTTY
fdis not associated with acharacter special device.- EINVAL
The
requestor the data pointedto byargpis not valid. This is a very commonerror code, see the individual ioctl requests listed inReference I,Function Reference for actual causes.- ENOMEM
Not enough physical or virtual memory was available tocomplete the request.
- ERANGE
The application attempted to set a control with the
VIDIOC_S_CTRLioctl to a value which is out of bounds.
ioctl VIDIOC_CROPCAP
Name
VIDIOC_CROPCAP -- Information about the video cropping and scaling abilities
Synopsis
int ioctl(int fd, int request, struct v4l2_cropcap*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_CROPCAP
argp
Description
Applications use this function to query the croppinglimits, the pixel
aspect of images and to calculate scale factors.They set thetype field of a v4l2_cropcapstructure to the respective buffer (stream) type and call theVIDIOC_CROPCAP
ioctl with a pointer to thisstructure. Drivers fill the rest of the
structure. The results areconstant except when switching the video
standard. Remember thisswitch can occur implicit when switching the
video input oroutput.
Table 1. struct v4l2_cropcap
| enum v4l2_buf_type | type | Type of the data stream, set by the application.Only these types are valid here:V4L2_BUF_TYPE_VIDEO_CAPTURE,V4L2_BUF_TYPE_VIDEO_OUTPUT,V4L2_BUF_TYPE_VIDEO_OVERLAY,and custom (driverdefined) types with code V4L2_BUF_TYPE_PRIVATEand higher. |
| struct v4l2_rect | bounds | Defines the window within capturing or output ispossible, this may exclude for example the horizontal and verticalblanking areas. The cropping rectangle cannot exceed these limits.Width and height are defined in pixels, the driver writer is free tochoose origin and units of the coordinate system in the analogdomain. |
| struct v4l2_rect | defrect | Default cropping rectangle, it shall cover the"whole picture". Assuming pixel aspect 1/1 this could be for example a640 × 480 rectangle for NTSC, a768 × 576 rectangle for PAL and SECAM centered overthe active picture area. The same co-ordinate system as for bounds is used. |
| struct v4l2_fract | pixelaspect | This is the pixel aspect (y / x) when noscaling is applied, the ratio When cropping coordinates refer to square pixels,the driver sets |
Table 2. struct v4l2_rect
| __s32 | left | Horizontal offset of the top, left corner of therectangle, in pixels. |
| __s32 | top | Vertical offset of the top, left corner of therectangle, in pixels. |
| __s32 | width | Width of the rectangle, in pixels. |
| __s32 | height | Height of the rectangle, in pixels. Widthand height cannot be negative, the fields are signed forhysterical reasons. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_cropcap
typeisinvalid or the ioctl is not supported. This is not permitted forvideo capture, output and overlay devices, which
must supportVIDIOC_CROPCAP.
ioctl VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER
Name
VIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER -- Read or write hardware registers
Synopsis
int ioctl(int fd, int request, struct v4l2_register *argp);
int ioctl(int fd, int request, const struct v4l2_register*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_DBG_G_REGISTER, VIDIOC_DBG_S_REGISTER
argp
Description
For driver debugging purposes these ioctls allow testapplications to
access hardware registers directly. Regularapplications should not use
them.
Since writing or even reading registers can jeopardize thesystem
security, its stability and damage the hardware, both ioctlsrequire
superuser privileges. Additionally the Linux kernel must becompiled with
theCONFIG_VIDEO_ADV_DEBUG
optionto enable these ioctls.
To write a register applications must initialize all fieldsof a struct v4l2_register and callVIDIOC_DBG_S_REGISTER with a pointer to thisstructure.
The match_type andmatch_chip fields select a chip on the TVcard, thereg field specifies a registernumber and theval field the
value to bewritten into the register.
To read a register applications must initialize thematch_type,match_chip andreg fields, and callVIDIOC_DBG_G_REGISTER with a
pointer to thisstructure. On success the driver stores the register value in theval field. On failure the structure remainsunchanged.
When match_type isV4L2_CHIP_MATCH_HOST,match_chip selects the nth non-IC chipon the TV card. Drivers may also interpretmatch_chip
as a random ID, but we recommendagainst that. The number zero always
selects the host chip, e. g. thechip connected to the PCI bus. You can
find out which chips arepresent with theVIDIOC_G_CHIP_IDENT
ioctl.
When match_type isV4L2_CHIP_MATCH_I2C_DRIVER,match_chip contains a driver ID as definedin thelinux/i2c-id.h header file. For instanceI2C_DRIVERID_SAA7127
will match any chipsupported by the saa7127 driver, regardless of its IC
bus address.When multiple chips supported by the same driver are
present, theeffect of these ioctls is undefined. Again with theVIDIOC_G_CHIP_IDENT
ioctl you can find out which IC chips arepresent.
When match_type isV4L2_CHIP_MATCH_I2C_ADDR,match_chip selects a chip by its 7 bit ICbus address.
These ioctls are optional, not all drivers may support them.However when a driver supports these ioctls it must also supportVIDIOC_G_CHIP_IDENT.
Conversely it may supportVIDIOC_G_CHIP_IDENT but not these ioctls.
VIDIOC_DBG_G_REGISTER andVIDIOC_DBG_S_REGISTER were introduced in Linux2.6.21.
We recommended the v4l2-dbgutility over calling these ioctls directly. It is available from theLinuxTV v4l-dvb repository; seehttp://linuxtv.org/repo/ foraccess instructions.
Table 1. struct v4l2_register
| __u32 | match_type | See Table 2 for a list of possible types. | |
| __u32 | match_chip | Match a chip by this number, interpreted accordingto the | |
| __u64 | reg | A register number. | |
| __u64 | val | The value read from, or to be written into theregister. |
Table 2. Chip Match Types
V4L2_CHIP_MATCH_HOST | 0 | Match the nth chip on the card, zero for the host chip. Does not match IC chips. |
V4L2_CHIP_MATCH_I2C_DRIVER | 1 | Match an IC chip by its driver ID from thelinux/i2c-id.h header file. |
V4L2_CHIP_MATCH_I2C_ADDR | 2 | Match a chip by its 7 bit IC bus address. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The driver does not support this ioctl, or the kernelwas not compiled with the
CONFIG_VIDEO_ADV_DEBUGoption, or thematch_typeis invalid, or theselected chip or register does not exist.- EPERM
Insufficient permissions. Root privileges are requiredto execute these ioctls.
ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD
Name
VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD -- Execute an encoder command
Synopsis
int ioctl(int fd, int request, struct v4l2_encoder_cmd *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD
argp
Description
These ioctls control an audio/video (usually MPEG-) encoder.VIDIOC_ENCODER_CMD sends a command to theencoder,VIDIOC_TRY_ENCODER_CMD can be used totry a command without actually executing it.
To send a command applications must initialize all fields of a struct v4l2_encoder_cmd and callVIDIOC_ENCODER_CMD orVIDIOC_TRY_ENCODER_CMD
with a pointer to this structure.
The cmd field must contain thecommand code. Theflags field is currentlyonly used by the STOP command and contains one bit: If theV4L2_ENC_CMD_STOP_AT_GOP_END
flag is set,encoding will continue until the end of the currentGroupOf Pictures, otherwise it will stop immediately.
A read() call sends a START command tothe encoder if it has not been started yet. After a STOP command,read() calls will read the remaining databuffered by the driver. When the buffer is empty,read()
will return zero and the nextread() call will restart the encoder.
A close() call sends an immediate STOPto the encoder, and all buffered data is discarded.
These ioctls are optional, not all drivers may supportthem. They were introduced in Linux 2.6.21.
Table 1. struct v4l2_encoder_cmd
| __u32 | cmd | The encoder command, see Table 2. |
| __u32 | flags | Flags to go with the command, see Table 3. If no flags are defined forthis command, drivers and applications must set this field tozero. |
| __u32 | data[8] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
Table 2. Encoder Commands
V4L2_ENC_CMD_START | 0 | Start the encoder. When the encoder is alreadyrunning or paused, this command does nothing. No flags are defined forthis command. |
V4L2_ENC_CMD_STOP | 1 | Stop the encoder. When theV4L2_ENC_CMD_STOP_AT_GOP_END flag is set,encoding will continue until the end of the currentGroupOf Pictures, otherwise encoding will stop immediately.When the encoder isalready stopped, this command doesnothing. |
V4L2_ENC_CMD_PAUSE | 2 | Pause the encoder. When the encoder has not beenstarted yet, the driver will return anEPERM error code. When the encoder isalready paused, this command does nothing. No flags are defined forthis command. |
V4L2_ENC_CMD_RESUME | 3 | Resume encoding after a PAUSE command. When theencoder has not been started yet, the driver will return anEPERM error code.When the encoder is already running, this command does nothing. Noflags are defined for this command. |
Table 3. Encoder Command Flags
V4L2_ENC_CMD_STOP_AT_GOP_END | 0x0001 | Stop encoding at the end of the current Group OfPictures, rather than immediately. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The driver does not support this ioctl, or the
cmdfield is invalid.- EPERM
The application sent a PAUSE or RESUME command whenthe encoder was not running.
ioctl VIDIOC_ENUMAUDIO
Name
VIDIOC_ENUMAUDIO -- Enumerate audio inputs
Synopsis
int ioctl(int fd, int request, struct v4l2_audio *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUMAUDIO
argp
Description
To query the attributes of an audio input applicationsinitialize the
indexreserved array of a struct v4l2_audioand call theVIDIOC_ENUMAUDIO ioctl with a pointerto
this structure. Drivers fill the rest of the structure or return anEINVAL
error code when the index is out of bounds. To enumerate all
audioinputs applications shall begin at index zero, incrementing by
oneuntil the driver returnsEINVAL.
See ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO(2) for a description ofstruct v4l2_audio.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The number of the audio input is out of bounds, orthere are no audio inputs at all and this ioctl is notsupported.
ioctl VIDIOC_ENUMAUDOUT
Name
VIDIOC_ENUMAUDOUT -- Enumerate audio outputs
Synopsis
int ioctl(int fd, int request, struct v4l2_audioout *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUMAUDOUT
argp
Description
To query the attributes of an audio output applicationsinitialize the
indexreserved array of a struct v4l2_audioout andcall theVIDIOC_G_AUDOUT ioctl with
a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL
error code when the index is out of bounds. To enumerate all
audiooutputs applications shall begin at index zero, incrementing by
oneuntil
the driver returns EINVAL.
Note connectors on a TV card to loop back the received audiosignal to a sound card are not audio outputs in this sense.
See ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT(2) for a description ofstruct v4l2_audioout.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The number of the audio output is out of bounds, orthere are no audio outputs at all and this ioctl is notsupported.
ioctl VIDIOC_ENUM_FMT
Name
VIDIOC_ENUM_FMT -- Enumerate image formats
Synopsis
int ioctl(int fd, int request, struct v4l2_fmtdesc*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUM_FMT
argp
Description
To enumerate image formats applications initialize thetype andindexfield of struct v4l2_fmtdesc and call theVIDIOC_ENUM_FMT
ioctl with a pointer to thisstructure. Drivers fill the rest of the structure or return anEINVAL error code. All formats are enumerable by beginning at index zero andincrementing by one untilEINVAL
isreturned.
Table 1. struct v4l2_fmtdesc
| __u32 | index | Number of the format in the enumeration, set bythe application. This is in no way related to thepixelformat field. |
| enum v4l2_buf_type | type | Type of the data stream, set by the application.Only these types are valid here:V4L2_BUF_TYPE_VIDEO_CAPTURE,V4L2_BUF_TYPE_VIDEO_OUTPUT,V4L2_BUF_TYPE_VIDEO_OVERLAY,and custom (driverdefined) types with code V4L2_BUF_TYPE_PRIVATEand higher. |
| __u32 | flags | See Table 2 |
| __u8 | description[32] | Description of the format, a NUL-terminated ASCIIstring. This information is intended for the user, for example: "YUV4:2:2". |
| __u32 | pixelformat | The image format identifier. This is afour character code as computed by the v4l2_fourcc()macro: |
#define v4l2_fourcc(a,b,c,d) (((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24)) Several image formats are alreadydefined by this specification in Chapter 2. Note thesecodes are not the same as those used in the Windows world. | ||
| __u32 | reserved[4] | Reserved for future extensions. Drivers must setthe array to zero. |
Table 2. Image Format Description Flags
V4L2_FMT_FLAG_COMPRESSED | 0x0001 | This is a compressed format. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_fmtdesc
typeis not supported or theindexis out ofbounds.
ioctl VIDIOC_ENUM_FRAMESIZES
Name
VIDIOC_ENUM_FRAMESIZES -- Enumerate frame sizes
Synopsis
int ioctl(int fd, int request, struct v4l2_frmsizeenum *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUM_FRAMESIZES
argpPointer to a struct v4l2_frmsizeenum that contains an indexand pixel format and receives a frame width and height.
Description
This ioctl allows applications to enumerate all frame sizes(i. e. width and height in pixels) that the device supports for thegiven pixel format.
The supported pixel formats can be obtained by using theVIDIOC_ENUM_FMT function.
The return value and the content of thev4l2_frmsizeenum.type field depend on thetype of frame sizes the device supports. Here are the semantics of thefunction for the different cases:
Discrete: The functionreturns success if the given index value (zero-based) is valid. Theapplication should increase the index by one for each call until
EINVALis returned. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_DISCRETEby the driver. Of theunion only thediscretemember isvalid.Step-wise: The functionreturns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_STEPWISEby the driver. Of theunion only thestepwisemember isvalid.Continuous: This is aspecial case of the step-wise type above. The function returns successif the given index value is zero and
EINVALforany other index value. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_CONTINUOUSby the driver. Ofthe union only thestepwisemember is validand thestep_widthandstep_heightvalues are set to 1.
When the application calls the function with index zero, itmust check the type field to determine thetype of frame size enumeration the device supports. Only for theV4L2_FRMSIZE_TYPE_DISCRETE type does it makesense to increase the index value to receive more frame sizes.
Note that the order in which the frame sizes are returnedhas no special meaning. In particular does it not say anything aboutpotential default format sizes.
Applications can assume that the enumeration data does notchange without any interaction from the application itself. This meansthat the enumeration data is consistent if the application does notperform any other ioctl calls while it runs the frame sizeenumeration.
Structs
In the structs below, IN denotes avalue that has to be filled in by the application,OUT denotes values that the driver fills in. Theapplication should zero out all members except for theIN fields.
Table 1. struct v4l2_frmsize_discrete
| __u32 | width | Width of the frame [pixel]. |
| __u32 | height | Height of the frame [pixel]. |
Table 2. struct v4l2_frmsize_stepwise
| __u32 | min_width | Minimum frame width [pixel]. |
| __u32 | max_width | Maximum frame width [pixel]. |
| __u32 | step_width | Frame width step size [pixel]. |
| __u32 | min_height | Minimum frame height [pixel]. |
| __u32 | max_height | Maximum frame height [pixel]. |
| __u32 | step_height | Frame height step size [pixel]. |
Table 3. struct v4l2_frmsizeenum
| __u32 | index | IN: Index of the given frame size in the enumeration. | |
| __u32 | pixel_format | IN: Pixel format for which the frame sizes are enumerated. | |
| __u32 | type | OUT: Frame size type the device supports. | |
| union | OUT: Frame size with the given index. | ||
| struct v4l2_frmsize_discrete | discrete | ||
| struct v4l2_frmsize_stepwise | stepwise | ||
| __u32 | reserved[2] | Reserved space for future use. |
Enums
Table 4. enum v4l2_frmsizetypes
V4L2_FRMSIZE_TYPE_DISCRETE | 1 | Discrete frame size. |
V4L2_FRMSIZE_TYPE_CONTINUOUS | 2 | Continuous frame size. |
V4L2_FRMSIZE_TYPE_STEPWISE | 3 | Step-wise defined frame size. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
See the description section above for a list of returnvalues that errno can have.
ioctl VIDIOC_ENUM_FRAMEINTERVALS
Name
VIDIOC_ENUM_FRAMEINTERVALS -- Enumerate frame intervals
Synopsis
int ioctl(int fd, int request, struct v4l2_frmivalenum *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUM_FRAMEINTERVALS
argpPointer to a struct v4l2_frmivalenum structure thatcontains a pixel format and size and receives a frame interval.
Description
This ioctl allows applications to enumerate all frameintervals that the device supports for the given pixel format andframe size.
The supported pixel formats and frame sizes can be obtainedby using the VIDIOC_ENUM_FMT and VIDIOC_ENUM_FRAMESIZESfunctions.
The return value and the content of thev4l2_frmivalenum.type field depend on thetype of frame intervals the device supports. Here are the semantics ofthe function for the different cases:
Discrete: The functionreturns success if the given index value (zero-based) is valid. Theapplication should increase the index by one for each call until
EINVALis returned. The `v4l2_frmivalenum.type`field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of theunion only the `discrete` member is valid.Step-wise: The functionreturns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmivalenum.typefield is set toV4L2_FRMIVAL_TYPE_STEPWISEby the driver. Of theunion only thestepwisemember isvalid.Continuous: This is aspecial case of the step-wise type above. The function returns successif the given index value is zero and
EINVALforany other index value. Thev4l2_frmivalenum.typefield is set toV4L2_FRMIVAL_TYPE_CONTINUOUSby the driver. Ofthe union only thestepwisemember is validand thestepvalue is set to 1.
When the application calls the function with index zero, itmust check the type field to determine thetype of frame interval enumeration the device supports. Only for theV4L2_FRMIVAL_TYPE_DISCRETE type does it makesense to increase the index value to receive more frameintervals.
Note that the order in which the frame intervals arereturned has no special meaning. In particular does it not sayanything about potential default frame intervals.
Applications can assume that the enumeration data does notchange without any interaction from the application itself. This meansthat the enumeration data is consistent if the application does notperform any other ioctl calls while it runs the frame intervalenumeration.
Notes
Frame intervals and framerates: The V4L2 API uses frame intervals instead of framerates. Given the frame interval the frame rate can be computed asfollows:
frame_rate = 1 / frame_interval
Structs
In the structs below, IN denotes avalue that has to be filled in by the application,OUT denotes values that the driver fills in. Theapplication should zero out all members except for theIN fields.
Table 1. struct v4l2_frmival_stepwise
| struct v4l2_fract | min | Minimum frame interval [s]. |
| struct v4l2_fract | max | Maximum frame interval [s]. |
| struct v4l2_fract | step | Frame interval step size [s]. |
Table 2. struct v4l2_frmivalenum
| __u32 | index | IN: Index of the given frame interval in theenumeration. | |
| __u32 | pixel_format | IN: Pixel format for which the frame intervals areenumerated. | |
| __u32 | width | IN: Frame width for which the frame intervals areenumerated. | |
| __u32 | height | IN: Frame height for which the frame intervals areenumerated. | |
| __u32 | type | OUT: Frame interval type the device supports. | |
| union | OUT: Frame interval with the given index. | ||
| struct v4l2_fract | discrete | Frame interval [s]. | |
| struct v4l2_frmival_stepwise | stepwise | ||
| __u32 | reserved[2] | Reserved space for future use. |
Enums
Table 3. enum v4l2_frmivaltypes
V4L2_FRMIVAL_TYPE_DISCRETE | 1 | Discrete frame interval. |
V4L2_FRMIVAL_TYPE_CONTINUOUS | 2 | Continuous frame interval. |
V4L2_FRMIVAL_TYPE_STEPWISE | 3 | Step-wise defined frame interval. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
See the description section above for a list of returnvalues that errno can have.
ioctl VIDIOC_ENUMINPUT
Name
VIDIOC_ENUMINPUT -- Enumerate video inputs
Synopsis
int ioctl(int fd, int request, struct v4l2_input*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUMINPUT
argp
Description
To query the attributes of a video input applicationsinitialize the
indexVIDIOC_ENUMINPUT ioctl with apointer to this structure. Drivers fill the rest of the structure orreturn
anEINVAL error code when the index is
out of bounds. To enumerate allinputs applications shall begin at index
zero, incrementing by oneuntil the driver returnsEINVAL.
Table 1. struct v4l2_input
| __u32 | index | Identifies the input, set by theapplication. |
| __u8 | name[32] | Name of the video input, a NUL-terminated ASCIIstring, for example: "Vin (Composite 2)". This information is intendedfor the user, preferably the connector label on the device itself. |
| __u32 | type | Type of the input, see Table 2. |
| __u32 | audioset | Drivers can enumerate up to 32 video andaudio inputs. This field When the driverdoes not enumerate audio inputs no bits must be set. For details on audio inputs andhow to select the current input see Section 1.5. |
| __u32 | tuner | Capture devices can have zero or more tuners (RFdemodulators). When the V4L2_INPUT_TYPE_TUNER this is an RF connector andthis field identifies the tuner. It corresponds tostruct v4l2_tuner fieldindex.For details ontuners see Section 1.6. |
| v4l2_std_id | std | Every video input supports one or more differentvideo standards. This field is a set of all supported standards. Fordetails on video standards and how to switch seeSection 1.7. |
| __u32 | status | This field provides status information about theinput. See Table 3 for flags.status is only valid when this is thecurrent input. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers must setthe array to zero. |
Table 2. Input Types
V4L2_INPUT_TYPE_TUNER | 1 | This input uses a tuner (RF demodulator). |
V4L2_INPUT_TYPE_CAMERA | 2 | Analog baseband input, for example CVBS /Composite Video, S-Video, RGB. |
Table 3. Input Status Flags
| General | ||
V4L2_IN_ST_NO_POWER | 0x00000001 | Attached device is off. |
V4L2_IN_ST_NO_SIGNAL | 0x00000002 | |
V4L2_IN_ST_NO_COLOR | 0x00000004 | The hardware supports color decoding, but does notdetect color modulation in the signal. |
| Analog Video | ||
V4L2_IN_ST_NO_H_LOCK | 0x00000100 | No horizontal sync lock. |
V4L2_IN_ST_COLOR_KILL | 0x00000200 | A color killer circuit automatically disables colordecoding when it detects no color modulation. When this flag is setthe color killer is enabledand has shut offcolor decoding. |
| Digital Video | ||
V4L2_IN_ST_NO_SYNC | 0x00010000 | No synchronization lock. |
V4L2_IN_ST_NO_EQU | 0x00020000 | No equalizer lock. |
V4L2_IN_ST_NO_CARRIER | 0x00040000 | Carrier recovery failed. |
| VCR and Set-Top Box | ||
V4L2_IN_ST_MACROVISION | 0x01000000 | Macrovision is an analog copy prevention systemmangling the video signal to confuse video recorders. When thisflag is set Macrovision has been detected. |
V4L2_IN_ST_NO_ACCESS | 0x02000000 | Conditional access denied. |
V4L2_IN_ST_VTR | 0x04000000 | VTR time constant. [?] |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_input
indexisout of bounds.
ioctl VIDIOC_ENUMOUTPUT
Name
VIDIOC_ENUMOUTPUT -- Enumerate video outputs
Synopsis
int ioctl(int fd, int request, struct v4l2_output *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUMOUTPUT
argp
Description
To query the attributes of a video outputs applicationsinitialize the
indexVIDIOC_ENUMOUTPUT ioctl with apointer to this structure. Drivers fill the rest of the structure
orreturn anEINVAL error code when the
index is out of bounds. To enumerate alloutputs applications shall begin
at index zero, incrementing by oneuntil the driver returnsEINVAL.
Table 1. struct v4l2_output
| __u32 | index | Identifies the output, set by theapplication. |
| __u8 | name[32] | Name of the video output, a NUL-terminated ASCIIstring, for example: "Vout". This information is intended for theuser, preferably the connector label on the device itself. |
| __u32 | type | Type of the output, see Table 2. |
| __u32 | audioset | Drivers can enumerate up to 32 video andaudio outputs. This field When the driver does not enumerate audio outputs nobits must be set. For details on audio outputs and how toselect the current output see Section 1.5. |
| __u32 | modulator | Output devices can have zero or more RF modulators.When the V4L2_OUTPUT_TYPE_MODULATOR this is an RFconnector and this field identifies the modulator. It corresponds tostruct v4l2_modulatorfield index. For detailson modulators see Section 1.6. |
| v4l2_std_id | std | Every video output supports one or more differentvideo standards. This field is a set of all supported standards. Fordetails on video standards and how to switch seeSection 1.7. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers must setthe array to zero. |
Table 2. Output Type
V4L2_OUTPUT_TYPE_MODULATOR | 1 | This output is an analog TV modulator. |
V4L2_OUTPUT_TYPE_ANALOG | 2 | Analog baseband output, for example Composite /CVBS, S-Video, RGB. |
V4L2_OUTPUT_TYPE_ANALOGVGAOVERLAY | 3 | [?] |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_output
indexis out of bounds.
ioctl VIDIOC_ENUMSTD
Name
VIDIOC_ENUMSTD -- Enumerate supported video standards
Synopsis
int ioctl(int fd, int request, struct v4l2_standard *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_ENUMSTD
argp
Description
To query the attributes of a video standard,especially a custom (driver defined) one, applications initialize theindex field of struct v4l2_standard
and call theVIDIOC_ENUMSTD ioctl with a pointer to thisstructure. Drivers fill the rest of the structure or return anEINVAL error code when the index is out of bounds. To enumerate all standardsapplications
shall begin at index zero, incrementing by one until thedriver returns
EINVAL. Drivers may enumerate adifferent set of standards after switching the video input oroutput.[23]
Table 1. struct v4l2_standard
| __u32 | index | Number of the video standard, set by theapplication. |
| v4l2_std_id | id | The bits in this field identify the standard asone of the common standards listed inTable 3,or if bits 32 to 63 are set as custom standards. Multiple bits can beset if the hardware does not distinguish between these standards,however separate indices do not indicate the opposite. The id must be unique. No other enumeratedv4l2_standard structure, for this input oroutput anyway,can contain the same set of bits. |
| __u8 | name[24] | Name of the standard, a NUL-terminated ASCIIstring, for example: "PAL-B/G", "NTSC Japan". This information isintended for the user. |
| struct v4l2_fract | frameperiod | The frame period (not field period) is numerator/ denominator. For example M/NTSC has a frame period of 1001 /30000 seconds. |
| __u32 | framelines | Total lines per frame including blanking,e. g. 625 for B/PAL. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers must setthe array to zero. |
Table 2. struct v4l2_fract
| __u32 | numerator | |
| __u32 | denominator |
Table 3. typedef v4l2_std_id
| __u64 | v4l2_std_id | This type is a set, each bit representing anothervideo standard as listed below and inTable 4. The 32 most significant bits are reservedfor custom (driver defined) video standards. |
#define V4L2_STD_PAL_B ((v4l2_std_id)0x00000001)
#define V4L2_STD_PAL_B1 ((v4l2_std_id)0x00000002)
#define V4L2_STD_PAL_G ((v4l2_std_id)0x00000004)
#define V4L2_STD_PAL_H ((v4l2_std_id)0x00000008)
#define V4L2_STD_PAL_I ((v4l2_std_id)0x00000010)
#define V4L2_STD_PAL_D ((v4l2_std_id)0x00000020)
#define V4L2_STD_PAL_D1 ((v4l2_std_id)0x00000040)
#define V4L2_STD_PAL_K ((v4l2_std_id)0x00000080) #define V4L2_STD_PAL_M ((v4l2_std_id)0x00000100)
#define V4L2_STD_PAL_N ((v4l2_std_id)0x00000200)
#define V4L2_STD_PAL_Nc ((v4l2_std_id)0x00000400)
#define V4L2_STD_PAL_60 ((v4l2_std_id)0x00000800)
V4L2_STD_PAL_60 isa hybrid standard with 525 lines, 60 Hz refresh rate, and PAL colormodulation with a 4.43 MHz color subcarrier. Some PAL video recorderscan play back NTSC tapes in this mode for display on a 50/60 Hz agnosticPAL TV.
#define V4L2_STD_NTSC_M ((v4l2_std_id)0x00001000)
#define V4L2_STD_NTSC_M_JP ((v4l2_std_id)0x00002000)
#define V4L2_STD_NTSC_443 ((v4l2_std_id)0x00004000)
V4L2_STD_NTSC_443is a hybrid standard with 525 lines, 60 Hz refresh rate, and NTSCcolor modulation with a 4.43 MHz colorsubcarrier.
#define V4L2_STD_NTSC_M_KR ((v4l2_std_id)0x00008000) #define V4L2_STD_SECAM_B ((v4l2_std_id)0x00010000)
#define V4L2_STD_SECAM_D ((v4l2_std_id)0x00020000)
#define V4L2_STD_SECAM_G ((v4l2_std_id)0x00040000)
#define V4L2_STD_SECAM_H ((v4l2_std_id)0x00080000)
#define V4L2_STD_SECAM_K ((v4l2_std_id)0x00100000)
#define V4L2_STD_SECAM_K1 ((v4l2_std_id)0x00200000)
#define V4L2_STD_SECAM_L ((v4l2_std_id)0x00400000)
#define V4L2_STD_SECAM_LC ((v4l2_std_id)0x00800000) /* ATSC/HDTV */
#define V4L2_STD_ATSC_8_VSB ((v4l2_std_id)0x01000000)
#define V4L2_STD_ATSC_16_VSB ((v4l2_std_id)0x02000000)
V4L2_STD_ATSC_8_VSB andV4L2_STD_ATSC_16_VSB are U.S. terrestrial digitalTV standards. Presently the V4L2 API does not support digital TV. Seealso the Linux DVB API athttp://linuxtv.org.
#define V4L2_STD_PAL_BG (V4L2_STD_PAL_B |\
V4L2_STD_PAL_B1 |\
V4L2_STD_PAL_G)
#define V4L2_STD_B (V4L2_STD_PAL_B |\
V4L2_STD_PAL_B1 |\
V4L2_STD_SECAM_B)
#define V4L2_STD_GH (V4L2_STD_PAL_G |\
V4L2_STD_PAL_H |\
V4L2_STD_SECAM_G |\
V4L2_STD_SECAM_H)
#define V4L2_STD_PAL_DK (V4L2_STD_PAL_D |\
V4L2_STD_PAL_D1 |\
V4L2_STD_PAL_K)
#define V4L2_STD_PAL (V4L2_STD_PAL_BG |\
V4L2_STD_PAL_DK |\
V4L2_STD_PAL_H |\
V4L2_STD_PAL_I)
#define V4L2_STD_NTSC (V4L2_STD_NTSC_M |\
V4L2_STD_NTSC_M_JP |\
V4L2_STD_NTSC_M_KR)
#define V4L2_STD_MN (V4L2_STD_PAL_M |\
V4L2_STD_PAL_N |\
V4L2_STD_PAL_Nc |\
V4L2_STD_NTSC)
#define V4L2_STD_SECAM_DK (V4L2_STD_SECAM_D |\
V4L2_STD_SECAM_K |\
V4L2_STD_SECAM_K1)
#define V4L2_STD_DK (V4L2_STD_PAL_DK |\
V4L2_STD_SECAM_DK) #define V4L2_STD_SECAM (V4L2_STD_SECAM_B |\
V4L2_STD_SECAM_G |\
V4L2_STD_SECAM_H |\
V4L2_STD_SECAM_DK |\
V4L2_STD_SECAM_L |\
V4L2_STD_SECAM_LC) #define V4L2_STD_525_60 (V4L2_STD_PAL_M |\
V4L2_STD_PAL_60 |\
V4L2_STD_NTSC |\
V4L2_STD_NTSC_443)
#define V4L2_STD_625_50 (V4L2_STD_PAL |\
V4L2_STD_PAL_N |\
V4L2_STD_PAL_Nc |\
V4L2_STD_SECAM) #define V4L2_STD_UNKNOWN 0
#define V4L2_STD_ALL (V4L2_STD_525_60 |\
V4L2_STD_625_50)
Table 4. Video Standards (based on [])
| Frame lines | 525 | 625 | |||||||||
| Frame period (s) | 1001/30000 | 1/25 | |||||||||
| Chrominance sub-carrier frequency (Hz) | 3579545 ± 10 | 3579611.49 ± 10 | 4433618.75 ± 5 (3582056.25± 5) | 4433618.75 ± 5 | 4433618.75 ± 1 | f =4406250 ± 2000, f = 4250000± 2000 | |||||
| Nominal radio-frequency channel bandwidth(MHz) | 6 | 6 | 6 | B: 7; B1, G: 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 |
| Sound carrier relative to vision carrier(MHz) | + 4.5 | + 4.5 | + 4.5 | + 5.5 ± 0.001 | + 6.5 ± 0.001 | + 5.5 | + 5.9996 ± 0.0005 | + 5.5 ± 0.001 | + 6.5 ± 0.001 | + 6.5 | + 6.5 |
| Notes: a. Japan uses a standardsimilar to M/NTSC(V4L2_STD_NTSC_M_JP). b. The values inbrackets apply to the combination N/PAL a.k.a.N used in Argentina(V4L2_STD_PAL_Nc). c. In the Federal Republic of Germany, Austria, Italy,the Netherlands, Slovakia and Switzerland a system of two soundcarriers is used, the frequency of the second carrier being242.1875 kHz above the frequency of the first sound carrier. Forstereophonic sound transmissions a similar system is used inAustralia. d. New Zealand uses a soundcarrier displaced 5.4996 ± 0.0005 MHz from the visioncarrier. e. In Denmark, Finland, NewZealand, Sweden and Spain a system of two sound carriers is used. InIceland, Norway and Poland the same system is being introduced. Thesecond carrier is 5.85 MHz above the vision carrier and is DQPSKmodulated with 728 kbit/s sound and data multiplex. (NICAMsystem) f. In the United Kingdom, asystem of two sound carriers is used. The second sound carrier is6.552 MHz above the vision carrier and is DQPSK modulated with a728 kbit/s sound and data multiplex able to carry two soundchannels. (NICAM system) g. In France, adigital carrier 5.85 MHz away from the vision carrier may be used inaddition to the main sound carrier. It is modulated in differentiallyencoded QPSK with a 728 kbit/s sound and data multiplexer capable ofcarrying two sound channels. (NICAMsystem) |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_standard
indexis out of bounds.
ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO
Name
VIDIOC_G_AUDIO, VIDIOC_S_AUDIO -- Query or select the current audio input and itsattributes
Synopsis
int ioctl(int fd, int request, struct v4l2_audio *argp);
int ioctl(int fd, int request, const struct v4l2_audio *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_AUDIO, VIDIOC_S_AUDIO
argp
Description
To query the current audio input applications zero out thereserved array of a struct v4l2_audioand call theVIDIOC_G_AUDIO
ioctl with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL error code when the device has no audio inputs, or none which combinewith the current video input.
Audio inputs have one writable property, the audio mode. Toselect the current audio inputand change theaudio mode, applications initialize theindex andmodefields,
and thereserved array of av4l2_audio structure and call theVIDIOC_S_AUDIO ioctl. Drivers may switch to adifferent audio mode if the request cannot be satisfied.
However, thisis a write-only ioctl, it does not return the actual new audiomode.
Table 1. struct v4l2_audio
| __u32 | index | Identifies the audio input, set by thedriver or application. |
| __u8 | name[32] | Name of the audio input, a NUL-terminated ASCIIstring, for example: "Line In". This information is intended for theuser, preferably the connector label on the device itself. |
| __u32 | capability | Audio capability flags, see Table 2. |
| __u32 | mode | Audio mode flags set by drivers and applications (on VIDIOC_S_AUDIO ioctl), seeTable 3. |
| __u32 | reserved[2] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
Table 2. Audio Capability Flags
V4L2_AUDCAP_STEREO | 0x00001 | This is a stereo input. The flag is intended toautomatically disable stereo recording etc. when the signal is alwaysmonaural. The API provides no means to detect if stereo isreceived, unless the audio input belongs to atuner. |
V4L2_AUDCAP_AVL | 0x00002 | Automatic Volume Level mode is supported. |
Table 3. Audio Mode Flags
V4L2_AUDMODE_AVL | 0x00001 | AVL mode is on. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
No audio inputs combine with the current video input,or the number of
the selected audio input is out of bounds or it doesnot combine, or
there are no audio inputs at all and the ioctl is notsupported.- EBUSY
I/O is in progress, the input cannot beswitched.
ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT
Name
VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT -- Query or select the current audio output
Synopsis
int ioctl(int fd, int request, struct v4l2_audioout *argp);
int ioctl(int fd, int request, const struct v4l2_audioout *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT
argp
Description
To query the current audio output applications zero out thereserved array of a struct v4l2_audioout andcall theVIDIOC_G_AUDOUT
ioctl with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL error code when the device has no audio inputs, or none which combinewith the current video output.
Audio outputs have no writable properties. Nevertheless, toselect the current audio output applications can initialize theindex field andreserved array (which in the future maycontain writable
properties) of av4l2_audioout structure and call theVIDIOC_S_AUDOUT ioctl. Drivers switch to therequested output or return theEINVAL error code when the index is out
ofbounds. This is a write-only ioctl, it does not return the currentaudio output attributes asVIDIOC_G_AUDOUTdoes.
Note connectors on a TV card to loop back the received audiosignal to a sound card are not audio outputs in this sense.
Table 1. struct v4l2_audioout
| __u32 | index | Identifies the audio output, set by thedriver or application. |
| __u8 | name[32] | Name of the audio output, a NUL-terminated ASCIIstring, for example: "Line Out". This information is intended for theuser, preferably the connector label on the device itself. |
| __u32 | capability | Audio capability flags, none defined yet. Driversmust set this field to zero. |
| __u32 | mode | Audio mode, none defined yet. Drivers andapplications (on |
| __u32 | reserved[2] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
No audio outputs combine with the current videooutput, or the number
of the selected audio output is out of bounds orit does not combine, or
there are no audio outputs at all and theioctl is not supported.- EBUSY
I/O is in progress, the output cannot beswitched.
ioctl VIDIOC_G_CHIP_IDENT
Name
VIDIOC_G_CHIP_IDENT -- Identify the chips on a TV card
Synopsis
int ioctl(int fd, int request, struct v4l2_chip_ident*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_CHIP_IDENT
argp
Description
For driver debugging purposes this ioctl allows testapplications to
query the driver about the chips present on the TVcard. Regular
applications should not use it. When you found a chipspecific bug,
please contact the Video4Linux mailing list (https://listman.redhat.com/mailman/listinfo/video4linux-list)so
it can be fixed.
To query the driver applications must initialize thematch_type andmatch_chip fields of a struct v4l2_chip_identand
call VIDIOC_G_CHIP_IDENT with a pointer tothis structure. On success the driver stores information about theselected chip in theident andrevision fields. On failure
the structureremains unchanged.
When match_type isV4L2_CHIP_MATCH_HOST,match_chip selects the nth non-IC chipon the TV card. You can enumerate all chips by starting at zero andincrementingmatch_chip
by one untilVIDIOC_G_CHIP_IDENT fails with anEINVAL error code.Drivers may also interpretmatch_chip as arandom ID, but we recommend against that. The number zero
alwaysselects the host chip, e. g. the chip connected to the PCI bus.
When match_type isV4L2_CHIP_MATCH_I2C_DRIVER,match_chip contains a driver ID as definedin thelinux/i2c-id.h header file. For instanceI2C_DRIVERID_SAA7127
will match any chipsupported by the saa7127 driver, regardless of its IC bus address.When multiple chips supported by the same driver are present, theioctl will returnV4L2_IDENT_AMBIGUOUS in theident
field.
When match_type isV4L2_CHIP_MATCH_I2C_ADDR,match_chip selects a chip by its 7 bitIC bus address.
On success, the ident field willcontain a chip ID from the Linuxmedia/v4l2-chip-ident.h header file, and therevision field will contain a driverspecific value, or
zero if no particular revision is associated withthis chip.
When the driver could not identify the selected chip,ident will containV4L2_IDENT_UNKNOWN. When no chip matchedmatch_type andmatch_chip,
the ioctl will succeed but theident field will containV4L2_IDENT_NONE. If multiple chips matched,ident will containV4L2_IDENT_AMBIGUOUS.
In all these cases therevision field remains unchanged.
This ioctl is optional, not all drivers may support it. Itwas introduced in Linux 2.6.21.
We recommended the v4l2-dbgutility over calling this ioctl directly. It is available from theLinuxTV v4l-dvb repository; seehttp://linuxtv.org/repo/ foraccess instructions.
Table 1. struct v4l2_chip_ident
| __u32 | match_type | See Table 2 for a list ofpossible types. |
| __u32 | match_chip | Match a chip by this number, interpreted accordingto the |
| __u32 | ident | A chip identifier as defined in the Linuxmedia/v4l2-chip-ident.h header file, or one ofthe values fromTable 3. |
| __u32 | revision | A chip revision, chip and driver specific. |
Table 2. Chip Match Types
V4L2_CHIP_MATCH_HOST | 0 | Match the nth chip on the card, zero for the host chip. Does not match IC chips. |
V4L2_CHIP_MATCH_I2C_DRIVER | 1 | Match an IC chip by its driver ID from thelinux/i2c-id.h header file. |
V4L2_CHIP_MATCH_I2C_ADDR | 2 | Match a chip by its 7 bit IC bus address. |
Table 3. Chip Identifiers
V4L2_IDENT_NONE | 0 | No chip matched. |
V4L2_IDENT_AMBIGUOUS | 1 | Multiple chips matched. |
V4L2_IDENT_UNKNOWN | 2 | A chip is present at this address, but the drivercould not identify it. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The driver does not support this ioctl, or the
match_typeis invalid.
ioctl VIDIOC_G_CROP, VIDIOC_S_CROP
Name
VIDIOC_G_CROP, VIDIOC_S_CROP -- Get or set the current cropping rectangle
Synopsis
int ioctl(int fd, int request, struct v4l2_crop *argp);
int ioctl(int fd, int request, const struct v4l2_crop *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_CROP, VIDIOC_S_CROP
argp
Description
To query the cropping rectangle size and positionapplications set the
typev4l2_crop structure to the respective buffer(stream) type and call theVIDIOC_G_CROP ioctlwith a pointer to this structure. The driver fills the rest of thestructure or returns
theEINVAL error code if cropping is not supported.
To change the cropping rectangle applications initialize thetype and struct v4l2_rect substructure namedc of a
v4l2_crop structure and call theVIDIOC_S_CROP ioctl with a pointer to thisstructure.
The driver first adjusts the requested dimensions againsthardware
limits, i. e. the bounds given by the capture/output window,and it
rounds to the closest possible values of horizontal andvertical offset,
width and height. In particular the driver must roundthe
vertical offset of the cropping rectangle to frame lines modulotwo,
such that the field order cannot be confused.
Second the driver adjusts the image size (the oppositerectangle of
the scaling process, source or target depending on thedata direction) to
the closest size possible while maintaining thecurrent horizontal and
vertical scaling factor.
Finally the driver programs the hardware with the actualcropping and image parameters.VIDIOC_S_CROP is awrite-only ioctl, it does not return the actual parameters. To querythem applications must callVIDIOC_G_CROP
andVIDIOC_G_FMT.
When the parameters are unsuitable the application maymodify the
cropping or image parameters and repeat the cycle untilsatisfactory
parameters have been negotiated.
When cropping is not supported then no parameters arechanged and
VIDIOC_S_CROP
Table 1. struct v4l2_crop
| enum v4l2_buf_type | type | Type of the data stream, set by the application.Only these types are valid here:V4L2_BUF_TYPE_VIDEO_CAPTURE,V4L2_BUF_TYPE_VIDEO_OUTPUT,V4L2_BUF_TYPE_VIDEO_OVERLAY,and custom (driverdefined) types with code V4L2_BUF_TYPE_PRIVATEand higher. |
| struct v4l2_rect | c | Cropping rectangle. The same co-ordinate system asfor struct v4l2_cropcapbounds is used. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
Cropping is not supported.
ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL
Name
VIDIOC_G_CTRL, VIDIOC_S_CTRL -- Get or set the value of a control
Synopsis
int ioctl(int fd, int request, struct v4l2_control*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_CTRL, VIDIOC_S_CTRL
argp
Description
To get the current value of a control applicationsinitialize the
idv4l2_control and call theVIDIOC_G_CTRL ioctl with a pointer to thisstructure. To change the value of a control applications initializetheid
and valuefields of a structv4l2_control and call theVIDIOC_S_CTRL ioctl.
When the id is invalid driversreturn an
EINVAL error code. When the value is outof bounds drivers can choose to take the closest valid value or returnanERANGE error code, whatever seems more appropriate. However,VIDIOC_S_CTRL
is a write-only ioctl, it does notreturn the actual new value.
These ioctls work only with user controls. For othercontrol classes the VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS orVIDIOC_TRY_EXT_CTRLS must be used.
Table 1. struct v4l2_control
| __u32 | id | Identifies the control, set by theapplication. |
| __s32 | value | New value or current value. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_control
idisinvalid.- ERANGE
The struct v4l2_control
valueis out of bounds.- EBUSY
The control is temporarily not changeable, possiblybecause another
applications took over control of the device functionthis control
belongs to.
ioctl VIDIOC_G_ENC_INDEX
Name
VIDIOC_G_ENC_INDEX -- Get meta data about a compressed video stream
Synopsis
int ioctl(int fd, int request, struct v4l2_enc_idx *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_ENC_INDEX
argp
Description
The VIDIOC_G_ENC_INDEX ioctl
providesmeta data about a compressed video stream the same or
anotherapplication currently reads from the driver, which is useful
forrandom access into the stream without decoding it.
To read the data applications must callVIDIOC_G_ENC_INDEX with a pointer to astruct v4l2_enc_idx. On success the driver fills theentry
array, stores the number of elementswritten in the entries field, andinitializes theentries_cap field.
Each element of the entry arraycontains meta data about one picture. AVIDIOC_G_ENC_INDEX call reads up toV4L2_ENC_IDX_ENTRIES entries from a driverbuffer, which
can hold up to entries_capentries. This number can be lower or higher thanV4L2_ENC_IDX_ENTRIES, but not zero. When theapplication fails to read the meta data in time the oldest entrieswill be lost.
When the buffer is empty or no capturing/encoding is inprogress,
entries
Currently this ioctl is only defined for MPEG-2 programstreams and video elementary streams.
Table 1. struct v4l2_enc_idx
| __u32 | entries | The number of entries the driver stored in theentry array. |
| __u32 | entries_cap | The number of entries the driver canbuffer. Must be greater than zero. |
| __u32 | reserved[4] | Reserved for future extensions.Drivers must set the array to zero. |
| struct v4l2_enc_idx_entry | entry[V4L2_ENC_IDX_ENTRIES] | Meta data about a compressed video stream. Eachelement of the array corresponds to one picture, sorted in ascendingorder by their offset. |
Table 2. struct v4l2_enc_idx_entry
| __u64 | offset | The offset in bytes from the beginning of thecompressed video stream to the beginning of this picture, that is aPES packet header as defined in or apictureheader as defined in . Whenthe encoder is stopped, the driver resets the offset to zero. |
| __u64 | pts | The 33 bit Presentation TimeStamp of this picture as defined in. |
| __u32 | length | The length of this picture in bytes. |
| __u32 | flags | Flags containing the coding type of this picture, see Table 3. |
| __u32 | reserved[2] | Reserved for future extensions.Drivers must set the array to zero. |
Table 3. Index Entry Flags
V4L2_ENC_IDX_FRAME_I | 0x00 | This is an Intra-coded picture. |
V4L2_ENC_IDX_FRAME_P | 0x01 | This is a Predictive-coded picture. |
V4L2_ENC_IDX_FRAME_B | 0x02 | This is a Bidirectionally predictive-codedpicture. |
V4L2_ENC_IDX_FRAME_MASK | 0x0F | AND the flags field withthis mask to obtain the picture coding type. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The driver does not support this ioctl.
ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,VIDIOC_TRY_EXT_CTRLS
Name
VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS -- Get or set the value of several controls, try controlvalues
Synopsis
int ioctl(int fd, int request, struct v4l2_ext_controls*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS,VIDIOC_TRY_EXT_CTRLS
argp
Description
These ioctls allow the caller to get or set multiplecontrols atomically. Control IDs are grouped into control classes (seeTable 3) and all controls in the control arraymust belong
to the same control class.
Applications must always fill in thecount,ctrl_class,controls andreserved fields of struct v4l2_ext_controls,
andinitialize the struct v4l2_ext_control array pointed to by thecontrols fields.
To get the current value of a set of controls applicationsinitialize the
idVIDIOC_G_EXT_CTRLS ioctl.
To change the value of a set of controls applicationsinitialize the
idvalue fields of a struct v4l2_ext_control andcall theVIDIOC_S_EXT_CTRLS ioctl. The controlswill
only be set ifall control values arevalid.
To check if the a set of controls have correct valuesapplications initialize theid andvalue fields of a struct v4l2_ext_control
andcall theVIDIOC_TRY_EXT_CTRLS ioctl. It is up tothe driver whether wrong values are automatically adjusted to a validvalue or if an error is returned.
When the id orctrl_class is invalid drivers return anEINVAL error code. When the value is out of bounds drivers can choose to takethe closest valid value or
return an ERANGE error code, whatever seems moreappropriate. In the first case the new value is set instruct v4l2_ext_control.
The driver will only set/get these controls if all controlvalues are
correct. This prevents the situation where only some of thecontrols were
set/get. Only low-level errors (e. g. a failed i2ccommand) can still
cause this situation.
Table 1. struct v4l2_ext_control
| __u32 | id | Identifies the control, set by theapplication. | |
| __u32 | reserved2[2] | Reserved for future extensions. Drivers andapplications must set the array to zero. | |
| union | (anonymous) | ||
| __s32 | value | New value or current value. | |
| __s64 | value64 | New value or current value. | |
| void * | reserved | Reserved for future pointer-type controls. Currently unused. |
Table 2. struct v4l2_ext_controls
| __u32 | ctrl_class | The control class to which all controls belong, seeTable 3. |
| __u32 | count | The number of controls in the controls array. Mayalso be zero. |
| __u32 | error_idx | Set by the driver in case of an error. It is theindex of the control causing the error or equal to 'count' when theerror is not associated with a particular control. Undefined when theioctl returns 0 (success). |
| __u32 | reserved[2] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
| struct v4l2_ext_control * | controls | Pointer to an array ofcount v4l2_ext_control structures. Ignoredifcount equals zero. |
Table 3. Control classes
V4L2_CTRL_CLASS_USER | 0x980000 | The class containing user controls. These controlsare described in Section 1.8. All controls that can be setusing the VIDIOC_S_CTRL and VIDIOC_G_CTRL ioctl belong to thisclass. |
V4L2_CTRL_CLASS_MPEG | 0x990000 | The class containing MPEG compression controls.These controls are described in sectionSection 1.9.5. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_ext_control
idis invalid or the struct v4l2_ext_controlsctrl_class
is invalid. This error code isalso returned by theVIDIOC_S_EXT_CTRLSandVIDIOC_TRY_EXT_CTRLSioctls if two or morecontrol values are in conflict.- ERANGE
The struct v4l2_ext_control
valueis out of bounds.- EBUSY
The control is temporarily not changeable, possiblybecause another
applications took over control of the device functionthis control
belongs to.
ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF
Name
VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters
Synopsis
int ioctl(int fd, int request, struct v4l2_framebuffer *argp);
int ioctl(int fd, int request, const struct v4l2_framebuffer *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_FBUF, VIDIOC_S_FBUF
argp
Description
Applications can use the VIDIOC_G_FBUF andVIDIOC_S_FBUF ioctl to get and set theframebuffer parameters for aVideoOverlay
orVideo Output Overlay(OSD). The type of overlay is implied by the device type (capture oroutput device) and can be determined with theVIDIOC_QUERYCAP
ioctl.One/dev/videoN device must not support bothkinds of overlay.
The V4L2 API distinguishes destructive and non-destructiveoverlays. A
destructive overlay copies captured video images into thevideo memory
of a graphics card. A non-destructive overlay blendsvideo images into a
VGA signal or graphics into a video signal.Video
Output Overlays are alwaysnon-destructive.
To get the current parameters applications call theVIDIOC_G_FBUF ioctl with a pointer to av4l2_framebuffer structure. The driver fillsall fields of the structure or returns anEINVAL
error code when overlays arenot supported.
To set the parameters for a Video OutputOverlay, applications must initialize theflags field of a structv4l2_framebuffer. Since the framebuffer isimplemented on
the TV card all other parameters are determined by thedriver. When an application callsVIDIOC_S_FBUFwith a pointer to this structure, the driver prepares for the overlayand returns the framebuffer parameters asVIDIOC_G_FBUF
does, or it returns an errorcode.
To set the parameters for a non-destructiveVideo Overlay, applications must initialize theflags field, thefmt substructure, and callVIDIOC_S_FBUF.
Again the driver prepares for theoverlay and returns the framebuffer parameters asVIDIOC_G_FBUF does, or it returns an errorcode.
For a destructive Video Overlayapplications must additionally provide abase address. Setting up a DMA to arandom memory location can jeopardize the system security, itsstability or even damage
the hardware, therefore only the superusercan set the parameters for a destructive video overlay.
Table 1. struct v4l2_framebuffer
| __u32 | capability | Overlay capability flags set by the driver, seeTable 2. | |
| __u32 | flags | Overlay control flags set by application anddriver, see Table 3 | |
| void * | base | Physical base address of the framebuffer,that is the address of the pixel in the top left corner of theframebuffer. This field is irrelevant tonon-destructive Video Overlays. Fordestructive Video Overlays applications mustprovide a base address. The driver may accept only base addresseswhich are a multiple of two, | |
| struct v4l2_pix_format | fmt | Layout of the frame buffer. Thev4l2_pix_format structure is defined inChapter 2, for clarification the fields and acceptable values are listed below: | |
| __u32 | width | Width of the frame buffer in pixels. | |
| __u32 | height | Height of the frame buffer in pixels. | |
| __u32 | pixelformat | The pixel format of theframebuffer. For non-destructive VideoOverlays this field only defines a format for thestruct v4l2_window For destructive VideoOverlays applications must initialize this field. ForVideo Output Overlays the driver must returna valid format. Usually this is an RGB format (for example | |
| enum v4l2_field | field | Drivers and applications shall ignore this field.If applicable, the field order is selected with theVIDIOC_S_FMTioctl, using thefieldfield ofstruct v4l2_window. | |
| __u32 | bytesperline | Distance in bytes between the leftmost pixels intwo adjacent lines. | |
This field is irrelevant tonon-destructive VideoOverlays. For destructive VideoOverlays both For Video OutputOverlays the driver must return a validvalue. Video hardware may access padding bytes, thereforethey must reside in When the image format is planar the | |||
| __u32 | sizeimage | This field is irrelevant tonon-destructive Video Overlays. Fordestructive Video Overlays applications mustinitialize this field. ForVideo OutputOverlays the driver must return Together with | |
| enum v4l2_colorspace | colorspace | This information supplements thepixelformat and must be set by the driver,seeSection 2.2. | |
| __u32 | priv | Reserved for additional information about custom(driver defined) formats. When not used drivers and applications mustset this field to zero. | |
| Notes: a. A physical base address may not suit allplatforms. GK notes in theory we should pass something like PCI device+ memory region + offset instead. If you encounter problems pleasediscuss on the Video4Linux mailing list:https://listman.redhat.com/mailman/listinfo/video4linux-list. |
Table 2. Frame Buffer Capability Flags
V4L2_FBUF_CAP_EXTERNOVERLAY | 0x0001 | The device is capable of non-destructive overlays.When the driver clears this flag, only destructive overlays aresupported. There are no drivers yet which support both destructive andnon-destructive overlays. |
V4L2_FBUF_CAP_CHROMAKEY | 0x0002 | The device supports clipping by chroma-keying theimages. That is, image pixels replace pixels in the VGA or videosignal only where the latter assume a certain color. Chroma-keyingmakes no sense for destructive overlays. |
V4L2_FBUF_CAP_LIST_CLIPPING | 0x0004 | The device supports clipping using a list of cliprectangles. |
V4L2_FBUF_CAP_BITMAP_CLIPPING | 0x0008 | The device supports clipping using a bit mask. |
V4L2_FBUF_CAP_LOCAL_ALPHA | 0x0010 | The device supports clipping/blending using thealpha channel of the framebuffer or VGA signal. Alpha blending makesno sense for destructive overlays. |
V4L2_FBUF_CAP_GLOBAL_ALPHA | 0x0020 | The device supports alpha blending using a globalalpha value. Alpha blending makes no sense for destructive overlays. |
V4L2_FBUF_CAP_LOCAL_INV_ALPHA | 0x0040 | The device supports clipping/blending using theinverted alpha channel of the framebuffer or VGA signal. Alphablending makes no sense for destructive overlays. |
Table 3. Frame Buffer Flags
V4L2_FBUF_FLAG_PRIMARY | 0x0001 | The framebuffer is the primary graphics surface.In other words, the overlay is destructive. [?] |
V4L2_FBUF_FLAG_OVERLAY | 0x0002 | The frame buffer is an overlay surface the samesize as the capture. [?] |
The purpose ofV4L2_FBUF_FLAG_PRIMARY andV4L2_FBUF_FLAG_OVERLAY was never quite clear.Most drivers seem to ignore these flags. For compatibility with thebttvdriver applications should set the V4L2_FBUF_FLAG_OVERLAY flag. | ||
V4L2_FBUF_FLAG_CHROMAKEY | 0x0004 | Use chroma-keying. The chroma-key color isdetermined by the VIDIOC_S_FMTioctl, seeSection 4.2andSection 4.4. |
There are no flags to enableclipping using a list of clip rectangles or a bitmap. These methodsare negotiated with theVIDIOC_S_FMTioctl, seeSection 4.2 andSection 4.4. | ||
V4L2_FBUF_FLAG_LOCAL_ALPHA | 0x0008 | Use the alpha channel of the framebuffer to clip orblend framebuffer pixels with video images. The blendfunction is: output = framebuffer pixel * alpha + video pixel * (1 -alpha). The actual alpha depth depends on the framebuffer pixelformat. |
V4L2_FBUF_FLAG_GLOBAL_ALPHA | 0x0010 | Use a global alpha value to blend the framebufferwith video images. The blend function is: output = (framebuffer pixel* alpha + video pixel * (255 - alpha)) / 255. The alpha value isdetermined by the global_alpha field ofstruct v4l2_windowand negotiated with the VIDIOC_S_FMT ioctl, seeSection 4.2andSection4.4. |
V4L2_FBUF_FLAG_LOCAL_INV_ALPHA | 0x0020 | LikeV4L2_FBUF_FLAG_LOCAL_ALPHA, usethe alpha channelof the framebuffer to clip or blend framebuffer pixels with videoimages, but with an inverted alpha value. The blend function is:output = framebuffer pixel * (1 - alpha) + video pixel * alpha. Theactual alpha depth depends on the framebuffer pixel format. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EPERM
VIDIOC_S_FBUFcan only be calledby a privileged user to negotiate the parameters for a destructiveoverlay.- EBUSY
The framebuffer parameters cannot be changed at thistime because
overlay is already enabled, or capturing is enabledand the hardware
cannot capture and overlay simultaneously.- EINVAL
The ioctl is not supported or the
VIDIOC_S_FBUFparameters are unsuitable.
ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,VIDIOC_TRY_FMT
Name
VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT -- Get or set the data format, try a format
Synopsis
int ioctl(int fd, int request, struct v4l2_format*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT
argp
Description
These ioctls are used to negotiate the format of data(typically image format) exchanged between driver andapplication.
To query the current parameters applications set thetype field of a structv4l2_format to the respective buffer (stream)type. For example video capture devices useV4L2_BUF_TYPE_VIDEO_CAPTURE.
When the applicationcalls the VIDIOC_G_FMT ioctl with a pointer tothis structure the driver fills the respective member of thefmt union. In case of video capture devicesthat is the struct v4l2_pix_formatpix
member.When the requested buffer type is not supported drivers return anEINVAL error code.
To change the current format parameters applicationsinitialize the
typefmtunion member. For details see the documentation of the various devicestypes inChapter 4.
Good practice
is to query thecurrent parameters first, and tomodify only those
parameters not suitable for the application. Whenthe application calls
theVIDIOC_S_FMT ioctlwith a pointer to av4l2_format
structurethe
driver checksand adjusts the parameters against hardware abilities.
Driversshould not return an error code unless the input is ambiguous,
this isa mechanism to fathom device capabilities and to approach
parametersacceptable for both the application and driver.
On success the drivermay program the hardware, allocate resources and
generally prepare fordata exchange.Finally theVIDIOC_S_FMT ioctl returns thecurrent format parameters asVIDIOC_G_FMT
does.Very
simple, inflexible devices may even ignore all input and alwaysreturn
the default parameters. However all V4L2 devices exchangingdata with the
application must implement theVIDIOC_G_FMT andVIDIOC_S_FMT
ioctl. When the requested buffertype is not supported drivers return anEINVAL error code on aVIDIOC_S_FMT attempt. When I/O is already inprogress or the resource is not available for other reasons
driversreturn theEBUSY error code.
The VIDIOC_TRY_FMT ioctl is equivalentto
VIDIOC_S_FMT
This function is provided tonegotiate parameters, to learn about
hardware limitations, withoutdisabling
I/O or possibly time consuming hardware preparations.Although strongly
recommended drivers are not required to implementthis ioctl.
Table 1. struct v4l2_format
| enum v4l2_buf_type | type | Type of the data stream, see Table 3-2. | |
| union | fmt | ||
| struct v4l2_pix_format | pix | Definition of an image format, see Chapter 2, used by video capture and outputdevices. | |
| struct v4l2_window | win | Definition of an overlaid image, see Section 4.2, used by video overlay devices. | |
| struct v4l2_vbi_format | vbi | Raw VBI capture or output parameters. This isdiscussed in more detail in Section 4.7. Used by raw VBIcapture and output devices. | |
| struct v4l2_sliced_vbi_format | sliced | Sliced VBI capture or output parameters. SeeSection 4.8 for details. Used by sliced VBIcapture and output devices. | |
| __u8 | raw_data[200] | Place holder for future extensions and custom(driver defined) formats with V4L2_BUF_TYPE_PRIVATE and higher. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EBUSY
The data format cannot be changed at thistime, for example because I/O is already in progress.
- EINVAL
The struct v4l2_format
typefield is invalid, the requested buffer type not supported, orVIDIOC_TRY_FMTwas called
and is notsupported with this buffer type.
ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY
Name
VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY -- Get or set tuner or modulator radiofrequency
Synopsis
int ioctl(int fd, int request, struct v4l2_frequency*argp);
int ioctl(int fd, int request, const struct v4l2_frequency*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY
argp
Description
To get the current tuner or modulator radio frequencyapplications set the
tunerreserved
array andcall the VIDIOC_G_FREQUENCY ioctl with a pointerto this structure. The driver stores the current frequency in thefrequency field.
To change the current tuner or modulator radio frequencyapplications initialize thetuner,type andfrequency fields, and thereserved
array of a struct v4l2_frequency andcall theVIDIOC_S_FREQUENCY ioctl with a pointerto this structure. When the requested frequency is not possible
thedriver assumes the closest possible value. HoweverVIDIOC_S_FREQUENCY is a write-only ioctl, it doesnot return the actual new frequency.
Table 1. struct v4l2_frequency
| __u32 | tuner | The tuner or modulator index number. This is thesame value as in the struct v4l2_inputtunerfield and the struct v4l2_tunerindexfield, orthe struct v4l2_output modulator field and thestruct v4l2_modulatorindexfield. |
| enum v4l2_tuner_type | type | The tuner type. This is the same value as in thestruct v4l2_tunertype field. The field is notapplicable to modulators, i. e. ignored by drivers. |
| __u32 | frequency | Tuning frequency in units of 62.5 kHz, or if thestruct v4l2_tuner or struct v4l2_modulatorcapabilitiesflag V4L2_TUNER_CAP_LOW is set, in units of 62.5Hz. |
| __u32 | reserved[8]; | Reserved for future extensions. Drivers and applications must set the array to zero. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The
tunerindex is out ofbounds or the value in thetypefield iswrong.
ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT
Name
VIDIOC_G_INPUT, VIDIOC_S_INPUT -- Query or select the current video input
Synopsis
int ioctl(int fd, int request, int *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_INPUT, VIDIOC_S_INPUT
argp
Description
To query the current video input applications call theVIDIOC_G_INPUT ioctl with a pointer to an integerwhere the driver stores the number of the input, as in thestruct v4l2_inputindex
field. This ioctl willfail only when there are no video inputs, returningEINVAL.
To select a video input applications store the number of thedesired input in an integer and call theVIDIOC_S_INPUT
ioctl with a pointer to thisinteger. Side effects are possible. For
example inputs may supportdifferent video
standards, so the driver may implicitly switch thecurrent standard. It
is good practice to select an input beforequerying or negotiating any
other parameters.
Information about video inputs is available using theVIDIOC_ENUMINPUT ioctl.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The number of the video input is out of bounds, orthere are no video inputs at all and this ioctl is notsupported.
- EBUSY
I/O is in progress, the input cannot beswitched.
ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
Name
VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP --
Synopsis
int ioctl(int fd, int request, v4l2_jpegcompression *argp);
int ioctl(int fd, int request, const v4l2_jpegcompression *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP
argp
Description
[to do]
Ronald Bultje elaborates:
APP is some application-specific information. Theapplication can set
it itself, and it'll be stored in the JPEG-encodedfields (eg;
interlacing information for in an AVI or so). COM is thesame, but it's
comments, like 'encoded by me' or so.
jpeg_markers describes whether the huffman tables,quantization tables
and the restart interval information (allJPEG-specific stuff) should be
stored in the JPEG-encoded fields.These define how the JPEG field is
encoded. If you omit them,applications assume
you've used standard encoding. You usually do wantto add them.
Table 1. struct v4l2_jpegcompression
| int | quality | |
| int | APPn | |
| int | APP_len | |
| char | APP_data[60] | |
| int | COM_len | |
| char | COM_data[60] | |
| __u32 | jpeg_markers | See Table 2. |
Table 2. JPEG Markers Flags
V4L2_JPEG_MARKER_DHT | (1<<3) | Define Huffman Tables |
V4L2_JPEG_MARKER_DQT | (1<<4) | Define Quantization Tables |
V4L2_JPEG_MARKER_DRI | (1<<5) | Define Restart Interval |
V4L2_JPEG_MARKER_COM | (1<<6) | Comment segment |
V4L2_JPEG_MARKER_APP | (1<<7) | App segment, driver will always use APP0 |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
This ioctl is not supported.
ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR
Name
VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR -- Get or set modulator attributes
Synopsis
int ioctl(int fd, int request, struct v4l2_modulator*argp);
int ioctl(int fd, int request, const struct v4l2_modulator*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR
argp
Description
To query the attributes of a modulator applications initializethe
indexreserved array of a struct v4l2_modulator andcall theVIDIOC_G_MODULATOR ioctl
with a pointerto this structure. Drivers fill the rest of the structure or return anEINVAL
error code when the index is out of bounds. To enumerate all
modulatorsapplications shall begin at index zero, incrementing by one
until
thedriver returns EINVAL.
Modulators have two writable properties, an audiomodulation set and
the radio frequency. To change the modulated audiosubprograms,
applications initialize theindex andtxsubchans fields and
thereserved array and call theVIDIOC_S_MODULATOR
ioctl. Drivers may choose adifferent audio modulation if the request
cannot be satisfied. Howeverthis is a write-only ioctl, it does not
return
the actual audiomodulation selected.
To change the radio frequency the VIDIOC_S_FREQUENCY ioctlis available.
Table 1. struct v4l2_modulator
| __u32 | index | Identifies the modulator, set by theapplication. |
| __u8 | name[32] | Name of the modulator, a NUL-terminated ASCIIstring. This information is intended for the user. |
| __u32 | capability | Modulator capability flags. No flags are definedfor this field, the tuner flags in struct v4l2_tunerare used accordingly. The audio flags indicate the abilityto encode audio subprograms. They will notchange for example with the current video standard. |
| __u32 | rangelow | The lowest tunable frequency in units of 62.5KHz, or if the V4L2_TUNER_CAP_LOW is set, in units of 62.5Hz. |
| __u32 | rangehigh | The highest tunable frequency in units of 62.5KHz, or if the V4L2_TUNER_CAP_LOW is set, in units of 62.5Hz. |
| __u32 | txsubchans | With this field applications can determine howaudio sub-carriers shall be modulated. It contains a set of flags asdefined inTable 2. Note the tunerrxsubchansflags are reused, but thesemantics are different. Video output devices are assumed to have ananalog or PCM audio input with 1-3 channels. The txsubchans flags select one or morechannels for modulation, together with some audiosubprogramindicator, for example a stereo pilot tone. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
Table 2. Modulator Audio Transmission Flags
V4L2_TUNER_SUB_MONO | 0x0001 | Modulate channel 1 as mono audio, when the inputhas more channels, a down-mix of channel 1 and 2. This flag does notcombine withV4L2_TUNER_SUB_STEREO orV4L2_TUNER_SUB_LANG1. |
V4L2_TUNER_SUB_STEREO | 0x0002 | Modulate channel 1 and 2 as left and rightchannel of a stereo audio signal. When the input has only one channelor two channels and V4L2_TUNER_SUB_SAP is alsoset, channel 1 is encoded as left and right channel. This flag doesnotcombine with V4L2_TUNER_SUB_MONO orV4L2_TUNER_SUB_LANG1. When the driver does notsupport stereo audio it shall fall back to mono. |
V4L2_TUNER_SUB_LANG1 | 0x0008 | Modulate channel 1 and 2 as primary and secondarylanguage of a bilingual audio signal. When the input has only onechannel it is used for both languages. It is not possible to encodethe primary or secondary language only. This flag does not combinewith V4L2_TUNER_SUB_MONOorV4L2_TUNER_SUB_STEREO.If the hardware does notsupport the respective audio matrix, or the current video standarddoes not permit bilingual audio the VIDIOC_S_MODULATOR ioctl shall return anEINVAL error codeand the drivershall fall back to mono or stereo mode. |
V4L2_TUNER_SUB_LANG2 | 0x0004 | Same effect asV4L2_TUNER_SUB_LANG1. |
V4L2_TUNER_SUB_SAP | 0x0004 | When combined with V4L2_TUNER_SUB_MONOthe first channel is encoded as mono audio, the lastchannel as Second Audio Program. When the input has only one channelit is used for both audio tracks. When the input has three channelsthe mono track is a down-mix of channel 1 and 2. When combined with V4L2_TUNER_SUB_STEREOchannel1 and 2 areencoded as left and right stereo audio, channel 3 as Second AudioProgram. When the input has only two channels, the first is encoded asleft and right channel and the second as SAP. When the input has onlyone channel it is used for all audio tracks. It is not possible toencode a Second Audio Program only. This flag must combine with V4L2_TUNER_SUB_MONOor V4L2_TUNER_SUB_STEREO. If the hardware does notsupport the respective audio matrix, or the current video standarddoes not permit SAP theVIDIOC_S_MODULATOR ioctlshall return anEINVALerror code and driver shall fall back to mono or stereomode. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_modulator
indexis out of bounds.
ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT
Name
VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT -- Query or select the current video output
Synopsis
int ioctl(int fd, int request, int *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT
argp
Description
To query the current video output applications call theVIDIOC_G_OUTPUT ioctl with a pointer to an integerwhere the driver stores the number of the output, as in thestruct v4l2_outputindex
field. This ioctlwill fail only when there are no video outputs, returning theEINVAL error code.
To select a video output applications store the number of thedesired output in an integer and call theVIDIOC_S_OUTPUT
ioctl with a pointer to this integer.Side effects are possible. For
example outputs may support differentvideo
standards, so the driver may implicitly switch the currentstandard. It
is good practice to select an output before querying ornegotiating any
other parameters.
Information about video outputs is available using theVIDIOC_ENUMOUTPUT ioctl.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The number of the video output is out of bounds, orthere are no video outputs at all and this ioctl is notsupported.
- EBUSY
I/O is in progress, the output cannot beswitched.
ioctl VIDIOC_G_PARM, VIDIOC_S_PARM
Name
VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters
Synopsis
int ioctl(int fd, int request, v4l2_streamparm *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_PARM, VIDIOC_S_PARM
argp
Description
The current video standard determines a nominal number offrames per
second. If less than this number of frames is to becaptured or output,
applications can request frame skipping orduplicating on the driver
side. This is especially useful when usingtheread()
or write(), whichare not augmented by timestamps or sequence counters, and to avoidunneccessary data copying.
Further these ioctls can be used to determine the number ofbuffers
used internally by a driver in read/write mode. Forimplications see the
section discussing theread()function.
To get and set the streaming parameters applications callthe
VIDIOC_G_PARMVIDIOC_S_PARM ioctl, respectively. They take apointer to a structv4l2_streamparm whichcontains a union holding separate parameters for input and outputdevices.
Table 1. struct v4l2_streamparm
| enum v4l2_buf_type | type | The buffer (stream) type, same as struct v4l2_formattype, set by the application. | |
| union | parm | ||
| struct v4l2_captureparm | capture | Parameters for capture devices, used whentype isV4L2_BUF_TYPE_VIDEO_CAPTURE. | |
| struct v4l2_outputparm | output | Parameters for output devices, used whentype isV4L2_BUF_TYPE_VIDEO_OUTPUT. | |
| __u8 | raw_data[200] | A place holder for future extensions and custom(driver defined) buffer types |
Table 2. struct v4l2_captureparm
| __u32 | capability | See Table 4. |
| __u32 | capturemode | Set by drivers and applications, see Table 5. |
| struct v4l2_fract | timeperframe | This is is the desired period betweensuccessive frames captured by Applications store here the desired frameperiod, drivers return the Drivers support this function only when they set the |
| __u32 | extendedmode | Custom (driver specific) streaming parameters. Whenunused, applications and drivers must set this field to zero.Applications using this field should check the driver name andversion, seeSection 1.2. |
| __u32 | readbuffers | Applications set this field to the desired numberof buffers used internally by the driver inread()mode. Driversreturn the actual number of buffers. When an application requests zerobuffers, drivers should just return the current setting rather thanthe minimum or an error code. For details seeSection 3.1. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
Table 3. struct v4l2_outputparm
| __u32 | capability | See Table 4. |
| __u32 | outputmode | Set by drivers and applications, see Table 5. |
| struct v4l2_fract | timeperframe | This is is the desired period betweensuccessive frames output by the driver, in seconds. |
The field is intended torepeat frames on the driver side in Applications store here the desired frameperiod, drivers return the Drivers support this function only when they set the | ||
| __u32 | extendedmode | Custom (driver specific) streaming parameters. Whenunused, applications and drivers must set this field to zero.Applications using this field should check the driver name andversion, seeSection 1.2. |
| __u32 | writebuffers | Applications set this field to the desired numberof buffers used internally by the driver inwrite() mode. Drivers return the actual number ofbuffers. When an application requests zero buffers, drivers shouldjust return thecurrent setting rather than the minimum or an errorcode. For details see Section 3.1. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
Table 4. Streaming Parameters Capabilites
V4L2_CAP_TIMEPERFRAME | 0x1000 | The frame skipping/repeating controlled by thetimeperframe field is supported. |
Table 5. Capture Parameters Flags
V4L2_MODE_HIGHQUALITY | 0x0001 | High quality imaging mode. High quality modeis intended for still
|
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
This ioctl is not supported.
ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY
Name
VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY -- Query or request the access priority associated with afile descriptor
Synopsis
int ioctl(int fd, int request, enum v4l2_priority *argp);
int ioctl(int fd, int request, const enum v4l2_priority *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY
argpPointer to an enum v4l2_priority type.
Description
To query the current access priorityapplications call the
VIDIOC_G_PRIORITY
To request an access priority applications store thedesired priority in an enum v4l2_priority variable and callVIDIOC_S_PRIORITY ioctl with a pointer to thisvariable.
Table 1. enum v4l2_priority
V4L2_PRIORITY_UNSET | 0 | |
V4L2_PRIORITY_BACKGROUND | 1 | Lowest priority, usually applications running inbackground, for example monitoring VBI transmissions. A proxyapplication running in user space will be necessary if multipleapplications want to read from a device at this priority. |
V4L2_PRIORITY_INTERACTIVE | 2 | |
V4L2_PRIORITY_DEFAULT | 2 | Medium priority, usually applications started andinteractively controlled by the user. For example TV viewers, Teletextbrowsers, or just "panel" applications to change the channel or videocontrols. This is the default priority unless an application requestsanother. |
V4L2_PRIORITY_RECORD | 3 | Highest priority. Only one file descriptor can havethis priority, it blocks any other fd from changing device properties.Usually applications which must not be interrupted, like videorecording. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The requested priority value is invalid, or thedriver does not support access priorities.
- EBUSY
Another application already requested higherpriority.
ioctl VIDIOC_G_SLICED_VBI_CAP
Name
VIDIOC_G_SLICED_VBI_CAP -- Query sliced VBI capabilities
Synopsis
int ioctl(int fd, int request, struct v4l2_sliced_vbi_cap *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_SLICED_VBI_CAP
argp
Description
To find out which data services are supported by a slicedVBI capture or output device, applications initialize thetype field of a struct v4l2_sliced_vbi_cap,clear
the reserved array andcall the
VIDIOC_G_SLICED_VBI_CAPtypeis invalid.
Note the type field was added,and the ioctl changed from read-only to write-read, in Linux 2.6.19.
Table 1. struct v4l2_sliced_vbi_cap
| __u16 | service_set | A set of all data servicessupported by the driver. Equal to the union of all elements of theservice_linesarray. | ||
| __u16 | service_lines[2][24] | Each element of this arraycontains a set of data services the hardware can look for or insertinto a particular scan line. Data services are defined inTable 2. Array indices map to ITU-Rline numbers (see alsoFigure 4-2 andFigure 4-3) as follows: | ||
| Element | 525 line systems | 625 line systems | ||
service_lines[0][1] | 1 | 1 | ||
service_lines[0][23] | 23 | 23 | ||
service_lines[1][1] | 264 | 314 | ||
service_lines[1][23] | 286 | 336 | ||
| The number of VBI lines thehardware can capture or output per frame, or the number of services itcan identify on a given line may be limited. For example on PAL line16 the hardware may be able to look for a VPS or Teletext signal, butnot both at the same time. Applications can learn about these limitsusing the VIDIOC_S_FMT ioctl as described in Section 4.8. | ||||
Drivers must setservice_lines[0][0] andservice_lines[1][0] to zero. | ||||
| enum v4l2_buf_type | type | Type of the data stream, see Table 3-2. Should beV4L2_BUF_TYPE_SLICED_VBI_CAPTURE orV4L2_BUF_TYPE_SLICED_VBI_OUTPUT. | ||
| __u32 | reserved[3] | This array is reserved for futureextensions. Applications and drivers must set it to zero. |
Table 2. Sliced VBI services
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately:
- EINVAL
The device does not support sliced VBI capturing oroutput, or the value in the
typefield iswrong.
ioctl VIDIOC_G_STD, VIDIOC_S_STD
Name
VIDIOC_G_STD, VIDIOC_S_STD -- Query or select the video standard of the current input
Synopsis
int ioctl(int fd, int request, v4l2_std_id*argp);
int ioctl(int fd, int request, const v4l2_std_id*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_STD, VIDIOC_S_STD
argp
Description
To query and select the current video standard applicationsuse the
VIDIOC_G_STDVIDIOC_S_STD ioctls which take a pointer to av4l2_std_id type as argument.VIDIOC_G_STD canreturn
a single flag or a set of flags as in struct v4l2_standard fieldid. The flags must be unambiguous suchthat they appear in only one enumeratedv4l2_standard
structure.
VIDIOC_S_STD accepts one or moreflags, being a write-only ioctl it does not return the actual new standard asVIDIOC_G_STD does. When no flags are given orthe current input does not support the requested
standard the driverreturns an EINVAL error code. When the standard set is ambiguous drivers mayreturnEINVAL or choose any of the requestedstandards.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
This ioctl is not supported, or the
VIDIOC_S_STDparameter was unsuitable.
ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER
Name
VIDIOC_G_TUNER, VIDIOC_S_TUNER -- Get or set tuner attributes
Synopsis
int ioctl(int fd, int request, struct v4l2_tuner*argp);
int ioctl(int fd, int request, const struct v4l2_tuner*argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_G_TUNER, VIDIOC_S_TUNER
argp
Description
To query the attributes of a tuner applications initialize theindex field and zero out thereserved array of a struct v4l2_tuner
and call theVIDIOC_G_TUNER ioctl with a pointer to thisstructure. Drivers fill the rest of the structure or return anEINVAL error code when the index is out of bounds. To enumerate all tunersapplications
shall begin at index zero, incrementing by one until thedriver returns
EINVAL.
Tuners have two writable properties, the audio mode andthe radio frequency. To change the audio mode, applications initializetheindex,audmode andreserved fields
and call theVIDIOC_S_TUNER ioctl. This willnot
change the current tuner, which is determinedby the current video
input. Drivers may choose a different audio modeif the requested
mode is invalid or unsupported. Since this is awrite-only ioctl, it
does not return the actuallyselected audio mode.
To change the radio frequency the VIDIOC_S_FREQUENCY ioctlis available.
Table 1. struct v4l2_tuner
| __u32 | index | Identifies the tuner, set by theapplication. | |
| __u8 | name[32] | Name of the tuner, aNUL-terminated ASCII string. This information is intended for theuser. | |
| enum v4l2_tuner_type | type | Type of the tuner, see Table 2. | |
| __u32 | capability | Tuner capability flags, seeTable 3. Audio flags indicate the abilityto decode audio subprograms. They willnotchange, Whenthe structure refers to a radio tuner only the | |
| __u32 | rangelow | The lowest tunable frequency inunits of 62.5 kHz, or if the V4L2_TUNER_CAP_LOW is set, in units of 62.5Hz. | |
| __u32 | rangehigh | The highest tunable frequency inunits of 62.5 kHz, or if the V4L2_TUNER_CAP_LOW is set, in units of 62.5Hz. | |
| __u32 | rxsubchans | Some tuners or audiodecoders can determine the received audio | |
V4L2_TUNER_SUB_MONO | receiving mono audio | ||
STEREO | SAP | receiving stereo audio and a secondary audioprogram | ||
MONO | STEREO | receiving mono or stereo audio, the hardware cannotdistinguish | ||
LANG1 | LANG2 | receiving bilingual audio | ||
MONO | STEREO | LANG1 | LANG2 | receiving mono, stereo or bilingualaudio | ||
When the This field is valid only if this is the tuner of thecurrent video input, or when the structure refers to a radiotuner. | |||
| __u32 | audmode | The selected audio mode, seeTable 5 for valid values. The audio mode doesnot affect audio subprogram detection, and like acontrol Currently this is the only field of struct | |
| __u32 | signal | The signal strength if known, rangingfrom 0 to 65535. Higher values indicate a better signal. | |
| __s32 | afc | Automatic frequency control: When theafc value is negative, the frequency is toolow, when positive too high. | |
| __u32 | reserved[4] | Reserved for future extensions. Drivers andapplications must set the array to zero. |
Table 2. enum v4l2_tuner_type
V4L2_TUNER_RADIO | 1 | |
V4L2_TUNER_ANALOG_TV | 2 |
Table 3. Tuner and Modulator Capability Flags
V4L2_TUNER_CAP_LOW | 0x0001 | When set, tuning frequencies are expressed in units of62.5 Hz, otherwise in units of 62.5 kHz. |
V4L2_TUNER_CAP_NORM | 0x0002 | This is a multi-standard tuner; the video standardcan or must be switched. (B/G PAL tuners for example are typically not considered multi-standard because the video standard is automatically determined from the frequency band.) The set of supported video standards is available from the struct v4l2_input pointing to this tuner, see the description of ioctl VIDIOC_ENUMINPUTfor details. Only V4L2_TUNER_ANALOG_TV tuners can have this capability. |
V4L2_TUNER_CAP_STEREO | 0x0010 | Stereo audio reception is supported. |
V4L2_TUNER_CAP_LANG1 | 0x0040 | Reception of the primary language of a bilingualaudio program is supported. Bilingual audio is a feature oftwo-channel systems, transmitting the primary language monaural on themain audio carrier and a secondary language monaural on a secondcarrier. Only V4L2_TUNER_ANALOG_TVtuners can have this capability. |
V4L2_TUNER_CAP_LANG2 | 0x0020 | Reception of the secondary language of a bilingualaudio program is supported. OnlyV4L2_TUNER_ANALOG_TV tuners can have this capability. |
V4L2_TUNER_CAP_SAP | 0x0020 | Reception of a secondary audio program issupported. This is a feature Note the |
Table 4. Tuner Audio Reception Flags
V4L2_TUNER_SUB_MONO | 0x0001 | The tuner receives a mono audio signal. |
V4L2_TUNER_SUB_STEREO | 0x0002 | The tuner receives a stereo audio signal. |
V4L2_TUNER_SUB_LANG1 | 0x0008 | The tuner receives the primary language of abilingual audio signal. Drivers must clear this flag when the currentvideo standard is V4L2_STD_NTSC_M. |
V4L2_TUNER_SUB_LANG2 | 0x0004 | The tuner receives the secondary language of abilingual audio signal (or a second audio program). |
V4L2_TUNER_SUB_SAP | 0x0004 | The tuner receives a Second Audio Program. Note theV4L2_TUNER_SUB_LANG2 andV4L2_TUNER_SUB_SAP flags are synonyms. TheV4L2_TUNER_SUB_SAP flag applies when thecurrentvideo standard is V4L2_STD_NTSC_M. |
Table 5. Tuner Audio Modes
V4L2_TUNER_MODE_MONO | 0 | Play mono audio. When the tuner receives a stereosignal this a down-mix of the left and right channel. When the tunerreceives a bilingual or SAP signal this mode selects the primarylanguage. |
V4L2_TUNER_MODE_STEREO | 1 | Play stereo audio. When the tuner receivesbilingual audio it may play Playing different languages in this mode isdeprecated. New drivers should do this only in When the tunerreceives no stereo signal or does not support stereo reception thedriver shall fall back to |
V4L2_TUNER_MODE_LANG1 | 3 | Play the primary language, mono or stereo. OnlyV4L2_TUNER_ANALOG_TV tuners support thismode. |
V4L2_TUNER_MODE_LANG2 | 2 | Play the secondary language, mono. When the tunerreceives no bilingual audio or SAP, or their reception is notsupported the driver shall fall back to mono or stereo mode. Only V4L2_TUNER_ANALOG_TV tuners support thismode. |
V4L2_TUNER_MODE_SAP | 2 | Play the Second Audio Program. When the tunerreceives no bilingual audio or SAP, or their reception is notsupported the driver shall fall back to mono or stereo mode. Only V4L2_TUNER_ANALOG_TV tuners support this mode.Note theV4L2_TUNER_MODE_LANG2and V4L2_TUNER_MODE_SAP are synonyms. |
V4L2_TUNER_MODE_LANG1_LANG2 | 4 | Play the primary language on the left channel, thesecondary language on the right channel. When the tuner receives nobilingual audio or SAP, it shall fall back to MODE_LANG1 orMODE_MONO.OnlyV4L2_TUNER_ANALOG_TVtuners support thismode. |
Table 6. Tuner Audio Matrix
MONO | Mono | Mono/Mono | Mono | Mono | Mono/Mono |
MONO | SAP | Mono | Mono/Mono | Mono | SAP | Mono/SAP (preferred) or Mono/Mono |
STEREO | L+R | L/R | Stereo L/R (preferred) or Mono L+R | Stereo L/R (preferred) or Mono L+R | L/R (preferred) or L+R/L+R |
STEREO | SAP | L+R | L/R | Stereo L/R (preferred) or Mono L+R | SAP | L+R/SAP (preferred) or L/R or L+R/L+R |
LANG1 | LANG2 | Language 1 | Lang1/Lang2 (deprecated[b]) orLang1/Lang1 | Language 1 | Language 2 | Lang1/Lang2 (preferred) or Lang1/Lang1 |
| Notes: a. This mode has been added in Linux 2.6.17 and may not be supported by olderdrivers. b. Playback ofboth languages in MODE_LANG1_LANG2 to record both languages or astereo signal. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_tuner
indexisout of bounds.
ioctl VIDIOC_LOG_STATUS
Name
VIDIOC_LOG_STATUS -- Log driver status information
Synopsis
int ioctl(int fd, int request);
Description
As the video/audio devices become more complicated itbecomes harder
to debug problems. When this ioctl is called the driverwill output the
current device status to the kernel log. This isparticular useful when
dealing with problems like no sound, no videoand
incorrectly tuned channels. Also many modern devices autodetectvideo
and audio standards and this ioctl will report what the devicethinks
what the standard is. Mismatches may give an indication wherethe problem
is.
This ioctl is optional and not all drivers support it. Itwas introduced in Linux 2.6.15.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The driver does not support this ioctl.
ioctl VIDIOC_OVERLAY
Name
VIDIOC_OVERLAY -- Start or stop video overlay
Synopsis
int ioctl(int fd, int request, const int *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_OVERLAY
argp
Description
This ioctl is part of the video overlay I/O method. Applications call VIDIOC_OVERLAY
to start or stop the overlay. It takes a pointer to an integer which
must be set to zero by the application to stop overlay, to one to start.
Drivers do not support VIDIOC_STREAMON orVIDIOC_STREAMOFF withV4L2_BUF_TYPE_VIDEO_OVERLAY.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
Video overlay is not supported, or theparameters have not been set up. See Section 4.2 for the necessary steps.
ioctl VIDIOC_QBUF, VIDIOC_DQBUF
Name
VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver
Synopsis
int ioctl(int fd, int request, struct v4l2_buffer *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_QBUF, VIDIOC_DQBUF
argp
Description
Applications call the VIDIOC_QBUF
ioctlto enqueue an empty (capturing) or filled (output) buffer in
thedriver's incoming queue. The semantics depend on the selected
I/Omethod.
To enqueue a memory mappedbuffer applications set the type field of astruct v4l2_buffer to the same buffer type as previously struct v4l2_formattype
and struct v4l2_requestbufferstype, thememoryfield toV4L2_MEMORY_MMAP and
theindex field. Valid index numbers range fromzero to the number of buffers allocated withVIDIOC_REQBUFS(struct v4l2_requestbufferscount)
minus one. Thecontents of the struct v4l2_buffer returnedby aVIDIOC_QUERYBUF ioctl will do as well. When the buffer
isintended for output (type isV4L2_BUF_TYPE_VIDEO_OUTPUT orV4L2_BUF_TYPE_VBI_OUTPUT) applications must alsoinitialize thebytesused,field
andtimestamp fields. SeeSection 3.5 for details. WhenVIDIOC_QBUF is called with a pointer to thisstructure the driver
sets theV4L2_BUF_FLAG_MAPPED andV4L2_BUF_FLAG_QUEUED flags and clears theV4L2_BUF_FLAG_DONE flag in theflags field, or it returns anEINVAL
error code.
To enqueue a user pointerbuffer applications set the type field of astruct v4l2_buffer to the same buffer type as previously struct v4l2_formattype
and struct v4l2_requestbufferstype, thememoryfield toV4L2_MEMORY_USERPTR
and them.userptr field to the address of thebuffer andlength to its size. When thebuffer is intended for output additional fields must be set as above.WhenVIDIOC_QBUF
is called with a pointer to thisstructure the driver sets theV4L2_BUF_FLAG_QUEUEDflag and clears theV4L2_BUF_FLAG_MAPPED andV4L2_BUF_FLAG_DONE flags in theflags
field, or it returns an error code.This ioctl locks the memory pages of
the buffer in physical memory,they cannot be swapped out to disk.
Buffers remain locked untildequeued, until theVIDIOC_STREAMOFF
orVIDIOC_REQBUFS ioctl arecalled, or until the device is closed.
Applications call the VIDIOC_DQBUFioctl to dequeue a filled (capturing) or displayed (output) bufferfrom the driver's outgoing queue. They just set thetype andmemoryfields
of a struct v4l2_buffer as above, whenVIDIOC_DQBUFis called with a pointer to this structure the driver fills theremaining fields or returns an error
code.
By default VIDIOC_DQBUF blocks when nobuffer is in the outgoing queue. When theO_NONBLOCK flag was given to theopen()function,VIDIOC_DQBUF
returns immediatelywith an EAGAIN error code when no buffer is available.
The v4l2_buffer structure isspecified in Section 3.5.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EAGAIN
Non-blocking I/O has been selected using
O_NONBLOCKand no buffer was in the outgoingqueue.- EINVAL
The buffer
typeis notsupported, or the
indexuserptrorlengthare invalid.- ENOMEM
Not enough physical or virtual memory was available toenqueue a user pointer buffer.
- EIO
VIDIOC_DQBUFfailed due to aninternal
error. Can also indicate temporary problems like signalloss. Note the
driver might dequeue an (empty) buffer despitereturning an error, or
even stop capturing.
ioctl VIDIOC_QUERYBUF
Name
VIDIOC_QUERYBUF -- Query the status of a buffer
Synopsis
int ioctl(int fd, int request, struct v4l2_buffer *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_QUERYBUF
argp
Description
This ioctl is part of the memorymapping I/O method. It can be used to query the status of abuffer at any time after buffers have been allocated with theVIDIOC_REQBUFS
ioctl.
Applications set the type field of a struct v4l2_buffer to the same buffer type as previouslystruct v4l2_formattype
and struct v4l2_requestbufferstype, and theindex field. Valid index numbers range from zeroto the number
of buffers allocated withVIDIOC_REQBUFS (struct v4l2_requestbufferscount)
minus one.After calling VIDIOC_QUERYBUF with a pointer to this structure drivers return an error code or fill the rest ofthe structure.
In the flags field theV4L2_BUF_FLAG_MAPPED,V4L2_BUF_FLAG_QUEUED andV4L2_BUF_FLAG_DONE flags will be valid. Thememory
field will be set toV4L2_MEMORY_MMAP, the
m.offsetlength field its size. The driver mayor may not set the remaining fields and flags, they are meaningless inthis context.
The v4l2_buffer structure is specified in Section 3.5.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The buffer
typeis notsupported, or the
index
ioctl VIDIOC_QUERYCAP
Name
VIDIOC_QUERYCAP -- Query device capabilities
Synopsis
int ioctl(int fd, int request, struct v4l2_capability *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_QUERYCAP
argp
Description
All V4L2 devices support theVIDIOC_QUERYCAP
ioctl. It is used to identifykernel devices compatible with this
specification and to obtaininformation about driver and hardware
capabilities. The ioctl takes apointer to a struct v4l2_capability
which is filled by the driver. When thedriver is not compatible with this specification the ioctl returns anEINVAL error code.
Table 1. struct v4l2_capability
| __u8 | driver[16] | Name of the driver, a unique NUL-terminatedASCII string. For example: Storing strings in fixed sized arrays is badpractice but unavoidable |
| __u8 | card[32] | Name of the device, a NUL-terminated ASCII string.For example: "Yoyodyne TV/FM". One driver may support different brandsor models of video hardware. This information is intended for users,for example in a menu of available devices. Since multiple TV cards ofthe same brand may be installed which are supported by the samedriver, this name should be combined with the character device filename (e. g./dev/video2) or the bus_info string to avoidambiguities. |
| __u8 | bus_info[32] | Location of the device in the system, aNUL-terminated ASCII string. For example: "PCI Slot 4". Thisinformation is intended for users, to distinguish multipleidentical devices. If no such information is available the field maysimply count the devices controlled by the driver, or contain theempty string ( bus_info[0] = 0). |
| __u32 | version | Version number of the driver. Together withthe |
#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))
__u32 version = KERNEL_VERSION(0, 8, 1);
printf ("Version: %u.%u.%u\n", | ||
| __u32 | capabilities | Device capabilities, see Table 2. |
| __u32 | reserved[4] | Reserved for future extensions. Drivers must setthis array to zero. |
Table 2. Device Capabilities Flags
V4L2_CAP_VIDEO_CAPTURE | 0x00000001 | The device supports the Video Capture interface. |
V4L2_CAP_VIDEO_OUTPUT | 0x00000002 | The device supports the Video Output interface. |
V4L2_CAP_VIDEO_OVERLAY | 0x00000004 | The device supports the Video Overlay interface. A video overlay devicetypically stores captured images directly in the video memory of agraphics card, with hardware clipping and scaling. |
V4L2_CAP_VBI_CAPTURE | 0x00000010 | The device supports the RawVBI Capture interface, providing Teletext and Closed Captiondata. |
V4L2_CAP_VBI_OUTPUT | 0x00000020 | The device supports the Raw VBI Output interface. |
V4L2_CAP_SLICED_VBI_CAPTURE | 0x00000040 | The device supports the Sliced VBI Capture interface. |
V4L2_CAP_SLICED_VBI_OUTPUT | 0x00000080 | The device supports the Sliced VBI Output interface. |
V4L2_CAP_RDS_CAPTURE | 0x00000100 | [to be defined] |
V4L2_CAP_VIDEO_OUTPUT_OVERLAY | 0x00000200 | The device supports the VideoOutput Overlay (OSD) interface. Unlike the VideoOverlay interface, this is a secondary function of videooutput devices and overlays an image onto an outgoing video signal.When the driver sets this flag, it must clear theV4L2_CAP_VIDEO_OVERLAY flag and viceversa.[a] |
V4L2_CAP_TUNER | 0x00010000 | The device has some sort of tuner or modulator toreceive or emit RF-modulated video signals. For more information abouttuner and modulator programming seeSection 1.6. |
V4L2_CAP_AUDIO | 0x00020000 | The device has audio inputs or outputs. It may ormay not support audio recording or playback, in PCM or compressedformats. PCM audio support must be implemented as ALSA or OSSinterface. For more information on audio inputs and outputs seeSection 1.5. |
V4L2_CAP_RADIO | 0x00040000 | This is a radio receiver. |
V4L2_CAP_READWRITE | 0x01000000 | The device supports the read() and/or write()I/O methods. |
V4L2_CAP_ASYNCIO | 0x02000000 | The device supports the asynchronous I/O methods. |
V4L2_CAP_STREAMING | 0x04000000 | The device supports the streaming I/O method. |
| Notes: a. The struct v4l2_framebuffer lacks anenum v4l2_buf_type field, therefore the type of overlay is implied by thedriver capabilities. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The device is not compatible with thisspecification.
ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU
Name
VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items
Synopsis
int ioctl(int fd, int request, struct v4l2_queryctrl *argp);
int ioctl(int fd, int request, struct v4l2_querymenu *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_QUERYCTRL, VIDIOC_QUERYMENU
argp
Description
To query the attributes of a control applications set theid field of a struct v4l2_queryctrl and call theVIDIOC_QUERYCTRL
ioctl with a pointer to thisstructure. The driver fills the rest of the structure or returns anEINVAL error code when theid is invalid.
It is possible to enumerate controls by callingVIDIOC_QUERYCTRL with successiveid values starting fromV4L2_CID_BASE up to and exclusiveV4L2_CID_BASE_LASTP1.
Drivers may returnEINVAL if a control in
this range is notsupported. Further applications can enumerate private
controls, whichare not defined in this specification, by starting atV4L2_CID_PRIVATE_BASE
and incrementingid until the driver returnsEINVAL.
In both cases, when the driver sets theV4L2_CTRL_FLAG_DISABLED flag in theflags field this control is permanentlydisabled and should be ignored by the application.[24]
When the application ORs id withV4L2_CTRL_FLAG_NEXT_CTRL the driver returns thenext supported control, orEINVAL if there isnone. Drivers which do not support this
flag yet always returnEINVAL.
Additional information is required for menu controls, thename of menu items. To query them applications set theid andindexfields of struct v4l2_querymenu
and call theVIDIOC_QUERYMENU ioctl with a pointer to thisstructure. The driver fills the rest of the structure or returns anEINVAL error code when theid orindex
is invalid. Menu items are enumeratedby callingVIDIOC_QUERYMENU with successiveindex values from struct v4l2_queryctrlminimum
(0) tomaximum, inclusive.
See also the examples in Section 1.8.
Table 1. struct v4l2_queryctrl
| __u32 | id | Identifies the control, set by the application. SeeTable 1-1 for predefined IDs. When the ID is ORedwith V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returnsthe first control with a higher ID. Drivers which do not support thisflag yet always return anEINVAL error code. |
| enum v4l2_ctrl_type | type | Type of control, see Table 3. |
| __u8 | name[32] | Name of the control, a NUL-terminated ASCIIstring. This information is intended for the user. |
| __s32 | minimum | Minimum value, inclusive. This field gives a lowerbound for V4L2_CTRL_TYPE_INTEGER64 controls. Note this is asigned value. |
| __s32 | maximum | Maximum value, inclusive. This field gives an upperbound for V4L2_CTRL_TYPE_INTEGER64 controls. Note this is asigned value. |
| __s32 | step | This field gives a step size for Generally drivers should not scale hardwarecontrol values. It may be necessary for example when the This field gives the smallest change ofan integer control actually Note although signed, the step value is supposed tobe always positive. |
| __s32 | default_value | The default value of aV4L2_CTRL_TYPE_INTEGER,_BOOLEAN or_MENU control.Not valid for other types of controls. Drivers reset controls onlywhen the driver is loaded,not later, in particular not when thefunc-open; is called. |
| __u32 | flags | Control flags, see Table 4. |
| __u32 | reserved[2] | Reserved for future extensions. Drivers must setthe array to zero. |
Table 2. struct v4l2_querymenu
| __u32 | id | Identifies the control, set by the applicationfrom the respective struct v4l2_queryctrlid. |
| __u32 | index | Index of the menu item, starting at zero, set by the application. |
| __u8 | name[32] | Name of the menu item, a NUL-terminated ASCIIstring. This information is intended for the user. |
| __u32 | reserved | Reserved for future extensions. Drivers must setthe array to zero. |
Table 3. enum v4l2_ctrl_type
V4L2_CTRL_TYPE_INTEGER | any | any | any | An integer-valued control ranging from minimum tomaximum inclusive. The step value indicates the increment betweenvalues which are actually different on the hardware. |
V4L2_CTRL_TYPE_BOOLEAN | 0 | 1 | 1 | A boolean-valued control. Zero corresponds to"disabled", and one means "enabled". |
V4L2_CTRL_TYPE_MENU | 0 | 1 | N-1 | The control has a menu of N choices. The names ofthe menu items can be enumerated with theVIDIOC_QUERYMENU ioctl. |
V4L2_CTRL_TYPE_BUTTON | 0 | 0 | 0 | A control which performs an action when set.Drivers must ignore the value passed withVIDIOC_S_CTRL and return anEINVAL error code on aVIDIOC_G_CTRL attempt. |
V4L2_CTRL_TYPE_INTEGER64 | n/a | n/a | n/a | A 64-bit integer valued control. Minimum, maximumand step size cannot be queried. |
V4L2_CTRL_TYPE_CTRL_CLASS | n/a | n/a | n/a | This is not a control. WhenVIDIOC_QUERYCTRL is called with a control IDequal to a control class code (seeTable 3), theioctl returns the name ofthe control class and this control type.Older drivers which do not support this feature return anEINVAL error code. |
Table 4. Control Flags
V4L2_CTRL_FLAG_DISABLED | 0x0001 | This control is permanently disabled and should beignored by the application. Any attempt to change the control willresult in anEINVAL error code. |
V4L2_CTRL_FLAG_GRABBED | 0x0002 | This control is temporarily unchangeable, forexample because another application took over control of therespective resource. Such controls may be displayed specially in auser interface. Attempts to change the control may result in anEBUSY error code. |
V4L2_CTRL_FLAG_READ_ONLY | 0x0004 | This control is permanently readable only. Anyattempt to change the control will result in anEINVAL error code. |
V4L2_CTRL_FLAG_UPDATE | 0x0008 | A hint that changing this control may affect thevalue of other controls within the same control class. Applicationsshould update their user interface accordingly. |
V4L2_CTRL_FLAG_INACTIVE | 0x0010 | This control is not applicable to the currentconfiguration and should be displayed accordingly in a user interface.For example the flag may be set on a MPEG audio level 2 bitratecontrol when MPEG audio encoding level 1 was selected with anothercontrol. |
V4L2_CTRL_FLAG_SLIDER | 0x0020 | A hint that this control is best represented as aslider-like element in a user interface. |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
The struct v4l2_queryctrl
idis invalid. The struct v4l2_querymenuid
orindexis invalid.
ioctl VIDIOC_QUERYSTD
Name
VIDIOC_QUERYSTD -- Sense the video standard received by the currentinput
Synopsis
int ioctl(int fd, int request, v4l2_std_id *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_QUERYSTD
argp
Description
The hardware may be able to detect the current videostandard automatically. To do so, applications callVIDIOC_QUERYSTD with a pointer to av4l2_std_id
type. Thedriver stores here a set of candidates, this can be a single
flag or aset of supported standards if for example the hardware can
onlydistinguish between 50 and 60 Hz systems. When detection is
notpossible or fails, the set must contain all standards
supported by thecurrent video input or output.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
This ioctl is not supported.
ioctl VIDIOC_REQBUFS
Name
VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O
Synopsis
int ioctl(int fd, int request, struct v4l2_requestbuffers *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_REQBUFS
argp
Description
This ioctl is used to initiate memorymapped or user pointerI/O. Memory mapped buffers are located in device memory
and must beallocated with this ioctl before they can be mapped into
theapplication's address space. User buffers are allocated
byapplications themselves, and this ioctl is merely used to
switch thedriver into user pointer I/O mode.
To allocate device buffers applications initialize threefields of a
v4l2_requestbufferstype field to the respectivestream or buffer type, thecount field tothe desired number of buffers, andmemorymust
be set to V4L2_MEMORY_MMAP. When the
ioctlis called with a pointer to this structure the driver attempts
toallocate the requested number of buffers and stores the actual
numberallocated in thecount
field. It can besmaller than the number requested, even zero, when the
driver runs outof free memory. A larger number is possible when the
driver requiresmore buffers to function correctly.[25]
When memory mapping I/O is not supported the ioctlreturns an
EINVAL error code.
Applications can call VIDIOC_REQBUFSagain to change the number of buffers, however this cannot succeedwhen any buffers are still mapped. Acountvalue of zero frees all buffers, after aborting
or finishing any DMAin progress, an implicitVIDIOC_STREAMOFF.
To negotiate user pointer I/O, applications initialize onlythe
typememory toV4L2_MEMORY_USERPTR.
When the ioctl is calledwith a pointer to this structure the driver
prepares for user pointerI/O, when this I/O method is not supported the
ioctl returns anEINVAL error code.
Table 1. struct v4l2_requestbuffers
| __u32 | count | The number of buffers requested or granted. Thisfield is only used when V4L2_MEMORY_MMAP. |
| enum v4l2_buf_type | type | Type of the stream or buffers, this is the sameas the struct v4l2_formattype field. SeeTable3-2 for valid values. |
| enum v4l2_memory | memory | Applications set this field toV4L2_MEMORY_MMAP orV4L2_MEMORY_USERPTR. |
| __u32 | reserved[2] | A place holder for future extensions and custom(driver defined) buffer types |
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EBUSY
The driver supports multiple opens and I/O is alreadyin progress, or
reallocation of buffers was attempted although one ormore are still
mapped.- EINVAL
The buffer type (
typefield) or therequested I/O method (memory) is notsupported.
ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF
Name
VIDIOC_STREAMON, VIDIOC_STREAMOFF -- Start or stop streaming I/O
Synopsis
int ioctl(int fd, int request, const int *argp);
Arguments
fdFile descriptor returned by
open().requestVIDIOC_STREAMON, VIDIOC_STREAMOFF
argp
Description
The VIDIOC_STREAMON andVIDIOC_STREAMOFF ioctl start and stop the captureor output process during streaming (memorymapping
oruser pointer) I/O.
Specifically the capture hardware is disabled and no inputbuffers are
filled (if there are any empty buffers in the incomingqueue) untilVIDIOC_STREAMON has been called.Accordingly the output hardware is disabled, no video signal
isproduced untilVIDIOC_STREAMON has been called.The ioctl will succeed only when at least one output buffer is in theincoming queue.
The VIDIOC_STREAMOFF ioctl, apart
ofaborting or finishing any DMA in progress, unlocks any user
pointerbuffers locked in physical memory, and it removes all buffers
from theincoming and outgoing queues. That means all images
captured but notdequeued yet will be lost, likewise all images enqueued
for output butnot transmitted yet. I/O returns to the same state as
after callingVIDIOC_REQBUFS
and can be restarted accordingly.
Both ioctls take a pointer to an integer, the desired buffer orstream type. This is the same as struct v4l2_requestbufferstype.
Note applications can be preempted for unknown periods rightbefore or after theVIDIOC_STREAMON orVIDIOC_STREAMOFF calls, there is no notion ofstarting or stopping "now". Buffer timestamps can be
used tosynchronize with other events.
Return Value
On success 0 is returned, on error
-1 and the errno variable is set appropriately:
- EINVAL
Streaming I/O is not supported, the buffer
typeis not supported, or no buffers havebeen allocated (memory mapping) or enqueued (output) yet.
V4L2 mmap()
Name
v4l2-mmap -- Map device memory into application address space
Synopsis
#include <unistd.h>
#include <sys/mman.h>
void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);
Arguments
startMap the buffer to this address in the application's address space. When the
MAP_FIXEDflag is specified,startmust be a multiple of thepagesize and mmap will fail when the specified addresscannot be used. Use of this option is discouraged; applications shouldjust specify aNULLpointer here.lengthLength of the memory area to map. This must be thesame value as returned by the driver in the struct v4l2_buffer
lengthfield.protThe
protargument describes thedesired memory protection. Regardless of the device type and thedirection of data exchange it should be set toPROT_READ|PROT_WRITE,permitting read and write access to image buffers. Drivers shouldsupport at least this combination of flags. Note the Linuxvideo-buf kernel module, which is used by thebttv, saa7134, saa7146, cx88 and vivi driver supports onlyPROT_READ|PROT_WRITE. Whenthe driver does not support the desired protection themmap()function fails.Note device memory accesses (e. g. the memory on agraphics card with video capturing hardware) may incur a performancepenalty compared to main memory accesses, or reads may besignificantly slower than writes or vice versa. Other I/O methods maybe more efficient in this case.
flagsThe
flagsparameterspecifies the type of the mapped object, mapping options and whethermodifications made to the mapped copy of the page are private to theprocess or are to be shared with other references.MAP_FIXEDrequests that thedriver selects no other address than the one specified. If thespecified address cannot be used,mmap()will fail. IfMAP_FIXEDis specified,startmust be a multiple of the pagesize. Useof this option is discouraged.One of the
MAP_SHAREDorMAP_PRIVATEflags must be set.MAP_SHAREDallows applications to share themapped memory with other (e. g. child-) processes. Note the Linuxvideo-buf module which is used by the bttv,saa7134, saa7146, cx88 and vivi driver supports onlyMAP_SHARED.MAP_PRIVATErequests copy-on-write semantics. V4L2 applications should not set theMAP_PRIVATE,MAP_DENYWRITE,MAP_EXECUTABLEorMAP_ANONflag.fdFile descriptor returned by
open().offsetOffset of the buffer in device memory. This must be thesame value as returned by the driver in the struct v4l2_buffer
munionoffsetfield.
Description
The mmap() function asks to maplength bytes starting atoffset in the memory of the device specified byfd into the application address space,preferably at address start. This latteraddress is a hint only, and is usually specified as 0.
Suitable length and offset parameters are queried with theVIDIOC_QUERYBUF ioctl. Buffers must be allocated with theVIDIOC_REQBUFS ioctl before they can be queried.
To unmap buffers the munmap() function is used.
Return Value
On success mmap() returns a pointer tothe mapped buffer. On errorMAP_FAILED (-1) isreturned, and theerrno variable is setappropriately. Possible error codes are:
- EBADF
fdis not a valid filedescriptor.- EACCES
fdisnot open for reading and writing.- EINVAL
The
startorlengthoroffsetare notsuitable. (E. g. they are too large, or not aligned on aPAGESIZEboundary.)The
flagsorprotvalue is not supported.No buffers have been allocated with the
VIDIOC_REQBUFSioctl.- ENOMEM
Not enough physical or virtual memory was available tocomplete the request.
V4L2 munmap()
Name
v4l2-munmap -- Unmap device memory
Synopsis
#include <unistd.h>
#include <sys/mman.h>
int munmap(void *start, size_t length);
Arguments
startAddress of the mapped buffer as returned by the
mmap()function.lengthLength of the mapped buffer. This must be the samevalue as given to
mmap()and returned by thedriver in the struct v4l2_bufferlengthfield.
Description
Unmaps a previously with the mmap() function mappedbuffer and frees it, if possible.
Return Value
On success munmap() returns 0, onfailure -1 and theerrno variable is setappropriately:
- EINVAL
The
startorlengthis incorrect, or no buffers have beenmapped yet.
V4L2 open()
Name
v4l2-open -- Open a V4L2 device
Synopsis
#include <fcntl.h>
int open(const char *device_name, int flags);
Arguments
device_nameDevice to be opened.
flagsOpen flags. Access mode must be
O_RDWR. This is just a technicality, input devicesstill support only reading and output devices only writing.When the
O_NONBLOCKflag isgiven, the read() function and theVIDIOC_DQBUFioctl will returntheEAGAIN error code when no data is available or no buffer is in the driveroutgoing queue, otherwise these functions block until data becomesavailable. All V4L2 drivers exchanging data with applications mustsupport theO_NONBLOCKflag.Other flags have no effect.
Description
To open a V4L2 device applications callopen() with the desired device name. Thisfunction has no side effects; all data format parameters, currentinput or output, control values or other properties remain unchanged.At the firstopen() call after loading the driverthey will be reset to default values, drivers are never in anundefined state.
Return Value
On success open returns the new filedescriptor. On error -1 is returned, and theerrnovariable is set appropriately. Possible error codes are:
- EACCES
The caller has no permission to access thedevice.
- EBUSY
The driver does not support multiple opens and thedevice is already in use.
- ENXIO
No device corresponding to this device special fileexists.
- ENOMEM
Not enough kernel memory was available to complete therequest.
- EMFILE
The process already has the maximum number offiles open.
- ENFILE
The limit on the total number of files open on thesystem has been reached.
V4L2 poll()
Name
v4l2-poll -- Wait for some event on a file descriptor
Synopsis
#include <sys/poll.h>
int poll(struct pollfd *ufds, unsigned int nfds, int timeout);
Description
With the poll() function applicationscan suspend execution until the driver has captured data or is readyto accept data for output.
When streaming I/O has been negotiated this function waitsuntil a buffer has been filled or displayed and can be dequeued withtheVIDIOC_DQBUF ioctl. When buffers are already in the outgoingqueue of the driver the function returns immediately.
On success poll() returns the number offile descriptors that have been selected (that is, file descriptorsfor which therevents field of therespectivepollfd structure is non-zero).Capture devices set thePOLLIN andPOLLRDNORM flags in therevents field, output devices thePOLLOUT andPOLLWRNORMflags. When the function timed out it returns a value of zero, onfailure it returns-1 and theerrno variable is set appropriately. When theapplication did not callVIDIOC_QBUF orVIDIOC_STREAMON yet thepoll() function succeeds, but sets thePOLLERR flag in therevents field.
When use of the read() function hasbeen negotiated and the driver does not capture yet, thepoll function starts capturing. When that failsit returns aPOLLERR as above. Otherwise it waitsuntil data has been captured and can be read. When the driver capturescontinuously (as opposed to, for example, still images) the functionmay return immediately.
When use of the write() function hasbeen negotiated thepoll function just waitsuntil the driver is ready for a non-blockingwrite() call.
All drivers implementing the read() orwrite() function or streaming I/O must alsosupport thepoll() function.
For more details see thepoll() manual page.
Return Value
On success, poll() returns the numberstructures which have non-zeroreventsfields, or zero if the call timed out. On error-1 is returned, and theerrno variable is set appropriately:
- EBADF
One or more of the
ufdsmembersspecify an invalid file descriptor.- EBUSY
The driver does not support multiple read or writestreams and the device is already in use.
- EFAULT
ufdsreferences an inaccessiblememory area.- EINTR
The call was interrupted by a signal.
- EINVAL
The
nfdsargument is greaterthanOPEN_MAX.
V4L2 read()
Name
v4l2-read -- Read from a V4L2 device
Synopsis
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
Arguments
fdFile descriptor returned by
open().bufcount
Description
read() attempts to read up tocount bytes from file descriptorfd into the buffer starting atbuf. The layout of the data in the buffer
isdiscussed in the respective device interface section, see ##. If
countread() returns zero and has no other results. Ifcount is greater thanSSIZE_MAX, the result is unspecified. Regardlessof thecount
value eachread() call will provide at most one frame (twofields) worth of data.
By default read() blocks until databecomes available. When theO_NONBLOCK flag wasgiven to theopen()
function itreturns immediately with anEAGAIN error code when no data is available. Theselect() orpoll()
functionscan always be used to suspend execution until data becomes available. Alldrivers supporting theread() function must alsosupportselect() andpoll().
Drivers can implement read functionality in differentways, using a
single or multiple buffers and discarding the oldest ornewest frames
once the internal buffers are filled.
read() never returns a "snapshot" of
abuffer being filled. Using a single buffer the driver will
stopcapturing when the application starts reading the buffer until
theread is finished. Thus only the period of the vertical
blankinginterval
is available for reading, or the capture rate must fall belowthe
nominal frame rate of the video standard.
The behavior ofread() when called
during the active pictureperiod or the vertical blanking separating the
top and bottom fielddepends on the discarding policy. A driver
discarding the oldestframes keeps capturing into an internal
buffer, continuouslyoverwriting the previously, not read frame, and
returns the framebeing received at the time of theread() call assoon as it is complete.
A driver discarding the newest frames stops capturing untilthe next
read()read()
time is discarded, returning the followingframe instead. Again this
implies a reduction of the capture rate toone half or less of the
nominal frame rate. An example of this
modelis the video read mode of the bttv driver, initiating a DMA to
usermemory whenread() is called and returning whenthe DMA finished.
In the multiple buffer model drivers maintain a ring ofinternal
buffers, automatically advancing to the next free buffer.This allows
continuous capturing when the application can empty thebuffers fast
enough. Again, the behavior when the driver runs out
offree buffers depends on the discarding policy.
Applications can get and set the number of buffers usedinternally by the driver with theVIDIOC_G_PARM andVIDIOC_S_PARMioctls.
They are optional, however. The discarding policy is notreported and cannot be changed. For minimum requirements seeChapter 4.
Return Value
On success, the number of bytes read is returned. It is notan error
if this number is smaller than the number of bytes requested,or the
amount of data required for one frame. This may happen forexample
becauseread() was interrupted
by asignal. On error, -1 is returned, and theerrnovariable is set appropriately. In this case the next read will startat the beginning of a new frame. Possible error codes are:
- EAGAIN
Non-blocking I/O has been selected usingO_NONBLOCK and no data was immediately available for reading.
- EBADF
fdis not a valid filedescriptor or is not open for reading, or the process already has themaximum number of files open.- EBUSY
The driver does not support multiple read streams and thedevice is already in use.
- EFAULT
bufreferences an inaccessiblememory area.- EINTR
The call was interrupted by a signal before anydata was read.
- EIO
I/O error. This indicates some hardware problem or afailure to communicate with a remote device (USB camera etc.).
- EINVAL
The
read()function is notsupported by this driver, not on this device, or generally not on thistype of device.
V4L2 select()
Name
v4l2-select -- Synchronous I/O multiplexing
Synopsis
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
Description
With the select() function applicationscan suspend execution until the driver has captured data or is readyto accept data for output.
When streaming I/O has been negotiated this function waitsuntil a buffer has been filled or displayed and can be dequeued withtheVIDIOC_DQBUF ioctl. When buffers are already in the outgoingqueue of the driver the function returns immediately.
On success select() returns the totalnumber of bits set in thefd_sets. When thefunction timed out it returns a value of zero. On failure it returns-1 and theerrnovariable is set appropriately. When the application did not callVIDIOC_QBUF orVIDIOC_STREAMON yet theselect() function succeeds, setting the bit ofthe file descriptor inreadfds orwritefds, but subsequentVIDIOC_DQBUF callswill fail.[26]
When use of the read() function hasbeen negotiated and the driver does not capture yet, theselect() function starts capturing. When thatfails,select() returns successful and asubsequent read() call, which also attempts tostart capturing, will return an appropriate error code. When thedriver captures continuously (as opposed to, for example, stillimages) and data is already available theselect() function returns immediately.
When use of the write() function hasbeen negotiated theselect() function just waitsuntil the driver is ready for a non-blockingwrite() call.
All drivers implementing the read() orwrite() function or streaming I/O must alsosupport theselect() function.
For more details see the select()manual page.
Return Value
On success, select() returns the numberof descriptors contained in the three returned descriptor sets, whichwill be zero if the timeout expired. On error-1 is returned, and theerrno variable is set appropriately; the sets andtimeout are undefined. Possible error codesare:
- EBADF
One or more of the file descriptor sets specified afile descriptor that is not open.
- EBUSY
The driver does not support multiple read or writestreams and the device is already in use.
- EFAULT
The
readfds,writefds,exceptfdsortimeoutpointer references an inaccessible memoryarea.- EINTR
The call was interrupted by a signal.
- EINVAL
The
nfdsargument is less thanzero or greater thanFD_SETSIZE.
V4L2 write()
Name
v4l2-write -- Write to a V4L2 device
Synopsis
#include <unistd.h>
ssize_t write(int fd, void *buf, size_t count);
Arguments
fdFile descriptor returned by
open().bufcount
Description
write() writes up tocount bytes to the device referenced by thefile descriptorfd from the buffer starting atbuf. When the hardware
outputs are not activeyet, this function enables them. Whencount iszero,write() returns0 without any other effect.
When the application does not provide more data in time, theprevious
video frame, raw VBI image, sliced VPS or WSS data isdisplayed again.
Sliced Teletext or Closed Caption data is notrepeated, the driver
inserts a blank line instead.
Return Value
On success, the number of bytes written are returned. Zeroindicates nothing was written. On error,-1is returned, and theerrno variable is setappropriately. In this case the next write will start
at the beginningof a new frame. Possible error codes are:
- EAGAIN
Non-blocking I/O has been selected using the
O_NONBLOCKflag and nobuffer space was available to write the data immediately.- EBADF
fdis not a valid filedescriptor or is not open for writing.- EBUSY
The driver does not support multiple write streams and thedevice is already in use.
- EFAULT
bufreferences an inaccessiblememory area.- EINTR
The call was interrupted by a signal before anydata was written.
- EIO
I/O error. This indicates some hardware problem.
- EINVAL
The
write()function is notsupported by this driver, not on this device, or generally not on thistype of device.
Chapter 5. V4L2 Driver Programming
to do
Chapter 6. Changes
The following chapters document the evolution of the V4L2 API,errata
or extensions. They are also intended to help application anddriver
writers to port or update their code.
6.1. Differences between V4L and V4L2
The Video For Linux API was first introduced in Linux 2.1 tounify and
replace various TV and radio device related interfaces,developed
independently by driver writers in prior years. Startingwith Linux 2.5
the much improved V4L2 API replaces the V4L API,although
existing drivers will continue to support V4L applications inthe
future, either directly or through the V4L2 compatibility layer inthevideodev kernel module translating ioctls onthe fly. For a transition period not all drivers will
support the V4L2API.
6.1.1. Opening and Closing Devices
For compatibility reasons the character device file namesrecommended
for V4L2 video capture, overlay, radio, teletext and rawvbi capture
devices did not change from those used by V4L. They arelisted inChapter
4 and below inTable 6-1.
The V4L videodev module
automaticallyassigns minor numbers to drivers in load order, depending
on theregistered device type. We recommend that V4L2 drivers by
defaultregister devices with the same numbers, but the system
administratorcan
assign arbitrary minor numbers using driver module options. Themajor
device number remains 81.
Table 6-1. V4L Device Types, Names and Numbers
| Video capture and overlay | /dev/video and/dev/bttv0,/dev/video0 to/dev/video63 | 0-63 |
| Radio receiver | /dev/radio, /dev/radio0 to/dev/radio63 | 64-127 |
| Teletext decoder | /dev/vtx,/dev/vtx0 to/dev/vtx31 | 192-223 |
| Raw VBI capture | /dev/vbi,/dev/vbi0 to/dev/vbi31 | 224-255 |
| Notes: a. According toDocumentation/devices.txt these should be symbolic links to/dev/video0. Note the original bttv interface isnot compatible with V4L or V4L2. b. According toDocumentation/devices.txt a symbolic link to/dev/radio0. |
V4L prohibits (or used to prohibit) multiple opens of adevice file. V4L2 driversmay support multipleopens, seeSection 1.1 for
details and consequences.
V4L drivers respond to V4L2 ioctls with an EINVAL error code. Thecompatibility layer in the V4L2videodev modulecan translate V4L ioctl requests to their V4L2 counterpart, however aV4L2 driver usually
needs more preparation to become fully V4Lcompatible. This is covered in more detail inChapter 5.
6.1.2. Querying Capabilities
The V4L VIDIOCGCAP ioctl isequivalent to V4L2's VIDIOC_QUERYCAP.
The name field in structvideo_capability becamecard in struct v4l2_capability,type
was replaced bycapabilities. Note V4L2
does notdistinguish between device types like this, better think of
basicvideo input, video output and radio devices supporting a set
ofrelated functions like video capturing, video overlay
and VBIcapturing. See Section 1.1 for anintroduction.
VID_TYPE_CAPTURE | V4L2_CAP_VIDEO_CAPTURE | The videocapture interface is supported. |
VID_TYPE_TUNER | V4L2_CAP_TUNER | The device has a tuner ormodulator. |
VID_TYPE_TELETEXT | V4L2_CAP_VBI_CAPTURE | The raw VBIcapture interface is supported. |
VID_TYPE_OVERLAY | V4L2_CAP_VIDEO_OVERLAY | The videooverlay interface is supported. |
VID_TYPE_CHROMAKEY | V4L2_FBUF_CAP_CHROMAKEY infield | Whether chromakey overlay is supported. Formore information on overlay seeSection 4.2. |
VID_TYPE_CLIPPING | V4L2_FBUF_CAP_LIST_CLIPPING and capability of struct v4l2_framebuffer | Whether clipping the overlaid image issupported, see Section 4.2. |
VID_TYPE_FRAMERAM | V4L2_FBUF_CAP_EXTERNOVERLAYnot set in field capability of struct v4l2_framebuffer | Whether overlay overwrites frame buffer memory,see Section 4.2. |
VID_TYPE_SCALES | - | This flag indicates if the hardware can scaleimages. The V4L2 API implies the scale factor by setting the croppingdimensions and image size with the VIDIOC_S_CROPand VIDIOC_S_FMTioctl, respectively. The driver returns the closest sizes possible.For more information on cropping and scaling seeSection1.11. |
VID_TYPE_MONOCHROME | - | Applications can enumerate the supported imageformats with the VIDIOC_ENUM_FMT ioctl to determine if the devicesupports grey scale capturing only. For more information on imageformats seeChapter 2. |
VID_TYPE_SUBCAPTURE | - | Applications can call the VIDIOC_G_CROP ioctlto determine if thedevice supports capturing a subsection of the fullpicture ("cropping" in V4L2). If not, the ioctl returns theEINVAL error code.For more information on cropping and scaling seeSection 1.11. |
VID_TYPE_MPEG_DECODER | - | Applications can enumerate the supported imageformats with the VIDIOC_ENUM_FMT ioctl to determine if the devicesupports MPEG streams. |
VID_TYPE_MPEG_ENCODER | - | See above. |
VID_TYPE_MJPEG_DECODER | - | See above. |
VID_TYPE_MJPEG_ENCODER | - | See above. |
The audios field was replacedby
capabilitiesV4L2_CAP_AUDIO, indicatingif the device has any audio inputs or outputs. Todetermine their number applications can enumerate audio inputs withtheVIDIOC_G_AUDIO
ioctl. The audio ioctls are described inSection 1.5.
The maxwidth,maxheight,minwidth andminheight fields were removed. Calling theVIDIOC_S_FMT
or VIDIOC_TRY_FMT ioctl with the desired
dimensionsreturns the closest size possible, taking into account the
currentvideo standard, cropping and scaling limitations.
6.1.3. Video Sources
V4L provides the VIDIOCGCHAN andVIDIOCSCHAN ioctl using structvideo_channel to enumeratethe video inputs of a V4L device. The equivalent V4L2 ioctlsareVIDIOC_ENUMINPUT,VIDIOC_G_INPUT
andVIDIOC_S_INPUTusing struct v4l2_input as discussed inSection
1.4.
The channel field countinginputs was renamed toindex, the videoinput types were renamed as follows:
VIDEO_TYPE_TV | V4L2_INPUT_TYPE_TUNER |
VIDEO_TYPE_CAMERA | V4L2_INPUT_TYPE_CAMERA |
Unlike the tuners fieldexpressing
the number of tuners of this input, V4L2 assumes each videoinput is
connected to at most one tuner. However a tuner can have morethan one
input, i. e. RF connectors, and a device can have
multipletuners. The index number of the tuner associated with the
input, ifany, is stored in fieldtuner ofstruct v4l2_input. Enumeration of tuners
is discussed inSection 1.6.
The redundant VIDEO_VC_TUNER flag wasdropped. Video inputs associated with a tuner are of typeV4L2_INPUT_TYPE_TUNER. TheVIDEO_VC_AUDIO flag was replaced by theaudioset
field. V4L2 considers devices withup to 32 audio inputs. Each set bit in theaudioset
field represents one audio inputthis video input combines with. For
information about audio inputs andhow to switch between them seeSection
1.5.
The norm field describing thesupported video standards was replaced bystd. The V4L specification mentions a flagVIDEO_VC_NORM indicating whether the standard
canbe changed. This flag was a later addition together with thenorm field and has been removed in themeantime. V4L2 has a similar, albeit more comprehensive approachto video standards, seeSection
1.7 for moreinformation.
6.1.4. Tuning
The V4L VIDIOCGTUNER andVIDIOCSTUNER ioctl and structvideo_tuner can be used to enumerate thetuners of a V4L TV or radio device. The equivalent V4L2 ioctls areVIDIOC_G_TUNER
and VIDIOC_S_TUNER using struct v4l2_tuner. Tuners arecovered inSection 1.6.
The tuner field counting tunerswas renamed toindex. The fieldsname,rangelowandrangehigh remained
unchanged.
The VIDEO_TUNER_PAL,VIDEO_TUNER_NTSC andVIDEO_TUNER_SECAM flags indicating the supportedvideo standards were dropped. This information is now contained in theassociated
struct v4l2_input. No replacement exists for theVIDEO_TUNER_NORM flag indicating whether thevideo standard can be switched. Themodefield
to select a different video standard was replaced by a whole newset of ioctls and structures described inSection 1.7.Due to its ubiquity it should be mentioned the BTTV driver supportsseveral
standards in addition to the regularVIDEO_MODE_PAL (0),VIDEO_MODE_NTSC,VIDEO_MODE_SECAM andVIDEO_MODE_AUTO (3). Namely N/PAL Argentina,M/PAL,
N/PAL, and NTSC Japan with numbers 3-6 (sic).
The VIDEO_TUNER_STEREO_ON flagindicating stereo reception becameV4L2_TUNER_SUB_STEREO in fieldrxsubchans. This field also permits thedetection of monaural and bilingual
audio, see the definition ofstruct v4l2_tuner for details. Presently no replacement exists for theVIDEO_TUNER_RDS_ON andVIDEO_TUNER_MBS_ON
flags.
The VIDEO_TUNER_LOW flag was renamedto
V4L2_TUNER_CAP_LOWcapability field.
The VIDIOCGFREQ andVIDIOCSFREQ ioctl to change the tuner frequencywhere renamed toVIDIOC_G_FREQUENCY
andVIDIOC_S_FREQUENCY. Theytake a pointer to a struct v4l2_frequency
instead of an unsigned longinteger.
6.1.5. Image Properties
V4L2 has no equivalent of theVIDIOCGPICT and
VIDIOCSPICTvideo_picture. The followingfields where replaced by V4L2 controls accessible with theVIDIOC_QUERYCTRL,VIDIOC_G_CTRL
andVIDIOC_S_CTRL ioctls:
brightness | V4L2_CID_BRIGHTNESS |
hue | V4L2_CID_HUE |
colour | V4L2_CID_SATURATION |
contrast | V4L2_CID_CONTRAST |
whiteness | V4L2_CID_WHITENESS |
The V4L picture controls are assumed to range from 0 to65535 with no
particular reset value. The V4L2 API permits arbitrarylimits and
defaults which can be queried with theVIDIOC_QUERYCTRLioctl.
For general information about controls seeSection 1.8.
The depth (average number ofbits per
pixel) of a video image is implied by the selected imageformat. V4L2
does not explicitely provide such information assumingapplications
recognizing the format are aware of the image depth
andothers need not know. The palette fieldmoved into the struct v4l2_pix_format:
VIDEO_PALETTE_GREY | |
VIDEO_PALETTE_HI240 | |
VIDEO_PALETTE_RGB565 | |
VIDEO_PALETTE_RGB555 | |
VIDEO_PALETTE_RGB24 | |
VIDEO_PALETTE_RGB32 | |
VIDEO_PALETTE_YUV422 | |
| |
VIDEO_PALETTE_UYVY | |
VIDEO_PALETTE_YUV420 | None |
VIDEO_PALETTE_YUV411 | |
VIDEO_PALETTE_RAW | None |
VIDEO_PALETTE_YUV422P | |
VIDEO_PALETTE_YUV411P | |
VIDEO_PALETTE_YUV420P | |
VIDEO_PALETTE_YUV410P | |
| Notes: a. This is a custom format used by the BTTVdriver, not one of the V4L2 standard formats. b. Presumably all V4L RGB formats arelittle-endian, although some drivers might interpret them according to machine endianess. V4L2 defines little-endian, big-endian and red/blueswapped variants. For details seeSection 2.4. c. VIDEO_PALETTE_YUV422andVIDEO_PALETTE_YUYV are the same formats. SomeV4L drivers respond to one, some to the other.d. Not to be confused with V4L2_PIX_FMT_YUV411P, which is a planarformat.e. V4L explains thisas: "RAW capture (BT848)" f. Not to be confused with V4L2_PIX_FMT_Y41P, which is a packedformat. |
V4L2 image formats are defined in Chapter 2. The image format can be selected with theVIDIOC_S_FMT ioctl.
6.1.6. Audio
The VIDIOCGAUDIO andVIDIOCSAUDIO ioctl and structvideo_audio are used to enumerate theaudio inputs of a V4L device. The equivalent V4L2 ioctls areVIDIOC_G_AUDIO
and VIDIOC_S_AUDIO using struct v4l2_audio asdiscussed inSection 1.5.
The audio "channel number"field counting audio inputs was renamed toindex.
On VIDIOCSAUDIO themode field selectsoneof theVIDEO_SOUND_MONO,VIDEO_SOUND_STEREO,VIDEO_SOUND_LANG1
orVIDEO_SOUND_LANG2 audio demodulation modes. Whenthe current audio standard is BTSCVIDEO_SOUND_LANG2 refers to SAP andVIDEO_SOUND_LANG1 is meaningless. Alsoundocumented
in the V4L specification, there is no way to query theselected mode. OnVIDIOCGAUDIO the driver returnstheactually received audio programmes in thisfield. In the V4L2 API
this information is stored in the struct v4l2_tunerrxsubchans andaudmode fields, respectively. SeeSection
1.6 for more information on tuners. Related to audiomodes struct v4l2_audio also reports if this is a mono or stereoinput, regardless if the source is a tuner.
The following fields where replaced by V4L2 controlsaccessible with the VIDIOC_QUERYCTRL, VIDIOC_G_CTRL andVIDIOC_S_CTRL ioctls:
volume | V4L2_CID_AUDIO_VOLUME |
bass | V4L2_CID_AUDIO_BASS |
treble | V4L2_CID_AUDIO_TREBLE |
balance | V4L2_CID_AUDIO_BALANCE |
To determine which of these controls are supported by adriver V4L provides theflagsVIDEO_AUDIO_VOLUME,VIDEO_AUDIO_BASS,VIDEO_AUDIO_TREBLE
andVIDEO_AUDIO_BALANCE. In the V4L2 API theVIDIOC_QUERYCTRL ioctl reports if the respective control issupported.
Accordingly the VIDEO_AUDIO_MUTABLEand
VIDEO_AUDIO_MUTEV4L2_CID_AUDIO_MUTE control.
All V4L2 controls have a stepattribute replacing the structvideo_audiostep field. The V4L audio controls areassumed to range from 0 to 65535 with no particular
reset value. TheV4L2 API permits arbitrary limits and defaults which can be queriedwith theVIDIOC_QUERYCTRL ioctl. For general information aboutcontrols
seeSection 1.8.
6.1.7. Frame Buffer Overlay
The V4L2 ioctls equivalent toVIDIOCGFBUF and
VIDIOCSFBUFVIDIOC_G_FBUF and VIDIOC_S_FBUF. Thebase field of structvideo_buffer remained unchanged, except V4L2defines a flag to indicate non-destructive overlays instead of aNULL
pointer. All other fields moved into thestruct v4l2_pix_formatfmt substructure ofstruct v4l2_framebuffer.
Thedepth field wasreplaced by
pixelformat
Instead of the special ioctlsVIDIOCGWIN and
VIDIOCSWINVIDIOC_G_FMT andVIDIOC_S_FMT.
They take a pointer to astruct v4l2_format as argument. Here thewinmember of thefmt union is used, astruct v4l2_window.
The x,y,
widthheight fields of structvideo_window moved into struct v4l2_rectsubstructurew
of structv4l2_window. Thechromakey,clips, andclipcount fields remained unchanged. Structvideo_clip
was renamed to struct v4l2_clip, alsocontaining a structv4l2_rect, but thesemantics are still the same.
The VIDEO_WINDOW_INTERLACE flag wasdropped. Instead applications must set thefield field toV4L2_FIELD_ANY orV4L2_FIELD_INTERLACED.
TheVIDEO_WINDOW_CHROMAKEY flag moved intostruct v4l2_framebuffer, under the new nameV4L2_FBUF_FLAG_CHROMAKEY.
In V4L, storing a bitmap pointer inclips and settingclipcount toVIDEO_CLIP_BITMAP (-1) requests bitmapclipping, using a fixed size bitmap of 1024 × 625 bits.
Structv4l2_window has a separatebitmap pointer field for this purpose andthe bitmap size is determined byw.width andw.height.
The VIDIOCCAPTURE ioctl to enable ordisable overlay was renamed toVIDIOC_OVERLAY.
6.1.8. Cropping
To capture only a subsection of the full picture V4Ldefines the
VIDIOCGCAPTUREVIDIOCSCAPTURE ioctls using structvideo_capture. The equivalent V4L2 ioctls areVIDIOC_G_CROP
and VIDIOC_S_CROP using struct v4l2_crop, and the relatedVIDIOC_CROPCAP
ioctl. This is a rather complex matter, seeSection 1.11 for details.
The x,y,
widthheight fields moved into struct v4l2_rectsubstructurec of structv4l2_crop.
Thedecimation field was dropped. In
the V4L2API the scaling factor is implied by the size of the
croppingrectangle and the size of the captured or overlaid image.
The VIDEO_CAPTURE_ODDand
VIDEO_CAPTURE_EVENV4L2_FIELD_TOP andV4L2_FIELD_BOTTOM in the field namedfield of struct v4l2_pix_format
andstruct v4l2_window. These structures are used to select a capture oroverlay format with theVIDIOC_S_FMT
ioctl.
6.1.9. Reading Images, Memory Mapping
6.1.9.1. Capturing using the read method
There is no essential difference between reading imagesfrom a V4L or V4L2 device using theread() function, however V4L2drivers are not required to
support this I/O method. Applications candetermine if the function is available with theVIDIOC_QUERYCAPioctl. All V4L2 devices exchanging data
with applications must supporttheselect() andpoll()
functions.
To select an image format and size, V4L provides theVIDIOCSPICT andVIDIOCSWINioctls. V4L2 uses the general-purpose data format negotiation ioctlsVIDIOC_G_FMT
andVIDIOC_S_FMT. They take a pointer to astruct v4l2_format as argument,
here the struct v4l2_pix_format namedpix of itsfmtunion is used.
For more information about the V4L2 read interface seeSection 3.1.
6.1.9.2. Capturing using memory mapping
Applications can read from V4L devices by mappingbuffers in device
memory, or more often just buffers allocated inDMA-able system memory,
into their address space. This avoids the datacopying overhead of the
read method. V4L2 supports memory mapping aswell,
with a few differences.
The image format must be selected beforebuffers are allocated, with the VIDIOC_S_FMT ioctl. When no formatis selected the driver may use the last, possibly by anotherapplication requested format. | |
Applications cannot change the number ofbuffers. The it is built into | The |
Drivers map all buffers as one contiguousrange of memory. The | Buffers are individually mapped. Theoffset and size of each buffer can be determined with the |
The The | Drivers maintain an incoming and outgoingqueue. |
For a more in-depth discussion of memory mapping andexamples, see Section 3.2.
6.1.10. Reading Raw VBI Data
Originally the V4L API did not specify a raw VBI captureinterface, only the device file/dev/vbi
wasreserved for this purpose. The only driver supporting this
interfacewas the BTTV driver, de-facto defining the V4L VBI interface.
Readingfrom the device yields a raw VBI image with the
followingparameters:
| sampling_rate | 28636363 Hz NTSC (or any other 525-linestandard); 35468950 Hz PAL and SECAM (625-line standards) |
| offset | ? |
| samples_per_line | 2048 |
| sample_format | V4L2_PIX_FMT_GREY. The last four bytes (amachine endianess integer) contain a frame counter. |
| start[] | 10, 273 NTSC; 22, 335 PAL and SECAM |
| count[] | 16, 16 |
| flags | 0 |
| Notes: a. Old driverversions used different values, eventually the custom BTTV_VBISIZE ioctl was added to query thecorrect values. |
Undocumented in the V4L specification, in Linux 2.3 theVIDIOCGVBIFMT andVIDIOCSVBIFMT ioctls using structvbi_format were added to determine the VBIimage parameters.
These ioctls are only partially compatible with theV4L2 VBI interface specified inSection 4.7.
An offset field does notexist,
sample_formatVIDEO_PALETTE_RAW, equivalent toV4L2_PIX_FMT_GREY. The remaining fields areprobably equivalent to struct v4l2_vbi_format.
Apparently only the Zoran (ZR 36120) driver implementsthese ioctls.
The semantics differ from those specified for V4L2 in twoways. The
parameters are reset onopen()
andVIDIOCSVBIFMT always returns anEINVAL error code if theparameters are invalid.
6.1.11. Miscellaneous
V4L2 has no equivalent of theVIDIOCGUNIT
ioctl. Applications can find the VBIdevice associated with a video
capture device (or vice versa) byreopening the device and requesting VBI
data. For details seeSection
1.1.
No replacement exists for VIDIOCKEY,and the V4L functions for microcode programming. A new interface forMPEG compression and playback devices is documented inSection
1.9.
6.2. Changes of the V4L2 API
Soon after the V4L API was added to the kernel it wascriticised as
too inflexible. In August 1998 Bill Dirks proposed anumber of
improvements and began to work on documentation, exampledrivers and
applications. With the help of other volunteers thiseventually
became the V4L2 API, not just an extension but areplacement for the V4L
API. However it took another four years andtwo stable kernel releases
until the new API was finally accepted forinclusion into the kernel in
its present form.
6.2.1. Early Versions
1998-08-20: First version.
1998-08-27: The select() function was introduced.
1998-09-10: New video standard interface.
1998-09-18: The VIDIOC_NONCAP ioctlwas replaced by the otherwise meaninglessO_TRUNCopen()
flag, and the aliasesO_NONCAP andO_NOIO
were defined. Applications can set thisflag if they intend to access
controls only, as opposed to captureapplications which need exclusive
access. TheVIDEO_STD_XXX
identifiers are now ordinalsinstead of flags, and the video_std_construct()helper function takes id and transmission arguments.
1998-09-28: Revamped video standard. Made video controlsindividually enumerable.
1998-10-02: The id field wasremoved from structvideo_standard and thecolor subcarrier fields were renamed. TheVIDIOC_QUERYSTD
ioctl wasrenamed toVIDIOC_ENUMSTD,VIDIOC_G_INPUT
toVIDIOC_ENUMINPUT. Afirst draft of the Codec API was released.
1998-11-08: Many minor changes. Most symbols have beenrenamed. Some material changes to struct v4l2_capability.
1998-11-12: The read/write directon of some ioctls was misdefined.
1998-11-14: V4L2_PIX_FMT_RGB24changed to
V4L2_PIX_FMT_BGR24V4L2_PIX_FMT_RGB32 changed toV4L2_PIX_FMT_BGR32. Audio controls are nowaccessible with theVIDIOC_G_CTRL
andVIDIOC_S_CTRL ioctls undernames starting withV4L2_CID_AUDIO. TheV4L2_MAJOR define
was removed fromvideodev.h since it was only used once in thevideodev kernel module. TheYUV422 andYUV411 planarimage formats were added.
1998-11-28: A few ioctl symbols changed. Interfaces for codecs andvideo output devices were added.
1999-01-14: A raw VBI capture interface was added.
1999-01-19: The VIDIOC_NEXTBUF ioctl was removed.
6.2.2. V4L2 Version 0.16 1999-01-31
1999-01-27: There is now one QBUF ioctl, VIDIOC_QWBUF and
VIDIOC_QRBUFare gone. VIDIOC_QBUF takes a v4l2_buffer as a parameter.
Addeddigital zoom (cropping) controls.
6.2.3. V4L2 Version 0.18 1999-03-16
Added a v4l to V4L2 ioctl compatibility layer tovideodev.c. Driver
writers, this changes how you implement your ioctlhandler. See the
Driver Writer's Guide. Added some more control idcodes.
6.2.4. V4L2 Version 0.19 1999-06-05
1999-03-18: Fill in the category and catname fields ofv4l2_queryctrl
objects before passing them to the driver. Required aminor change to the
VIDIOC_QUERYCTRL handlers in the sampledrivers.
1999-03-31: Better compatibility for v4l memory captureioctls.
Requires changes to drivers to fully support new compatibilityfeatures,
see Driver Writer's Guide and v4l2cap.c. Added new controlIDs:
V4L2_CID_HFLIP, _VFLIP. Changed V4L2_PIX_FMT_YUV422P to
_YUV422P,and _YUV411P to _YUV411P.
1999-04-04: Added a few more control IDs.
1999-04-07: Added the button control type.
1999-05-02: Fixed a typo in videodev.h, and added theV4L2_CTRL_FLAG_GRAYED (later V4L2_CTRL_FLAG_GRABBED) flag.
1999-05-20: Definition of VIDIOC_G_CTRL was wrong causinga malfunction of this ioctl.
1999-06-05: Changed the value ofV4L2_CID_WHITENESS.
6.2.5. V4L2 Version 0.20 (1999-09-10)
Version 0.20 introduced a number of changes which werenot backward compatible with 0.19 and earlierversions. Purpose of these changes was to simplify the API, whilemaking it more extensible and following
common Linux driver APIconventions.
Some typos in
V4L2_FMT_FLAGsymbols were fixed. struct v4l2_clip was changed for compatibility withv4l. (1999-08-30)V4L2_TUNER_SUB_LANG1was added.(1999-09-05)All ioctl() commands that used an integer argument nowtake a pointer
to an integer. Where it makes sense, ioctls will returnthe actual new
value in the integer pointed to by the argument, acommon convention in
the V4L2 API. The affected ioctls are:VIDIOC_PREVIEW,
VIDIOC_STREAMON, VIDIOC_STREAMOFF, VIDIOC_S_FREQ,VIDIOC_S_INPUT,
VIDIOC_S_OUTPUT, VIDIOC_S_EFFECT. For exampleerr = ioctl (fd, VIDIOC_XXX, V4L2_XXX);
becomes
int a = V4L2_XXX; err = ioctl(fd, VIDIOC_XXX, &a);
All the different get- and set-format commands wereswept into one
VIDIOC_G_FMTandVIDIOC_S_FMTioctl taking a unionand a type field selecting the union member as parameter. Purpose is tosimplify the API by eliminating several ioctls and to allow new anddriver private data streams without adding new ioctls.This change obsoletes the following ioctls:
VIDIOC_S_INFMT,VIDIOC_G_INFMT,VIDIOC_S_OUTFMT,VIDIOC_G_OUTFMT,VIDIOC_S_VBIFMTandVIDIOC_G_VBIFMT. The image format structurev4l2_formatwas renamed to struct v4l2_pix_format,while struct v4l2_format is now the envelopping structure for all formatnegotiations.Similar to the changes above, the
VIDIOC_G_PARMandVIDIOC_S_PARMioctls were merged withVIDIOC_G_OUTPARMandVIDIOC_S_OUTPARM. Atypefield in the new struct v4l2_streamparmselects the respective union member.This change obsoletes the
VIDIOC_G_OUTPARMandVIDIOC_S_OUTPARMioctls.Control enumeration was simplified, and two newcontrol flags were introduced and one dropped. The
catnamefield was replaced by agroupfield.Drivers can now flag unsupported and temporarilyunavailable controls with
V4L2_CTRL_FLAG_DISABLEDandV4L2_CTRL_FLAG_GRABBEDrespectively. Thegroupname indicates a possibly narrowerclassification than thecategory. In otherwords, there may be multiple groups within a category. Controls withina group would typically be drawn within a group box. Controls indifferent categories might have a greater separation, or may evenappear in separate windows.The struct v4l2_buffer
timestampwas changed to a 64 bit integer, containing the sampling or outputtime of the frame in nanoseconds. Additionally timestamps will be inabsolute system time, not starting from zero at the beginning of astream. The data type name for timestamps is stamp_t, defined as asigned 64-bit integer. Output devices should not send a buffer outuntil the time in the timestamp field has arrived. I would like tofollow SGI's lead, and adopt a multimedia timestamping system liketheir UST (Unadjusted System Time). Seehttp://reality.sgi.com/cpirazzi_engr/lg/time/intro.html. [This link isno longer valid.] UST uses timestamps that are 64-bit signed integers(not struct timeval's) and given in nanosecond units. The UST clockstarts at zero when the system is booted and runs continuously anduniformly. It takes a little over 292 years for UST to overflow. Thereis no way to set the UST clock. The regular Linux time-of-day clockcan be changed periodically, which would cause errors if it were beingused for timestamping a multimedia stream. A real UST style clock willrequire some support in the kernel that is not there yet. But inanticipation, I will change the timestamp field to a 64-bit integer,and I will change the v4l2_masterclock_gettime() function (used onlyby drivers) to return a 64-bit integer.A
sequencefield was addedto struct v4l2_buffer. Thesequencefield countscaptured frames, it is ignored by output devices. When a capturedriver drops a frame, the sequence number of that frame isskipped.
6.2.6. V4L2 Version 0.20 incremental changes
1999-12-23: In struct v4l2_vbi_format thereserved1 field becameoffset. Previously drivers were required toclear the reserved1 field.
2000-01-13: The V4L2_FMT_FLAG_NOT_INTERLACED flag was added.
2000-07-31: The linux/poll.h headeris now included byvideodev.h for compatibilitywith the originalvideodev.h file.
2000-11-20: V4L2_TYPE_VBI_OUTPUT andV4L2_PIX_FMT_Y41P were added.
2000-11-25: V4L2_TYPE_VBI_INPUT wasadded.
2000-12-04: A couple typos in symbol names were fixed.
2001-01-18: To avoid namespace conflicts thefourcc macro defined in thevideodev.h header file was renamed tov4l2_fourcc.
2001-01-25: A possible driver-level compatibility problembetween the videodev.h file in Linux 2.4.0 andthe videodev.h file included in thevideodevX patch was fixed. Users of an earlierversion ofvideodevX on Linux 2.4.0 shouldrecompile their V4L and V4L2 drivers.
2001-01-26: A possible kernel-level incompatibilitybetween the videodev.h file in thevideodevX patch and thevideodev.h file in Linux 2.2.x with devfs patchesapplied was fixed.
2001-03-02: Certain V4L ioctls which pass data in bothdirection although they are defined with read-only parameter, did notwork correctly through the backward compatibility layer.[Solution?]
2001-04-13: Big endian 16-bit RGB formats were added.
2001-09-17: New YUV formats and the VIDIOC_G_FREQUENCY andVIDIOC_S_FREQUENCY ioctls were added. (The oldVIDIOC_G_FREQ andVIDIOC_S_FREQ ioctls did not take multiple tunersinto account.)
2000-09-18: V4L2_BUF_TYPE_VBI wasadded. This maybreak compatibility as theVIDIOC_G_FMT andVIDIOC_S_FMT ioctls may fail now if the structv4l2_fmttypefield does not containV4L2_BUF_TYPE_VBI. In thedocumentation of the struct v4l2_vbi_formatoffset field the ambiguous phrase "risingedge" was changed to "leading edge".
6.2.7. V4L2 Version 0.20 2000-11-23
A number of changes were made to the raw VBIinterface.
Figures clarifying the line numbering scheme wereadded to the V4L2 API specification. The
start[0] andstart[1] fields no longer count linenumbers beginning at zero. Rationale: a) The previous definition wasunclear. b) Thestart[] values are ordinalnumbers. c) There is no point in inventing a new line numberingscheme. We now use line number as defined by ITU-R, period.Compatibility: Add one to the start values. Applications depending onthe previous semantics may not function correctly.The restriction "count[0] > 0 and count[1] > 0"has been relaxed to "(count[0] + count[1]) > 0". Rationale:Drivers may allocate resources at scan line granularity and some dataservices are transmitted only on the first field. The comment thatboth
countvalues will usually be equal ismisleading and pointless and has been removed. This changebreaks compatibility with earlier versions:Drivers may return EINVAL, applications may not functioncorrectly.Drivers are again permitted to return negative(unknown) start values as proposed earlier. Why this feature wasdropped is unclear. This change maybreakcompatibility with applications depending on the startvalues being positive. The use of
EBUSYandEINVALerror codes with theVIDIOC_S_FMTioctlwas clarified. TheEBUSY error code was finally documented, and thereserved2field which was previouslymentioned only in thevideodev.h headerfile.New buffer types
V4L2_TYPE_VBI_INPUTandV4L2_TYPE_VBI_OUTPUTwere added. The former is analias for the oldV4L2_TYPE_VBI, the latter wasmissing in thevideodev.h file.
6.2.8. V4L2 Version 0.20 2002-07-25
Added sliced VBI interface proposal.
6.2.9. V4L2 in Linux 2.5.46, 2002-10
Around October-November 2002, prior to an announcedfeature freeze of Linux 2.5, the API was revised, drawing fromexperience with V4L2 0.20. This unnamed version was finally mergedinto Linux 2.5.46.
As specified in Section 1.1.2, driversmust make related device functions available under all minor devicenumbers.
The
open()function requires access modeO_RDWRregardless of the device type. All V4L2drivers exchanging data with applications must support theO_NONBLOCKflag. TheO_NOIOflag, a V4L2 symbol which aliased the meaninglessO_TRUNCto indicate accesses without dataexchange (panel applications) was dropped. Drivers must stay in "panelmode" until the application attempts to initiate a data exchange, seeSection 1.1.The struct v4l2_capability changed dramatically. Note thatalso the size of the structure changed, which is encoded in the ioctlrequest code, thus older V4L2 devices will respond with an EINVAL error code tothe new
VIDIOC_QUERYCAPioctl.There are new fields to identify the driver, a new (asof yet unspecified) device function
V4L2_CAP_RDS_CAPTURE, theV4L2_CAP_AUDIOflag indicates if the device hasany audio connectors, another I/O capabilityV4L2_CAP_ASYNCIOcan be flagged. In response tothese changes thetypefield became a bitset and was merged into theflagsfield.V4L2_FLAG_TUNERwas renamed toV4L2_CAP_TUNER,V4L2_CAP_VIDEO_OVERLAYreplacedV4L2_FLAG_PREVIEWandV4L2_CAP_VBI_CAPTUREandV4L2_CAP_VBI_OUTPUTreplacedV4L2_FLAG_DATA_SERVICE.V4L2_FLAG_READandV4L2_FLAG_WRITEwere merged intoV4L2_CAP_READWRITE.The redundant fields
inputs,outputsandaudioswere removed. These propertiescan be determined as described inSection 1.4 andSection 1.5.The somewhat volatile and therefore barely usefulfields
maxwidth,maxheight,minwidth,minheight,maxframeratewere removed. This informationis available as described inSection 1.10 andSection 1.7.V4L2_FLAG_SELECTwas removed. Webelieve the select() function is important enough to require supportof it in all V4L2 drivers exchanging data with applications. TheredundantV4L2_FLAG_MONOCHROMEflag was removed,this information is available as described inSection 1.10.In struct v4l2_input the
assoc_audiofield and thecapabilityfield and its only flagV4L2_INPUT_CAP_AUDIOwas replaced by the newaudiosetfield. Instead of linking onevideo input to one audio input this field reports all audio inputsthis video input combines with.New fields are
tuner(reversing the former link from tuners to video inputs),stdandstatus.Accordingly struct v4l2_output lost its
capabilityandassoc_audiofields.audioset,modulatorandstdwhere added instead.The struct v4l2_audio field
audiowas renamed toindex, for consistency with otherstructures. A new capability flagV4L2_AUDCAP_STEREOwas added to indicated if theaudio input in question supports stereo sound.V4L2_AUDCAP_EFFECTSand the correspondingV4L2_AUDMODEflags where removed. This can beeasily implemented using controls. (However the same applies to AVLwhich is still there.)Again for consistency the struct v4l2_audioout field
audiowas renamed toindex.The struct v4l2_tuner
inputfield was replaced by anindexfield, permitting devices withmultiple tuners. The link between video inputs and tuners is nowreversed, inputs point to their tuner. Thestdsubstructure became asimple set (more about this below) and moved into struct v4l2_input. Atypefield was added.Accordingly in struct v4l2_modulator the
outputwas replaced by anindexfield.In struct v4l2_frequency the
portfield was replaced by atunerfield containing the respective tuneror modulator index number. A tunertypefield was added and thereservedfieldbecame larger for future extensions (satellite tuners inparticular).The idea of completely transparent video standards wasdropped. Experience showed that applications must be able to work withvideo standards beyond presenting the user a menu. Instead ofenumerating supported standards with an ioctl applications can nowrefer to standards by v4l2_std_id and symbols defined in thevideodev2.h header file. For details seeSection 1.7. The
VIDIOC_G_STDandVIDIOC_S_STDnow take a pointer to this type as argument.VIDIOC_QUERYSTDwas added to autodetect the received standard, ifthe hardware has this capability. In struct v4l2_standard anindexfield was added forVIDIOC_ENUMSTD.Av4l2_std_id field namedidwas added asmachine readable identifier, also replacing thetransmissionfield. The misleadingframeratefield was renamedtoframeperiod. The now obsoletecolorstandardinformation, originallyneeded to distguish between variations of standards, wereremoved.Struct
v4l2_enumstdceased tobe.VIDIOC_ENUMSTDnow takes a pointer to a struct v4l2_standarddirectly. The information which standards are supported by aparticular video input or output moved into struct v4l2_input andstruct v4l2_output fields namedstd,respectively.The struct v4l2_queryctrl fields
categoryandgroupdid not catch on and/or were notimplemented as expected and therefore removed.The
VIDIOC_TRY_FMTioctl was added to negotiate dataformats as withVIDIOC_S_FMT, but without the overhead ofprogramming the hardware and regardless of I/O in progress.In struct v4l2_format the
fmtunion was extended to contain struct v4l2_window. All image formatnegotiations are now possible withVIDIOC_G_FMT,VIDIOC_S_FMTandVIDIOC_TRY_FMT; ioctl. TheVIDIOC_G_WINandVIDIOC_S_WINioctls to prepare for a videooverlay were removed. Thetypefieldchanged to type enum v4l2_buf_type and the buffer type names changed asfollows.V4L2_BUF_TYPE_CAPTUREV4L2_BUF_TYPE_VIDEO_CAPTUREV4L2_BUF_TYPE_CODECINOmitted for now V4L2_BUF_TYPE_CODECOUTOmitted for now V4L2_BUF_TYPE_EFFECTSINOmitted for now V4L2_BUF_TYPE_EFFECTSIN2Omitted for now V4L2_BUF_TYPE_EFFECTSOUTOmitted for now V4L2_BUF_TYPE_VIDEOOUTV4L2_BUF_TYPE_VIDEO_OUTPUT-V4L2_BUF_TYPE_VIDEO_OVERLAY-V4L2_BUF_TYPE_VBI_CAPTURE-V4L2_BUF_TYPE_VBI_OUTPUT-V4L2_BUF_TYPE_SLICED_VBI_CAPTURE-V4L2_BUF_TYPE_SLICED_VBI_OUTPUTV4L2_BUF_TYPE_PRIVATE_BASEV4L2_BUF_TYPE_PRIVATEIn struct v4l2_fmtdesc a enum v4l2_buf_type field named
typewas added as in struct v4l2_format. TheVIDIOC_ENUM_FBUFFMTioctl is no longer needed andwas removed. These calls can be replaced byVIDIOC_ENUM_FMTwithtypeV4L2_BUF_TYPE_VIDEO_OVERLAY.In struct v4l2_pix_format the
depthfield was removed, assumingapplications which recognize the format by its four-character-codealready know the color depth, and others do not care about it. Thesame rationale lead to the removal of theV4L2_FMT_FLAG_COMPRESSEDflag. TheV4L2_FMT_FLAG_SWCONVECOMPRESSEDflag was removedbecause drivers are not supposed to convert images in kernel space. Auser library of conversion functions should be provided instead. TheV4L2_FMT_FLAG_BYTESPERLINEflag was redundant.Applications can set thebytesperlinefieldto zero to get a reasonable default. Since the remaining flags werereplaced as well, theflagsfield itselfwas removed.The interlace flags were replaced by a enum v4l2_fieldvalue in a newly added
fieldfield.V4L2_FMT_FLAG_NOT_INTERLACED? V4L2_FMT_FLAG_INTERLACED=V4L2_FMT_FLAG_COMBINEDV4L2_FIELD_INTERLACEDV4L2_FMT_FLAG_TOPFIELD=V4L2_FMT_FLAG_ODDFIELDV4L2_FIELD_TOPV4L2_FMT_FLAG_BOTFIELD=V4L2_FMT_FLAG_EVENFIELDV4L2_FIELD_BOTTOM-V4L2_FIELD_SEQ_TB-V4L2_FIELD_SEQ_BT-V4L2_FIELD_ALTERNATEThe color space flags were replaced by aenum v4l2_colorspace value in a newly added
colorspacefield, where one ofV4L2_COLORSPACE_SMPTE170M,V4L2_COLORSPACE_BT878,V4L2_COLORSPACE_470_SYSTEM_MorV4L2_COLORSPACE_470_SYSTEM_BGreplacesV4L2_FMT_CS_601YUV.In struct v4l2_requestbuffers the
typefield was properly defined asenum v4l2_buf_type. Buffer types changed as mentioned above. A newmemoryfield of type enum v4l2_memory wasadded to distinguish between I/O methods using buffers allocatedby the driver or the application. See Chapter 3 fordetails.In struct v4l2_buffer the
typefield was properly defined as enum v4l2_buf_type. Buffer types changed asmentioned above. Afieldfield of typeenum v4l2_field was added to indicate if a buffer contains a top orbottom field. The old field flags were removed. Since no unadjustedsystem time clock was added to the kernel as planned, thetimestampfield changed back from typestamp_t, an unsigned 64 bit integer expressing the sample time innanoseconds, to structtimeval. With theaddition of a second memory mapping method theoffsetfield moved into unionm, and a newmemoryfield of type enum v4l2_memory wasadded to distinguish between I/O methods. SeeChapter 3for details.The
V4L2_BUF_REQ_CONTIGflag was used by the V4L compatibility layer, after changes to thiscode it was no longer needed. TheV4L2_BUF_ATTR_DEVICEMEMflag would indicate ifthe buffer was indeed allocated in device memory rather than DMA-ablesystem memory. It was barely useful and so was removed.In struct v4l2_framebuffer the
base[3]array anticipating double- andtriple-buffering in off-screen video memory, however without defininga synchronization mechanism, was replaced by a single pointer. TheV4L2_FBUF_CAP_SCALEUPandV4L2_FBUF_CAP_SCALEDOWNflags were removed.Applications can determine this capability more accurately using thenew cropping and scaling interface. TheV4L2_FBUF_CAP_CLIPPINGflag was replaced byV4L2_FBUF_CAP_LIST_CLIPPINGandV4L2_FBUF_CAP_BITMAP_CLIPPING.In struct v4l2_clip the
x,y,widthandheightfield moved into acsubstructure of type struct v4l2_rect. Thexandyfieldswere renamed toleftandtop, i. e. offsets to a context dependentorigin.In struct v4l2_window the
x,y,widthandheightfield moved into awsubstructure as above. Afieldfield of type %v4l2-field; was addedto distinguish between field and frame (interlaced) overlay.The digital zoom interface, including struct
v4l2_zoomcap, structv4l2_zoom,V4L2_ZOOM_NONCAPandV4L2_ZOOM_WHILESTREAMINGwas replaced by a newcropping and scaling interface. The previously unused structv4l2_cropcapandv4l2_cropwhere redefined for this purpose.SeeSection 1.11 for details.In struct v4l2_vbi_format the
SAMPLE_FORMATfield now contains afour-character-code as used to identify video image formats andV4L2_PIX_FMT_GREYreplaces theV4L2_VBI_SF_UBYTEdefine. Thereservedfield was extended.In struct v4l2_captureparm the type of the
timeperframefield changed from unsignedlong to struct v4l2_fract. This allows the accurate expression of multiplesof the NTSC-M frame rate 30000 / 1001. A new fieldreadbufferswas added to control the driverbehaviour in read I/O mode.Similar changes were made to struct v4l2_outputparm.
The struct
v4l2_performanceandVIDIOC_G_PERFioctl were dropped. Except whenusing the read/write I/O method, which islimited anyway, this information is already available toapplications.The example transformation from RGB to YCbCr colorspace in the old V4L2 documentation was inaccurate, this has beencorrected inChapter 2.
6.2.10. V4L2 2003-06-19
A new capability flag
V4L2_CAP_RADIOwas added for radio devices. Priorto this change radio devices would identify solely by having exactly onetuner whose type field readsV4L2_TUNER_RADIO.An optional driver access priority mechanism wasadded, see Section 1.3 for details.
The audio input and output interface was found to beincomplete.
Previously the
VIDIOC_G_AUDIOioctl would enumerate the available audio inputs. An ioctl todetermine the current audio input, if more than one combines with thecurrent video input, did not exist. SoVIDIOC_G_AUDIOwas renamed toVIDIOC_G_AUDIO_OLD, this ioctl will be removed inthe future. TheVIDIOC_ENUMAUDIOioctl was added to enumerateaudio inputs, whileVIDIOC_G_AUDIOnow reports the current audioinput.The same changes were made to
VIDIOC_G_AUDOUTandVIDIOC_ENUMAUDOUT.Until further the "videodev" module will automaticallytranslate between the old and new ioctls, but drivers and applicationsmust be updated to successfully compile again.
The
VIDIOC_OVERLAYioctl was incorrectly defined withwrite-read parameter. It was changed to write-only, while the write-readversion was renamed toVIDIOC_OVERLAY_OLD. The oldioctl will be removed in the future. Until further the "videodev"kernel module will automatically translate to the new version, so driversmust be recompiled, but not applications.Section 4.2 incorrectly stated thatclipping rectangles define regions where the video can be seen.Correct is that clipping rectangles define regions whereno video shall be displayed and so the graphicssurface can be seen.
The
VIDIOC_S_PARMandVIDIOC_S_CTRLioctls weredefined with write-only parameter, inconsistent with other ioctlsmodifying their argument. They were changed to write-read, while a_OLDsuffix was added to the write-only versions.The old ioctls will be removed in the future. Drivers andapplications assuming a constant parameter need an update.
6.2.11. V4L2 2003-11-05
In Section 2.4 the following pixelformats were incorrectly transferred from Bill Dirks' V4L2specification. Descriptions below refer to bytes in memory, inascending address order.
V4L2_PIX_FMT_RGB24B, G, R R, G, B V4L2_PIX_FMT_BGR24R, G, B B, G, R V4L2_PIX_FMT_RGB32B, G, R, X R, G, B, X V4L2_PIX_FMT_BGR32R, G, B, X B, G, R, X The
V4L2_PIX_FMT_BGR24example was alwayscorrect.In Section 6.1.5 the mappingof the V4L
VIDEO_PALETTE_RGB24andVIDEO_PALETTE_RGB32formats to V4L2 pixel formatswas accordingly corrected.Unrelated to the fixes above, drivers may stillinterpret some V4L2 RGB pixel formats differently. These issues haveyet to be addressed, for details seeSection 2.4.
6.2.12. V4L2 in Linux 2.6.6, 2004-05-09
The
VIDIOC_CROPCAPioctl was incorrectly definedwith read-only parameter. It is now defined as write-read ioctl, whilethe read-only version was renamed toVIDIOC_CROPCAP_OLD. The old ioctl will be removedin the future.
6.2.13. V4L2 in Linux 2.6.8
A new field
input(formerreserved[0]) was added to the struct v4l2_bufferstructure. Purpose of this field is to alternate between video inputs(e. g. cameras) in step with the video capturing process. This functionmust be enabled with the newV4L2_BUF_FLAG_INPUTflag. Theflagsfield is no longerread-only.
6.2.14. V4L2 spec erratum 2004-08-01
The return value of theV4L2 open()(2) function was incorrectly documented.
Audio output ioctls end in -AUDOUT, not -AUDIOOUT.
In the Current Audio Input example the
VIDIOC_G_AUDIOioctl took the wrongargument.The documentation of the
VIDIOC_QBUFandVIDIOC_DQBUFioctls did not mention the struct v4l2_buffermemoryfield. It was also missing fromexamples. Also on theVIDIOC_DQBUFpage theEIO error codewas not documented.
6.2.15. V4L2 in Linux 2.6.14
A new sliced VBI interface was added. It is documentedin Section 4.8 and replaces the interface firstproposed in V4L2 specification 0.8.
6.2.16. V4L2 in Linux 2.6.15
The
VIDIOC_LOG_STATUSioctl was added.New video standards
V4L2_STD_NTSC_443,V4L2_STD_SECAM_LC,V4L2_STD_SECAM_DK(a set of SECAM D, K and K1),andV4L2_STD_ATSC(a set ofV4L2_STD_ATSC_8_VSBandV4L2_STD_ATSC_16_VSB) were defined. Note theV4L2_STD_525_60set now includesV4L2_STD_NTSC_443. See alsoTable 3.The
VIDIOC_G_COMPandVIDIOC_S_COMPioctl were renamed toVIDIOC_G_MPEGCOMPandVIDIOC_S_MPEGCOMPrespectively. Their argumentwas replaced by a structv4l2_mpeg_compressionpointer. (TheVIDIOC_G_MPEGCOMPandVIDIOC_S_MPEGCOMPioctls where removed in Linux2.6.25.)
6.2.17. V4L2 spec erratum 2005-11-27
The capture example in Appendix Bcalled the VIDIOC_S_CROP ioctl without checking if cropping issupported. In the video standard selection example inSection 1.7 theVIDIOC_S_STD call used the wrongargument type.
6.2.18. V4L2 spec erratum 2006-01-10
The
V4L2_IN_ST_COLOR_KILLflag instruct v4l2_input not only indicates if the color killer is enabled, butalso if it is active. (The color killer disables color decoding whenit detects no color in the video signal to improve the imagequality.)VIDIOC_S_PARMis a write-read ioctl, not write-only asstated on its reference page. The ioctl changed in 2003 as noted above.
6.2.19. V4L2 spec erratum 2006-02-03
In struct v4l2_captureparm and struct v4l2_outputparm the
timeperframefield gives the time inseconds, not microseconds.
6.2.20. V4L2 spec erratum 2006-02-04
The
clipsfield instruct v4l2_window must point to an array of struct v4l2_clip, not a linkedlist, because drivers ignore the structv4l2_clip.nextpointer.
6.2.21. V4L2 in Linux 2.6.17
New video standard macros were added:
V4L2_STD_NTSC_M_KR(NTSC M South Korea), and thesetsV4L2_STD_MN,V4L2_STD_B,V4L2_STD_GHandV4L2_STD_DK. TheV4L2_STD_NTSCandV4L2_STD_SECAMsets now includeV4L2_STD_NTSC_M_KRandV4L2_STD_SECAM_LCrespectively.A new
V4L2_TUNER_MODE_LANG1_LANG2was defined to record both languages of a bilingual program. Theuse ofV4L2_TUNER_MODE_STEREOfor this purposeis deprecated now. See theVIDIOC_G_TUNERsection fordetails.
6.2.22. V4L2 spec erratum 2006-09-23 (Draft 0.15)
In various places
V4L2_BUF_TYPE_SLICED_VBI_CAPTUREandV4L2_BUF_TYPE_SLICED_VBI_OUTPUTof the sliced VBIinterface were not mentioned along with other buffer types.In ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO(2) it was clarifiedthat the struct v4l2_audio
modefield is a flagsfield.ioctl VIDIOC_QUERYCAP(2) did not mention thesliced VBI and radio capability flags.
In ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY(2) it wasclarified that applications must initialize the tuner
typefield of struct v4l2_frequency beforecallingVIDIOC_S_FREQUENCY.The
reservedarrayin struct v4l2_requestbuffers has 2 elements, not 32.In Section 4.3 andSection 4.7 the device file names/dev/vout which never caught on were replacedby/dev/video.
With Linux 2.6.15 the possible range for VBI device minornumbers was extended from 224-239 to 224-255. Accordingly device file names/dev/vbi0 to/dev/vbi31 arepossible now.
6.2.23. V4L2 in Linux 2.6.18
New ioctls
VIDIOC_G_EXT_CTRLS,VIDIOC_S_EXT_CTRLSandVIDIOC_TRY_EXT_CTRLSwere added, a flag to skip unsupportedcontrols withVIDIOC_QUERYCTRL, new control typesV4L2_CTRL_TYPE_INTEGER64andV4L2_CTRL_TYPE_CTRL_CLASS(Table 3), and new control flagsV4L2_CTRL_FLAG_READ_ONLY,V4L2_CTRL_FLAG_UPDATE,V4L2_CTRL_FLAG_INACTIVEandV4L2_CTRL_FLAG_SLIDER(Table 4). SeeSection 1.9 for details.
6.2.24. V4L2 in Linux 2.6.19
In struct v4l2_sliced_vbi_cap a buffer type field was addedreplacing a reserved field. Note on architectures where the size ofenum types differs from int types the size of the structure changed.The
VIDIOC_G_SLICED_VBI_CAPioctl was redefined from being read-onlyto write-read. Applications must initialize the type field and clearthe reserved fields now. These changes maybreak thecompatibility with older drivers and applications.The ioctls
VIDIOC_ENUM_FRAMESIZESandVIDIOC_ENUM_FRAMEINTERVALSwere added.A new pixel format
V4L2_PIX_FMT_RGB444(Table 2-1) was added.
6.2.25. V4L2 spec erratum 2006-10-12 (Draft 0.17)
V4L2_PIX_FMT_HM12(Table 2-8) is a YUV 4:2:0, not 4:2:2 format.
6.2.26. V4L2 in Linux 2.6.21
The videodev2.h header file isnow dual licensed under GNU General Public License version two orlater, and under a 3-clause BSD-style license.
6.2.27. V4L2 in Linux 2.6.22
Two new field orders
V4L2_FIELD_INTERLACED_TBandV4L2_FIELD_INTERLACED_BTwere added. SeeTable 3-8 for details.Three new clipping/blending methods with a global orstraight or inverted local alpha value were added to the video overlayinterface. See the description of the
VIDIOC_G_FBUFandVIDIOC_S_FBUFioctls for details.A new
global_alphafieldwas added tov4l2_window,extending the structure. This maybreakcompatibility with applications using a structv4l2_windowdirectly. However theVIDIOC_G/S/TRY_FMT ioctls, which take apointer to av4l2_format parentstructure with padding bytes at the end, are not affected.The format of the
chromakeyfield in struct v4l2_window changed from "host order RGB32" to a pixelvalue in the same format as the framebuffer. This may breakcompatibility with existing applications. Driverssupporting the "host order RGB32" format are not known.
6.2.28. V4L2 in Linux 2.6.24
The pixel formats
V4L2_PIX_FMT_PAL8,V4L2_PIX_FMT_YUV444,V4L2_PIX_FMT_YUV555,V4L2_PIX_FMT_YUV565andV4L2_PIX_FMT_YUV32were added.
6.2.29. V4L2 in Linux 2.6.25
The pixel formats
V4L2_PIX_FMT_Y16andV4L2_PIX_FMT_SBGGR16were added.New controls
V4L2_CID_POWER_LINE_FREQUENCY,V4L2_CID_HUE_AUTO,V4L2_CID_WHITE_BALANCE_TEMPERATURE,V4L2_CID_SHARPNESSandV4L2_CID_BACKLIGHT_COMPENSATIONwere added. ThecontrolsV4L2_CID_BLACK_LEVEL,V4L2_CID_WHITENESS,V4L2_CID_HCENTERandV4L2_CID_VCENTERwere deprecated.A Camera controlsclass was added, with the new controls
V4L2_CID_EXPOSURE_AUTO,V4L2_CID_EXPOSURE_ABSOLUTE,V4L2_CID_EXPOSURE_AUTO_PRIORITY,V4L2_CID_PAN_RELATIVE,V4L2_CID_TILT_RELATIVE,V4L2_CID_PAN_RESET,V4L2_CID_TILT_RESET,V4L2_CID_PAN_ABSOLUTE,V4L2_CID_TILT_ABSOLUTE,V4L2_CID_FOCUS_ABSOLUTE,V4L2_CID_FOCUS_RELATIVEandV4L2_CID_FOCUS_AUTO.The
VIDIOC_G_MPEGCOMPandVIDIOC_S_MPEGCOMPioctls, which were supersededby theextended controlsinterface in Linux 2.6.18, where finally removed from thevideodev2.h header file.
6.3. Relation of V4L2 to other Linux multimedia APIs
6.3.1. X Video Extension
The X Video Extension (abbreviated XVideo or just Xv) isan extension of the X Window system, implemented for example by theXFree86 project. Its scope is similar to V4L2, an API to video captureand output devices for X clients. Xv allows applications to displaylive video in a window, send window contents to a TV output, andcapture or output still images in XPixmaps[27]. With their implementation XFree86 makes theextension available across many operating systems andarchitectures.
Because the driver is embedded into the X server Xv has anumber of advantages over the V4L2videooverlay interface. The driver can easily determine the overlaytarget, i. e. visible graphics memory or off-screen buffers for adestructive overlay. It can program the RAMDAC for a non-destructiveoverlay, scaling or color-keying, or the clipping functions of thevideo capture hardware, always in sync with drawing operations orwindows moving or changing their stacking order.
To combine the advantages of Xv and V4L a special Xvdriver exists in XFree86 and XOrg, just programming any overlay capableVideo4Linux device it finds. To enable it/etc/X11/XF86Config must contain these lines:
Section "Module"
Load "v4l"
EndSection
As of XFree86 4.2 this driver still supports only V4Lioctls, however it should work just fine with all V4L2 devices throughthe V4L2 backward-compatibility layer. Since V4L2 permits multipleopens it is possible (if supported by the V4L2 driver) to capturevideo while an X client requested video overlay. Restrictions ofsimultaneous capturing and overlay are discussed inSection 4.2 apply.
Only marginally related to V4L2, XFree86 extended Xv tosupport hardware YUV to RGB conversion and scaling for faster videoplayback, and added an interface to MPEG-2 decoding hardware. This APIis useful to display images captured with V4L2 devices.
6.3.2. Digital Video
V4L2 does not support digital terrestrial, cable orsatellite broadcast. A separate project aiming at digital receiversexists. You can find its homepage athttp://linuxtv.org. The Linux DVB APIhas no connection to the V4L2 API except that drivers for hybridhardware may support both.
6.3.3. Audio Interfaces
[to do - OSS/ALSA]
6.4. Experimental API Elements
The following V4L2 API elements are currently experimentaland may change in the future.
Video Output Overlay (OSD) Interface, Section 4.4.
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY, enum v4l2_buf_type,Table 3-2.V4L2_CAP_VIDEO_OUTPUT_OVERLAY,VIDIOC_QUERYCAPioctl,Table 2.VIDIOC_ENUM_FRAMESIZESandVIDIOC_ENUM_FRAMEINTERVALSioctls.VIDIOC_G_ENC_INDEXioctl.VIDIOC_ENCODER_CMDandVIDIOC_TRY_ENCODER_CMDioctls.VIDIOC_DBG_G_REGISTERandVIDIOC_DBG_S_REGISTERioctls.VIDIOC_G_CHIP_IDENTioctl.
6.5. Obsolete API Elements
The following V4L2 API elements were superseded by newinterfaces and should not be implemented in new drivers.
VIDIOC_G_MPEGCOMPandVIDIOC_S_MPEGCOMPioctls. Use Extended Controls,Section 1.9.
Appendix A. Video For Linux Two Header File
/*
* Video for Linux Two header file
*
* Copyright (C) 1999-2007 the contributors
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* Alternatively you can redistribute this file under the terms of the
* BSD license as stated below:
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
* 3. The names of its contributors may not be used to endorse or promote
* products derived from this software without specific prior written
* permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
* TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* Header file for v4l or V4L2 drivers and applications
* with public API.
* All kernel-specific stuff were moved to media/v4l2-dev.h, so
* no #if __KERNEL tests are allowed here
*
* See http://linuxtv.org for more info
*
* Author: Bill Dirks <[email protected]>
* Justin Schoeman
* Hans Verkuil <[email protected]>
* et al.
*/
#ifndef __LINUX_VIDEODEV2_H
#define __LINUX_VIDEODEV2_H
#ifdef __KERNEL__
#include <linux/time.h> /* need struct timeval */
#include <linux/compiler.h> /* need __user */
#else
#define __user
#include <sys/time.h>
#endif
#include <linux/ioctl.h>
#include <linux/types.h> /*
* Common stuff for both V4L1 and V4L2
* Moved from videodev.h
*/
#define VIDEO_MAX_FRAME 32 #define VID_TYPE_CAPTURE 1 /* Can capture */
#define VID_TYPE_TUNER 2 /* Can tune */
#define VID_TYPE_TELETEXT 4 /* Does teletext */
#define VID_TYPE_OVERLAY 8 /* Overlay onto frame buffer */
#define VID_TYPE_CHROMAKEY 16 /* Overlay by chromakey */
#define VID_TYPE_CLIPPING 32 /* Can clip */
#define VID_TYPE_FRAMERAM 64 /* Uses the frame buffer memory */
#define VID_TYPE_SCALES 128 /* Scalable */
#define VID_TYPE_MONOCHROME 256 /* Monochrome only */
#define VID_TYPE_SUBCAPTURE 512 /* Can capture subareas of the image */
#define VID_TYPE_MPEG_DECODER 1024 /* Can decode MPEG streams */
#define VID_TYPE_MPEG_ENCODER 2048 /* Can encode MPEG streams */
#define VID_TYPE_MJPEG_DECODER 4096 /* Can decode MJPEG streams */
#define VID_TYPE_MJPEG_ENCODER 8192 /* Can encode MJPEG streams */ /*
* M I S C E L L A N E O U S
*/ /* Four-character-code (FOURCC) */
#define v4l2_fourcc(a,b,c,d)\
(((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24)) /*
* E N U M S
*/
enum v4l2_field {
V4L2_FIELD_ANY = 0, /* driver can choose from none,
top, bottom, interlaced
depending on whatever it thinks
is approximate ... */
V4L2_FIELD_NONE = 1, /* this device has no fields ... */
V4L2_FIELD_TOP = 2, /* top field only */
V4L2_FIELD_BOTTOM = 3, /* bottom field only */
V4L2_FIELD_INTERLACED = 4, /* both fields interlaced */
V4L2_FIELD_SEQ_TB = 5, /* both fields sequential into one
buffer, top-bottom order */
V4L2_FIELD_SEQ_BT = 6, /* same as above + bottom-top order */
V4L2_FIELD_ALTERNATE = 7, /* both fields alternating into
separate buffers */
V4L2_FIELD_INTERLACED_TB = 8, /* both fields interlaced, top field
first and the top field is
transmitted first */
V4L2_FIELD_INTERLACED_BT = 9, /* both fields interlaced, top field
first and the bottom field is
transmitted first */
};
#define V4L2_FIELD_HAS_TOP(field) \
((field) == V4L2_FIELD_TOP ||\
(field) == V4L2_FIELD_INTERLACED ||\
(field) == V4L2_FIELD_INTERLACED_TB ||\
(field) == V4L2_FIELD_INTERLACED_BT ||\
(field) == V4L2_FIELD_SEQ_TB ||\
(field) == V4L2_FIELD_SEQ_BT)
#define V4L2_FIELD_HAS_BOTTOM(field) \
((field) == V4L2_FIELD_BOTTOM ||\
(field) == V4L2_FIELD_INTERLACED ||\
(field) == V4L2_FIELD_INTERLACED_TB ||\
(field) == V4L2_FIELD_INTERLACED_BT ||\
(field) == V4L2_FIELD_SEQ_TB ||\
(field) == V4L2_FIELD_SEQ_BT)
#define V4L2_FIELD_HAS_BOTH(field) \
((field) == V4L2_FIELD_INTERLACED ||\
(field) == V4L2_FIELD_INTERLACED_TB ||\
(field) == V4L2_FIELD_INTERLACED_BT ||\
(field) == V4L2_FIELD_SEQ_TB ||\
(field) == V4L2_FIELD_SEQ_BT) enum v4l2_buf_type {
V4L2_BUF_TYPE_VIDEO_CAPTURE = 1,
V4L2_BUF_TYPE_VIDEO_OUTPUT = 2,
V4L2_BUF_TYPE_VIDEO_OVERLAY = 3,
V4L2_BUF_TYPE_VBI_CAPTURE = 4,
V4L2_BUF_TYPE_VBI_OUTPUT = 5,
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE = 6,
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT = 7,
#if 1 /*KEEP*/
/* Experimental */
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY = 8,
#endif
V4L2_BUF_TYPE_PRIVATE = 0x80,
}; enum v4l2_ctrl_type {
V4L2_CTRL_TYPE_INTEGER = 1,
V4L2_CTRL_TYPE_BOOLEAN = 2,
V4L2_CTRL_TYPE_MENU = 3,
V4L2_CTRL_TYPE_BUTTON = 4,
V4L2_CTRL_TYPE_INTEGER64 = 5,
V4L2_CTRL_TYPE_CTRL_CLASS = 6,
}; enum v4l2_tuner_type {
V4L2_TUNER_RADIO = 1,
V4L2_TUNER_ANALOG_TV = 2,
V4L2_TUNER_DIGITAL_TV = 3,
}; enum v4l2_memory {
V4L2_MEMORY_MMAP = 1,
V4L2_MEMORY_USERPTR = 2,
V4L2_MEMORY_OVERLAY = 3,
}; /* see also http://vektor.theorem.ca/graphics/ycbcr/ */
enum v4l2_colorspace {
/* ITU-R 601 -- broadcast NTSC/PAL */
V4L2_COLORSPACE_SMPTE170M = 1, /* 1125-Line (US) HDTV */
V4L2_COLORSPACE_SMPTE240M = 2, /* HD and modern captures. */
V4L2_COLORSPACE_REC709 = 3, /* broken BT878 extents (601, luma range 16-253 instead of 16-235) */
V4L2_COLORSPACE_BT878 = 4, /* These should be useful. Assume 601 extents. */
V4L2_COLORSPACE_470_SYSTEM_M = 5,
V4L2_COLORSPACE_470_SYSTEM_BG = 6, /* I know there will be cameras that send this. So, this is
* unspecified chromaticities and full 0-255 on each of the
* Y'CbCr components
*/
V4L2_COLORSPACE_JPEG = 7, /* For RGB colourspaces, this is probably a good start. */
V4L2_COLORSPACE_SRGB = 8,
}; enum v4l2_priority {
V4L2_PRIORITY_UNSET = 0, /* not initialized */
V4L2_PRIORITY_BACKGROUND = 1,
V4L2_PRIORITY_INTERACTIVE = 2,
V4L2_PRIORITY_RECORD = 3,
V4L2_PRIORITY_DEFAULT = V4L2_PRIORITY_INTERACTIVE,
}; struct v4l2_rect {
__s32 left;
__s32 top;
__s32 width;
__s32 height;
}; struct v4l2_fract {
__u32 numerator;
__u32 denominator;
}; /*
* D R I V E R C A P A B I L I T I E S
*/
struct v4l2_capability
{
__u8 driver[16]; /* i.e.ie; "bttv" */
__u8 card[32]; /* i.e.ie; "Hauppauge WinTV" */
__u8 bus_info[32]; /* "PCI:" + pci_name(pci_dev) */
__u32 version; /* should use KERNEL_VERSION() */
__u32 capabilities; /* Device capabilities */
__u32 reserved[4];
}; /* Values for 'capabilities' field */
#define V4L2_CAP_VIDEO_CAPTURE 0x00000001 /* Is a video capture device */
#define V4L2_CAP_VIDEO_OUTPUT 0x00000002 /* Is a video output device */
#define V4L2_CAP_VIDEO_OVERLAY 0x00000004 /* Can do video overlay */
#define V4L2_CAP_VBI_CAPTURE 0x00000010 /* Is a raw VBI capture device */
#define V4L2_CAP_VBI_OUTPUT 0x00000020 /* Is a raw VBI output device */
#define V4L2_CAP_SLICED_VBI_CAPTURE 0x00000040 /* Is a sliced VBI capture device */
#define V4L2_CAP_SLICED_VBI_OUTPUT 0x00000080 /* Is a sliced VBI output device */
#define V4L2_CAP_RDS_CAPTURE 0x00000100 /* RDS data capture */
#define V4L2_CAP_VIDEO_OUTPUT_OVERLAY 0x00000200 /* Can do video output overlay */ #define V4L2_CAP_TUNER 0x00010000 /* has a tuner */
#define V4L2_CAP_AUDIO 0x00020000 /* has audio support */
#define V4L2_CAP_RADIO 0x00040000 /* is a radio device */ #define V4L2_CAP_READWRITE 0x01000000 /* read/write systemcalls */
#define V4L2_CAP_ASYNCIO 0x02000000 /* async I/O */
#define V4L2_CAP_STREAMING 0x04000000 /* streaming I/O ioctls */ /*
* V I D E O I M A G E F O R M A T
*/
struct v4l2_pix_format
{
__u32 width;
__u32 height;
__u32 pixelformat;
enum v4l2_field field;
__u32 bytesperline; /* for padding, zero if unused */
__u32 sizeimage;
enum v4l2_colorspace colorspace;
__u32 priv; /* private data, depends on pixelformat */
}; /* Pixel format FOURCC depth Description */
#define V4L2_PIX_FMT_RGB332 v4l2_fourcc('R','G','B','1') /* 8 RGB-3-3-2 */
#define V4L2_PIX_FMT_RGB444 v4l2_fourcc('R','4','4','4') /* 16 xxxxrrrr ggggbbbb */
#define V4L2_PIX_FMT_RGB555 v4l2_fourcc('R','G','B','O') /* 16 RGB-5-5-5 */
#define V4L2_PIX_FMT_RGB565 v4l2_fourcc('R','G','B','P') /* 16 RGB-5-6-5 */
#define V4L2_PIX_FMT_RGB555X v4l2_fourcc('R','G','B','Q') /* 16 RGB-5-5-5 BE */
#define V4L2_PIX_FMT_RGB565X v4l2_fourcc('R','G','B','R') /* 16 RGB-5-6-5 BE */
#define V4L2_PIX_FMT_BGR24 v4l2_fourcc('B','G','R','3') /* 24 BGR-8-8-8 */
#define V4L2_PIX_FMT_RGB24 v4l2_fourcc('R','G','B','3') /* 24 RGB-8-8-8 */
#define V4L2_PIX_FMT_BGR32 v4l2_fourcc('B','G','R','4') /* 32 BGR-8-8-8-8 */
#define V4L2_PIX_FMT_RGB32 v4l2_fourcc('R','G','B','4') /* 32 RGB-8-8-8-8 */
#define V4L2_PIX_FMT_GREY v4l2_fourcc('G','R','E','Y') /* 8 Greyscale */
#define V4L2_PIX_FMT_Y16 v4l2_fourcc('Y','1','6',' ') /* 16 Greyscale */
#define V4L2_PIX_FMT_PAL8 v4l2_fourcc('P','A','L','8') /* 8 8-bit palette */
#define V4L2_PIX_FMT_YVU410 v4l2_fourcc('Y','V','U','9') /* 9 YVU 4:1:0 */
#define V4L2_PIX_FMT_YVU420 v4l2_fourcc('Y','V','1','2') /* 12 YVU 4:2:0 */
#define V4L2_PIX_FMT_YUYV v4l2_fourcc('Y','U','Y','V') /* 16 YUV 4:2:2 */
#define V4L2_PIX_FMT_UYVY v4l2_fourcc('U','Y','V','Y') /* 16 YUV 4:2:2 */
#define V4L2_PIX_FMT_YUV422P v4l2_fourcc('4','2','2','P') /* 16 YVU422 planar */
#define V4L2_PIX_FMT_YUV411P v4l2_fourcc('4','1','1','P') /* 16 YVU411 planar */
#define V4L2_PIX_FMT_Y41P v4l2_fourcc('Y','4','1','P') /* 12 YUV 4:1:1 */
#define V4L2_PIX_FMT_YUV444 v4l2_fourcc('Y','4','4','4') /* 16 xxxxyyyy uuuuvvvv */
#define V4L2_PIX_FMT_YUV555 v4l2_fourcc('Y','U','V','O') /* 16 YUV-5-5-5 */
#define V4L2_PIX_FMT_YUV565 v4l2_fourcc('Y','U','V','P') /* 16 YUV-5-6-5 */
#define V4L2_PIX_FMT_YUV32 v4l2_fourcc('Y','U','V','4') /* 32 YUV-8-8-8-8 */ /* two planes -- one Y, one Cr + Cb interleaved */
#define V4L2_PIX_FMT_NV12 v4l2_fourcc('N','V','1','2') /* 12 Y/CbCr 4:2:0 */
#define V4L2_PIX_FMT_NV21 v4l2_fourcc('N','V','2','1') /* 12 Y/CrCb 4:2:0 */ /* The following formats are not defined in the V4L2 specification */
#define V4L2_PIX_FMT_YUV410 v4l2_fourcc('Y','U','V','9') /* 9 YUV 4:1:0 */
#define V4L2_PIX_FMT_YUV420 v4l2_fourcc('Y','U','1','2') /* 12 YUV 4:2:0 */
#define V4L2_PIX_FMT_YYUV v4l2_fourcc('Y','Y','U','V') /* 16 YUV 4:2:2 */
#define V4L2_PIX_FMT_HI240 v4l2_fourcc('H','I','2','4') /* 8 8-bit color */
#define V4L2_PIX_FMT_HM12 v4l2_fourcc('H','M','1','2') /* 8 YUV 4:2:0 16x16 macroblocks */ /* see http://www.siliconimaging.com/RGB%20Bayer.htm */
#define V4L2_PIX_FMT_SBGGR8 v4l2_fourcc('B','A','8','1') /* 8 BGBG.. GRGR.. */
#define V4L2_PIX_FMT_SBGGR16 v4l2_fourcc('B','Y','R','2') /* 16 BGBG.. GRGR.. */ /* compressed formats */
#define V4L2_PIX_FMT_MJPEG v4l2_fourcc('M','J','P','G') /* Motion-JPEG */
#define V4L2_PIX_FMT_JPEG v4l2_fourcc('J','P','E','G') /* JFIF JPEG */
#define V4L2_PIX_FMT_DV v4l2_fourcc('d','v','s','d') /* 1394 */
#define V4L2_PIX_FMT_MPEG v4l2_fourcc('M','P','E','G') /* MPEG-1/2/4 */ /* Vendor-specific formats */
#define V4L2_PIX_FMT_WNVA v4l2_fourcc('W','N','V','A') /* Winnov hw compress */
#define V4L2_PIX_FMT_SN9C10X v4l2_fourcc('S','9','1','0') /* SN9C10x compression */
#define V4L2_PIX_FMT_PWC1 v4l2_fourcc('P','W','C','1') /* pwc older webcam */
#define V4L2_PIX_FMT_PWC2 v4l2_fourcc('P','W','C','2') /* pwc newer webcam */
#define V4L2_PIX_FMT_ET61X251 v4l2_fourcc('E','6','2','5') /* ET61X251 compression */ /*
* F O R M A T E N U M E R A T I O N
*/
struct v4l2_fmtdesc
{
__u32 index; /* Format number */
enum v4l2_buf_type type; /* buffer type */
__u32 flags;
__u8 description[32]; /* Description string */
__u32 pixelformat; /* Format fourcc */
__u32 reserved[4];
}; #define V4L2_FMT_FLAG_COMPRESSED 0x0001 #if 1 /*KEEP*/
/* Experimental Frame Size and frame rate enumeration */
/*
* F R A M E S I Z E E N U M E R A T I O N
*/
enum v4l2_frmsizetypes
{
V4L2_FRMSIZE_TYPE_DISCRETE = 1,
V4L2_FRMSIZE_TYPE_CONTINUOUS = 2,
V4L2_FRMSIZE_TYPE_STEPWISE = 3,
}; struct v4l2_frmsize_discrete
{
__u32 width; /* Frame width [pixel] */
__u32 height; /* Frame height [pixel] */
}; struct v4l2_frmsize_stepwise
{
__u32 min_width; /* Minimum frame width [pixel] */
__u32 max_width; /* Maximum frame width [pixel] */
__u32 step_width; /* Frame width step size [pixel] */
__u32 min_height; /* Minimum frame height [pixel] */
__u32 max_height; /* Maximum frame height [pixel] */
__u32 step_height; /* Frame height step size [pixel] */
}; struct v4l2_frmsizeenum
{
__u32 index; /* Frame size number */
__u32 pixel_format; /* Pixel format */
__u32 type; /* Frame size type the device supports. */ union { /* Frame size */
struct v4l2_frmsize_discrete discrete;
struct v4l2_frmsize_stepwise stepwise;
}; __u32 reserved[2]; /* Reserved space for future use */
}; /*
* F R A M E R A T E E N U M E R A T I O N
*/
enum v4l2_frmivaltypes
{
V4L2_FRMIVAL_TYPE_DISCRETE = 1,
V4L2_FRMIVAL_TYPE_CONTINUOUS = 2,
V4L2_FRMIVAL_TYPE_STEPWISE = 3,
}; struct v4l2_frmival_stepwise
{
struct v4l2_fract min; /* Minimum frame interval [s] */
struct v4l2_fract max; /* Maximum frame interval [s] */
struct v4l2_fract step; /* Frame interval step size [s] */
}; struct v4l2_frmivalenum
{
__u32 index; /* Frame format index */
__u32 pixel_format; /* Pixel format */
__u32 width; /* Frame width */
__u32 height; /* Frame height */
__u32 type; /* Frame interval type the device supports. */ union { /* Frame interval */
struct v4l2_fract discrete;
struct v4l2_frmival_stepwise stepwise;
}; __u32 reserved[2]; /* Reserved space for future use */
};
#endif /*
* T I M E C O D E
*/
struct v4l2_timecode
{
__u32 type;
__u32 flags;
__u8 frames;
__u8 seconds;
__u8 minutes;
__u8 hours;
__u8 userbits[4];
}; /* Type */
#define V4L2_TC_TYPE_24FPS 1
#define V4L2_TC_TYPE_25FPS 2
#define V4L2_TC_TYPE_30FPS 3
#define V4L2_TC_TYPE_50FPS 4
#define V4L2_TC_TYPE_60FPS 5 /* Flags */
#define V4L2_TC_FLAG_DROPFRAME 0x0001 /* "drop-frame" mode */
#define V4L2_TC_FLAG_COLORFRAME 0x0002
#define V4L2_TC_USERBITS_field 0x000C
#define V4L2_TC_USERBITS_USERDEFINED 0x0000
#define V4L2_TC_USERBITS_8BITCHARS 0x0008
/* The above is based on SMPTE timecodes */ struct v4l2_jpegcompression
{
int quality; int APPn; /* Number of APP segment to be written,
* must be 0..15 */
int APP_len; /* Length of data in JPEG APPn segment */
char APP_data[60]; /* Data in the JPEG APPn segment. */ int COM_len; /* Length of data in JPEG COM segment */
char COM_data[60]; /* Data in JPEG COM segment */ __u32 jpeg_markers; /* Which markers should go into the JPEG
* output. Unless you exactly know what
* you do, leave them untouched.
* Inluding less markers will make the
* resulting code smaller, but there will
* be fewer aplications which can read it.
* The presence of the APP and COM marker
* is influenced by APP_len and COM_len
* ONLY, not by this property! */ #define V4L2_JPEG_MARKER_DHT (1<<3) /* Define Huffman Tables */
#define V4L2_JPEG_MARKER_DQT (1<<4) /* Define Quantization Tables */
#define V4L2_JPEG_MARKER_DRI (1<<5) /* Define Restart Interval */
#define V4L2_JPEG_MARKER_COM (1<<6) /* Comment segment */
#define V4L2_JPEG_MARKER_APP (1<<7) /* App segment, driver will
* allways use APP0 */
}; /*
* M E M O R Y - M A P P I N G B U F F E R S
*/
struct v4l2_requestbuffers
{
__u32 count;
enum v4l2_buf_type type;
enum v4l2_memory memory;
__u32 reserved[2];
}; struct v4l2_buffer
{
__u32 index;
enum v4l2_buf_type type;
__u32 bytesused;
__u32 flags;
enum v4l2_field field;
struct timeval timestamp;
struct v4l2_timecode timecode;
__u32 sequence; /* memory location */
enum v4l2_memory memory;
union {
__u32 offset;
unsigned long userptr;
} m;
__u32 length;
__u32 input;
__u32 reserved;
}; /* Flags for 'flags' field */
#define V4L2_BUF_FLAG_MAPPED 0x0001 /* Buffer is mapped (flag) */
#define V4L2_BUF_FLAG_QUEUED 0x0002 /* Buffer is queued for processing */
#define V4L2_BUF_FLAG_DONE 0x0004 /* Buffer is ready */
#define V4L2_BUF_FLAG_KEYFRAME 0x0008 /* Image is a keyframe (I-frame) */
#define V4L2_BUF_FLAG_PFRAME 0x0010 /* Image is a P-frame */
#define V4L2_BUF_FLAG_BFRAME 0x0020 /* Image is a B-frame */
#define V4L2_BUF_FLAG_TIMECODE 0x0100 /* timecode field is valid */
#define V4L2_BUF_FLAG_INPUT 0x0200 /* input field is valid */ /*
* O V E R L A Y P R E V I E W
*/
struct v4l2_framebuffer
{
__u32 capability;
__u32 flags;
/* FIXME: in theory we should pass something like PCI device + memory
* region + offset instead of some physical address */
void* base;
struct v4l2_pix_format fmt;
};
/* Flags for the 'capability' field. Read only */
#define V4L2_FBUF_CAP_EXTERNOVERLAY 0x0001
#define V4L2_FBUF_CAP_CHROMAKEY 0x0002
#define V4L2_FBUF_CAP_LIST_CLIPPING 0x0004
#define V4L2_FBUF_CAP_BITMAP_CLIPPING 0x0008
#define V4L2_FBUF_CAP_LOCAL_ALPHA 0x0010
#define V4L2_FBUF_CAP_GLOBAL_ALPHA 0x0020
#define V4L2_FBUF_CAP_LOCAL_INV_ALPHA 0x0040
/* Flags for the 'flags' field. */
#define V4L2_FBUF_FLAG_PRIMARY 0x0001
#define V4L2_FBUF_FLAG_OVERLAY 0x0002
#define V4L2_FBUF_FLAG_CHROMAKEY 0x0004
#define V4L2_FBUF_FLAG_LOCAL_ALPHA 0x0008
#define V4L2_FBUF_FLAG_GLOBAL_ALPHA 0x0010
#define V4L2_FBUF_FLAG_LOCAL_INV_ALPHA 0x0020 struct v4l2_clip
{
struct v4l2_rect c;
struct v4l2_clip __user *next;
}; struct v4l2_window
{
struct v4l2_rect w;
enum v4l2_field field;
__u32 chromakey;
struct v4l2_clip __user *clips;
__u32 clipcount;
void __user *bitmap;
__u8 global_alpha;
}; /*
* C A P T U R E P A R A M E T E R S
*/
struct v4l2_captureparm
{
__u32 capability; /* Supported modes */
__u32 capturemode; /* Current mode */
struct v4l2_fract timeperframe; /* Time per frame in .1us units */
__u32 extendedmode; /* Driver-specific extensions */
__u32 readbuffers; /* # of buffers for read */
__u32 reserved[4];
}; /* Flags for 'capability' and 'capturemode' fields */
#define V4L2_MODE_HIGHQUALITY 0x0001 /* High quality imaging mode */
#define V4L2_CAP_TIMEPERFRAME 0x1000 /* timeperframe field is supported */ struct v4l2_outputparm
{
__u32 capability; /* Supported modes */
__u32 outputmode; /* Current mode */
struct v4l2_fract timeperframe; /* Time per frame in seconds */
__u32 extendedmode; /* Driver-specific extensions */
__u32 writebuffers; /* # of buffers for write */
__u32 reserved[4];
}; /*
* I N P U T I M A G E C R O P P I N G
*/
struct v4l2_cropcap {
enum v4l2_buf_type type;
struct v4l2_rect bounds;
struct v4l2_rect defrect;
struct v4l2_fract pixelaspect;
}; struct v4l2_crop {
enum v4l2_buf_type type;
struct v4l2_rect c;
}; /*
* A N A L O G V I D E O S T A N D A R D
*/ typedef __u64 v4l2_std_id; /* one bit for each */
#define V4L2_STD_PAL_B ((v4l2_std_id)0x00000001)
#define V4L2_STD_PAL_B1 ((v4l2_std_id)0x00000002)
#define V4L2_STD_PAL_G ((v4l2_std_id)0x00000004)
#define V4L2_STD_PAL_H ((v4l2_std_id)0x00000008)
#define V4L2_STD_PAL_I ((v4l2_std_id)0x00000010)
#define V4L2_STD_PAL_D ((v4l2_std_id)0x00000020)
#define V4L2_STD_PAL_D1 ((v4l2_std_id)0x00000040)
#define V4L2_STD_PAL_K ((v4l2_std_id)0x00000080) #define V4L2_STD_PAL_M ((v4l2_std_id)0x00000100)
#define V4L2_STD_PAL_N ((v4l2_std_id)0x00000200)
#define V4L2_STD_PAL_Nc ((v4l2_std_id)0x00000400)
#define V4L2_STD_PAL_60 ((v4l2_std_id)0x00000800) #define V4L2_STD_NTSC_M ((v4l2_std_id)0x00001000)
#define V4L2_STD_NTSC_M_JP ((v4l2_std_id)0x00002000)
#define V4L2_STD_NTSC_443 ((v4l2_std_id)0x00004000)
#define V4L2_STD_NTSC_M_KR ((v4l2_std_id)0x00008000) #define V4L2_STD_SECAM_B ((v4l2_std_id)0x00010000)
#define V4L2_STD_SECAM_D ((v4l2_std_id)0x00020000)
#define V4L2_STD_SECAM_G ((v4l2_std_id)0x00040000)
#define V4L2_STD_SECAM_H ((v4l2_std_id)0x00080000)
#define V4L2_STD_SECAM_K ((v4l2_std_id)0x00100000)
#define V4L2_STD_SECAM_K1 ((v4l2_std_id)0x00200000)
#define V4L2_STD_SECAM_L ((v4l2_std_id)0x00400000)
#define V4L2_STD_SECAM_LC ((v4l2_std_id)0x00800000) /* ATSC/HDTV */
#define V4L2_STD_ATSC_8_VSB ((v4l2_std_id)0x01000000)
#define V4L2_STD_ATSC_16_VSB ((v4l2_std_id)0x02000000) /* FIXME:
Although std_id is 64 bits, there is an issue on PPC32 architecture that
makes switch(__u64) to break. So, there's a hack on v4l2-common.c rounding
this value to 32 bits.
As, currently, the max value is for V4L2_STD_ATSC_16_VSB (30 bits wide),
it should work fine. However, if needed to add more than two standards,
v4l2-common.c should be fixed.
*/ /* some merged standards */
#define V4L2_STD_MN (V4L2_STD_PAL_M|V4L2_STD_PAL_N|V4L2_STD_PAL_Nc|V4L2_STD_NTSC)
#define V4L2_STD_B (V4L2_STD_PAL_B|V4L2_STD_PAL_B1|V4L2_STD_SECAM_B)
#define V4L2_STD_GH (V4L2_STD_PAL_G|V4L2_STD_PAL_H|V4L2_STD_SECAM_G|V4L2_STD_SECAM_H)
#define V4L2_STD_DK (V4L2_STD_PAL_DK|V4L2_STD_SECAM_DK) /* some common needed stuff */
#define V4L2_STD_PAL_BG (V4L2_STD_PAL_B |\
V4L2_STD_PAL_B1 |\
V4L2_STD_PAL_G)
#define V4L2_STD_PAL_DK (V4L2_STD_PAL_D |\
V4L2_STD_PAL_D1 |\
V4L2_STD_PAL_K)
#define V4L2_STD_PAL (V4L2_STD_PAL_BG |\
V4L2_STD_PAL_DK |\
V4L2_STD_PAL_H |\
V4L2_STD_PAL_I)
#define V4L2_STD_NTSC (V4L2_STD_NTSC_M |\
V4L2_STD_NTSC_M_JP |\
V4L2_STD_NTSC_M_KR)
#define V4L2_STD_SECAM_DK (V4L2_STD_SECAM_D |\
V4L2_STD_SECAM_K |\
V4L2_STD_SECAM_K1)
#define V4L2_STD_SECAM (V4L2_STD_SECAM_B |\
V4L2_STD_SECAM_G |\
V4L2_STD_SECAM_H |\
V4L2_STD_SECAM_DK |\
V4L2_STD_SECAM_L |\
V4L2_STD_SECAM_LC) #define V4L2_STD_525_60 (V4L2_STD_PAL_M |\
V4L2_STD_PAL_60 |\
V4L2_STD_NTSC |\
V4L2_STD_NTSC_443)
#define V4L2_STD_625_50 (V4L2_STD_PAL |\
V4L2_STD_PAL_N |\
V4L2_STD_PAL_Nc |\
V4L2_STD_SECAM)
#define V4L2_STD_ATSC (V4L2_STD_ATSC_8_VSB |\
V4L2_STD_ATSC_16_VSB) #define V4L2_STD_UNKNOWN 0
#define V4L2_STD_ALL (V4L2_STD_525_60 |\
V4L2_STD_625_50) struct v4l2_standard
{
__u32 index;
v4l2_std_id id;
__u8 name[24];
struct v4l2_fract frameperiod; /* Frames, not fields */
__u32 framelines;
__u32 reserved[4];
}; /*
* V I D E O I N P U T S
*/
struct v4l2_input
{
__u32 index; /* Which input */
__u8 name[32]; /* Label */
__u32 type; /* Type of input */
__u32 audioset; /* Associated audios (bitfield) */
__u32 tuner; /* Associated tuner */
v4l2_std_id std;
__u32 status;
__u32 reserved[4];
}; /* Values for the 'type' field */
#define V4L2_INPUT_TYPE_TUNER 1
#define V4L2_INPUT_TYPE_CAMERA 2 /* field 'status' - general */
#define V4L2_IN_ST_NO_POWER 0x00000001 /* Attached device is off */
#define V4L2_IN_ST_NO_SIGNAL 0x00000002
#define V4L2_IN_ST_NO_COLOR 0x00000004 /* field 'status' - analog */
#define V4L2_IN_ST_NO_H_LOCK 0x00000100 /* No horizontal sync lock */
#define V4L2_IN_ST_COLOR_KILL 0x00000200 /* Color killer is active */ /* field 'status' - digital */
#define V4L2_IN_ST_NO_SYNC 0x00010000 /* No synchronization lock */
#define V4L2_IN_ST_NO_EQU 0x00020000 /* No equalizer lock */
#define V4L2_IN_ST_NO_CARRIER 0x00040000 /* Carrier recovery failed */ /* field 'status' - VCR and set-top box */
#define V4L2_IN_ST_MACROVISION 0x01000000 /* Macrovision detected */
#define V4L2_IN_ST_NO_ACCESS 0x02000000 /* Conditional access denied */
#define V4L2_IN_ST_VTR 0x04000000 /* VTR time constant */ /*
* V I D E O O U T P U T S
*/
struct v4l2_output
{
__u32 index; /* Which output */
__u8 name[32]; /* Label */
__u32 type; /* Type of output */
__u32 audioset; /* Associated audios (bitfield) */
__u32 modulator; /* Associated modulator */
v4l2_std_id std;
__u32 reserved[4];
};
/* Values for the 'type' field */
#define V4L2_OUTPUT_TYPE_MODULATOR 1
#define V4L2_OUTPUT_TYPE_ANALOG 2
#define V4L2_OUTPUT_TYPE_ANALOGVGAOVERLAY 3 /*
* C O N T R O L S
*/
struct v4l2_control
{
__u32 id;
__s32 value;
}; struct v4l2_ext_control
{
__u32 id;
__u32 reserved2[2];
union {
__s32 value;
__s64 value64;
void *reserved;
};
} __attribute__ ((packed)); struct v4l2_ext_controls
{
__u32 ctrl_class;
__u32 count;
__u32 error_idx;
__u32 reserved[2];
struct v4l2_ext_control *controls;
}; /* Values for ctrl_class field */
#define V4L2_CTRL_CLASS_USER 0x00980000 /* Old-style 'user' controls */
#define V4L2_CTRL_CLASS_MPEG 0x00990000 /* MPEG-compression controls */
#define V4L2_CTRL_CLASS_CAMERA 0x009a0000 /* Camera class controls */ #define V4L2_CTRL_ID_MASK (0x0fffffff)
#define V4L2_CTRL_ID2CLASS(id) ((id) & 0x0fff0000UL)
#define V4L2_CTRL_DRIVER_PRIV(id) (((id) & 0xffff) >= 0x1000) /* Used in the VIDIOC_QUERYCTRL ioctl for querying controls */
struct v4l2_queryctrl
{
__u32 id;
enum v4l2_ctrl_type type;
__u8 name[32]; /* Whatever */
__s32 minimum; /* Note signedness */
__s32 maximum;
__s32 step;
__s32 default_value;
__u32 flags;
__u32 reserved[2];
}; /* Used in the VIDIOC_QUERYMENU ioctl for querying menu items */
struct v4l2_querymenu
{
__u32 id;
__u32 index;
__u8 name[32]; /* Whatever */
__u32 reserved;
}; /* Control flags */
#define V4L2_CTRL_FLAG_DISABLED 0x0001
#define V4L2_CTRL_FLAG_GRABBED 0x0002
#define V4L2_CTRL_FLAG_READ_ONLY 0x0004
#define V4L2_CTRL_FLAG_UPDATE 0x0008
#define V4L2_CTRL_FLAG_INACTIVE 0x0010
#define V4L2_CTRL_FLAG_SLIDER 0x0020 /* Query flag, to be ORed with the control ID */
#define V4L2_CTRL_FLAG_NEXT_CTRL 0x80000000 /* User-class control IDs defined by V4L2 */
#define V4L2_CID_BASE (V4L2_CTRL_CLASS_USER | 0x900)
#define V4L2_CID_USER_BASE V4L2_CID_BASE
/* IDs reserved for driver specific controls */
#define V4L2_CID_PRIVATE_BASE 0x08000000 #define V4L2_CID_USER_CLASS (V4L2_CTRL_CLASS_USER | 1)
#define V4L2_CID_BRIGHTNESS (V4L2_CID_BASE+0)
#define V4L2_CID_CONTRAST (V4L2_CID_BASE+1)
#define V4L2_CID_SATURATION (V4L2_CID_BASE+2)
#define V4L2_CID_HUE (V4L2_CID_BASE+3)
#define V4L2_CID_AUDIO_VOLUME (V4L2_CID_BASE+5)
#define V4L2_CID_AUDIO_BALANCE (V4L2_CID_BASE+6)
#define V4L2_CID_AUDIO_BASS (V4L2_CID_BASE+7)
#define V4L2_CID_AUDIO_TREBLE (V4L2_CID_BASE+8)
#define V4L2_CID_AUDIO_MUTE (V4L2_CID_BASE+9)
#define V4L2_CID_AUDIO_LOUDNESS (V4L2_CID_BASE+10)
#define V4L2_CID_BLACK_LEVEL (V4L2_CID_BASE+11) /* Deprecated */
#define V4L2_CID_AUTO_WHITE_BALANCE (V4L2_CID_BASE+12)
#define V4L2_CID_DO_WHITE_BALANCE (V4L2_CID_BASE+13)
#define V4L2_CID_RED_BALANCE (V4L2_CID_BASE+14)
#define V4L2_CID_BLUE_BALANCE (V4L2_CID_BASE+15)
#define V4L2_CID_GAMMA (V4L2_CID_BASE+16)
#define V4L2_CID_WHITENESS (V4L2_CID_GAMMA) /* Deprecated */
#define V4L2_CID_EXPOSURE (V4L2_CID_BASE+17)
#define V4L2_CID_AUTOGAIN (V4L2_CID_BASE+18)
#define V4L2_CID_GAIN (V4L2_CID_BASE+19)
#define V4L2_CID_HFLIP (V4L2_CID_BASE+20)
#define V4L2_CID_VFLIP (V4L2_CID_BASE+21) /* Deprecated, use V4L2_CID_PAN_RESET and V4L2_CID_TILT_RESET */
#define V4L2_CID_HCENTER_DEPRECATED (V4L2_CID_BASE+22)
#define V4L2_CID_VCENTER_DEPRECATED (V4L2_CID_BASE+23) #define V4L2_CID_POWER_LINE_FREQUENCY (V4L2_CID_BASE+24)
enum v4l2_power_line_frequency {
V4L2_CID_POWER_LINE_FREQUENCY_DISABLED = 0,
V4L2_CID_POWER_LINE_FREQUENCY_50HZ = 1,
V4L2_CID_POWER_LINE_FREQUENCY_60HZ = 2,
};
#define V4L2_CID_HUE_AUTO (V4L2_CID_BASE+25)
#define V4L2_CID_WHITE_BALANCE_TEMPERATURE (V4L2_CID_BASE+26)
#define V4L2_CID_SHARPNESS (V4L2_CID_BASE+27)
#define V4L2_CID_BACKLIGHT_COMPENSATION (V4L2_CID_BASE+28)
#define V4L2_CID_LASTP1 (V4L2_CID_BASE+29) /* last CID + 1 */ /* MPEG-class control IDs defined by V4L2 */
#define V4L2_CID_MPEG_BASE (V4L2_CTRL_CLASS_MPEG | 0x900)
#define V4L2_CID_MPEG_CLASS (V4L2_CTRL_CLASS_MPEG | 1) /* MPEG streams */
#define V4L2_CID_MPEG_STREAM_TYPE (V4L2_CID_MPEG_BASE+0)
enum v4l2_mpeg_stream_type {
V4L2_MPEG_STREAM_TYPE_MPEG2_PS = 0, /* MPEG-2 program stream */
V4L2_MPEG_STREAM_TYPE_MPEG2_TS = 1, /* MPEG-2 transport stream */
V4L2_MPEG_STREAM_TYPE_MPEG1_SS = 2, /* MPEG-1 system stream */
V4L2_MPEG_STREAM_TYPE_MPEG2_DVD = 3, /* MPEG-2 DVD-compatible stream */
V4L2_MPEG_STREAM_TYPE_MPEG1_VCD = 4, /* MPEG-1 VCD-compatible stream */
V4L2_MPEG_STREAM_TYPE_MPEG2_SVCD = 5, /* MPEG-2 SVCD-compatible stream */
};
#define V4L2_CID_MPEG_STREAM_PID_PMT (V4L2_CID_MPEG_BASE+1)
#define V4L2_CID_MPEG_STREAM_PID_AUDIO (V4L2_CID_MPEG_BASE+2)
#define V4L2_CID_MPEG_STREAM_PID_VIDEO (V4L2_CID_MPEG_BASE+3)
#define V4L2_CID_MPEG_STREAM_PID_PCR (V4L2_CID_MPEG_BASE+4)
#define V4L2_CID_MPEG_STREAM_PES_ID_AUDIO (V4L2_CID_MPEG_BASE+5)
#define V4L2_CID_MPEG_STREAM_PES_ID_VIDEO (V4L2_CID_MPEG_BASE+6)
#define V4L2_CID_MPEG_STREAM_VBI_FMT (V4L2_CID_MPEG_BASE+7)
enum v4l2_mpeg_stream_vbi_fmt {
V4L2_MPEG_STREAM_VBI_FMT_NONE = 0, /* No VBI in the MPEG stream */
V4L2_MPEG_STREAM_VBI_FMT_IVTV = 1, /* VBI in private packets, IVTV format */
}; /* MPEG audio */
#define V4L2_CID_MPEG_AUDIO_SAMPLING_FREQ (V4L2_CID_MPEG_BASE+100)
enum v4l2_mpeg_audio_sampling_freq {
V4L2_MPEG_AUDIO_SAMPLING_FREQ_44100 = 0,
V4L2_MPEG_AUDIO_SAMPLING_FREQ_48000 = 1,
V4L2_MPEG_AUDIO_SAMPLING_FREQ_32000 = 2,
};
#define V4L2_CID_MPEG_AUDIO_ENCODING (V4L2_CID_MPEG_BASE+101)
enum v4l2_mpeg_audio_encoding {
V4L2_MPEG_AUDIO_ENCODING_LAYER_1 = 0,
V4L2_MPEG_AUDIO_ENCODING_LAYER_2 = 1,
V4L2_MPEG_AUDIO_ENCODING_LAYER_3 = 2,
};
#define V4L2_CID_MPEG_AUDIO_L1_BITRATE (V4L2_CID_MPEG_BASE+102)
enum v4l2_mpeg_audio_l1_bitrate {
V4L2_MPEG_AUDIO_L1_BITRATE_32K = 0,
V4L2_MPEG_AUDIO_L1_BITRATE_64K = 1,
V4L2_MPEG_AUDIO_L1_BITRATE_96K = 2,
V4L2_MPEG_AUDIO_L1_BITRATE_128K = 3,
V4L2_MPEG_AUDIO_L1_BITRATE_160K = 4,
V4L2_MPEG_AUDIO_L1_BITRATE_192K = 5,
V4L2_MPEG_AUDIO_L1_BITRATE_224K = 6,
V4L2_MPEG_AUDIO_L1_BITRATE_256K = 7,
V4L2_MPEG_AUDIO_L1_BITRATE_288K = 8,
V4L2_MPEG_AUDIO_L1_BITRATE_320K = 9,
V4L2_MPEG_AUDIO_L1_BITRATE_352K = 10,
V4L2_MPEG_AUDIO_L1_BITRATE_384K = 11,
V4L2_MPEG_AUDIO_L1_BITRATE_416K = 12,
V4L2_MPEG_AUDIO_L1_BITRATE_448K = 13,
};
#define V4L2_CID_MPEG_AUDIO_L2_BITRATE (V4L2_CID_MPEG_BASE+103)
enum v4l2_mpeg_audio_l2_bitrate {
V4L2_MPEG_AUDIO_L2_BITRATE_32K = 0,
V4L2_MPEG_AUDIO_L2_BITRATE_48K = 1,
V4L2_MPEG_AUDIO_L2_BITRATE_56K = 2,
V4L2_MPEG_AUDIO_L2_BITRATE_64K = 3,
V4L2_MPEG_AUDIO_L2_BITRATE_80K = 4,
V4L2_MPEG_AUDIO_L2_BITRATE_96K = 5,
V4L2_MPEG_AUDIO_L2_BITRATE_112K = 6,
V4L2_MPEG_AUDIO_L2_BITRATE_128K = 7,
V4L2_MPEG_AUDIO_L2_BITRATE_160K = 8,
V4L2_MPEG_AUDIO_L2_BITRATE_192K = 9,
V4L2_MPEG_AUDIO_L2_BITRATE_224K = 10,
V4L2_MPEG_AUDIO_L2_BITRATE_256K = 11,
V4L2_MPEG_AUDIO_L2_BITRATE_320K = 12,
V4L2_MPEG_AUDIO_L2_BITRATE_384K = 13,
};
#define V4L2_CID_MPEG_AUDIO_L3_BITRATE (V4L2_CID_MPEG_BASE+104)
enum v4l2_mpeg_audio_l3_bitrate {
V4L2_MPEG_AUDIO_L3_BITRATE_32K = 0,
V4L2_MPEG_AUDIO_L3_BITRATE_40K = 1,
V4L2_MPEG_AUDIO_L3_BITRATE_48K = 2,
V4L2_MPEG_AUDIO_L3_BITRATE_56K = 3,
V4L2_MPEG_AUDIO_L3_BITRATE_64K = 4,
V4L2_MPEG_AUDIO_L3_BITRATE_80K = 5,
V4L2_MPEG_AUDIO_L3_BITRATE_96K = 6,
V4L2_MPEG_AUDIO_L3_BITRATE_112K = 7,
V4L2_MPEG_AUDIO_L3_BITRATE_128K = 8,
V4L2_MPEG_AUDIO_L3_BITRATE_160K = 9,
V4L2_MPEG_AUDIO_L3_BITRATE_192K = 10,
V4L2_MPEG_AUDIO_L3_BITRATE_224K = 11,
V4L2_MPEG_AUDIO_L3_BITRATE_256K = 12,
V4L2_MPEG_AUDIO_L3_BITRATE_320K = 13,
};
#define V4L2_CID_MPEG_AUDIO_MODE (V4L2_CID_MPEG_BASE+105)
enum v4l2_mpeg_audio_mode {
V4L2_MPEG_AUDIO_MODE_STEREO = 0,
V4L2_MPEG_AUDIO_MODE_JOINT_STEREO = 1,
V4L2_MPEG_AUDIO_MODE_DUAL = 2,
V4L2_MPEG_AUDIO_MODE_MONO = 3,
};
#define V4L2_CID_MPEG_AUDIO_MODE_EXTENSION (V4L2_CID_MPEG_BASE+106)
enum v4l2_mpeg_audio_mode_extension {
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_4 = 0,
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_8 = 1,
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_12 = 2,
V4L2_MPEG_AUDIO_MODE_EXTENSION_BOUND_16 = 3,
};
#define V4L2_CID_MPEG_AUDIO_EMPHASIS (V4L2_CID_MPEG_BASE+107)
enum v4l2_mpeg_audio_emphasis {
V4L2_MPEG_AUDIO_EMPHASIS_NONE = 0,
V4L2_MPEG_AUDIO_EMPHASIS_50_DIV_15_uS = 1,
V4L2_MPEG_AUDIO_EMPHASIS_CCITT_J17 = 2,
};
#define V4L2_CID_MPEG_AUDIO_CRC (V4L2_CID_MPEG_BASE+108)
enum v4l2_mpeg_audio_crc {
V4L2_MPEG_AUDIO_CRC_NONE = 0,
V4L2_MPEG_AUDIO_CRC_CRC16 = 1,
};
#define V4L2_CID_MPEG_AUDIO_MUTE (V4L2_CID_MPEG_BASE+109) /* MPEG video */
#define V4L2_CID_MPEG_VIDEO_ENCODING (V4L2_CID_MPEG_BASE+200)
enum v4l2_mpeg_video_encoding {
V4L2_MPEG_VIDEO_ENCODING_MPEG_1 = 0,
V4L2_MPEG_VIDEO_ENCODING_MPEG_2 = 1,
};
#define V4L2_CID_MPEG_VIDEO_ASPECT (V4L2_CID_MPEG_BASE+201)
enum v4l2_mpeg_video_aspect {
V4L2_MPEG_VIDEO_ASPECT_1x1 = 0,
V4L2_MPEG_VIDEO_ASPECT_4x3 = 1,
V4L2_MPEG_VIDEO_ASPECT_16x9 = 2,
V4L2_MPEG_VIDEO_ASPECT_221x100 = 3,
};
#define V4L2_CID_MPEG_VIDEO_B_FRAMES (V4L2_CID_MPEG_BASE+202)
#define V4L2_CID_MPEG_VIDEO_GOP_SIZE (V4L2_CID_MPEG_BASE+203)
#define V4L2_CID_MPEG_VIDEO_GOP_CLOSURE (V4L2_CID_MPEG_BASE+204)
#define V4L2_CID_MPEG_VIDEO_PULLDOWN (V4L2_CID_MPEG_BASE+205)
#define V4L2_CID_MPEG_VIDEO_BITRATE_MODE (V4L2_CID_MPEG_BASE+206)
enum v4l2_mpeg_video_bitrate_mode {
V4L2_MPEG_VIDEO_BITRATE_MODE_VBR = 0,
V4L2_MPEG_VIDEO_BITRATE_MODE_CBR = 1,
};
#define V4L2_CID_MPEG_VIDEO_BITRATE (V4L2_CID_MPEG_BASE+207)
#define V4L2_CID_MPEG_VIDEO_BITRATE_PEAK (V4L2_CID_MPEG_BASE+208)
#define V4L2_CID_MPEG_VIDEO_TEMPORAL_DECIMATION (V4L2_CID_MPEG_BASE+209)
#define V4L2_CID_MPEG_VIDEO_MUTE (V4L2_CID_MPEG_BASE+210)
#define V4L2_CID_MPEG_VIDEO_MUTE_YUV (V4L2_CID_MPEG_BASE+211) /* MPEG-class control IDs specific to the CX2584x driver as defined by V4L2 */
#define V4L2_CID_MPEG_CX2341X_BASE (V4L2_CTRL_CLASS_MPEG | 0x1000)
#define V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE (V4L2_CID_MPEG_CX2341X_BASE+0)
enum v4l2_mpeg_cx2341x_video_spatial_filter_mode {
V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_MANUAL = 0,
V4L2_MPEG_CX2341X_VIDEO_SPATIAL_FILTER_MODE_AUTO = 1,
};
#define V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER (V4L2_CID_MPEG_CX2341X_BASE+1)
#define V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE (V4L2_CID_MPEG_CX2341X_BASE+2)
enum v4l2_mpeg_cx2341x_video_luma_spatial_filter_type {
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_OFF = 0,
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_HOR = 1,
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_1D_VERT = 2,
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_HV_SEPARABLE = 3,
V4L2_MPEG_CX2341X_VIDEO_LUMA_SPATIAL_FILTER_TYPE_2D_SYM_NON_SEPARABLE = 4,
};
#define V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE (V4L2_CID_MPEG_CX2341X_BASE+3)
enum v4l2_mpeg_cx2341x_video_chroma_spatial_filter_type {
V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_OFF = 0,
V4L2_MPEG_CX2341X_VIDEO_CHROMA_SPATIAL_FILTER_TYPE_1D_HOR = 1,
};
#define V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE (V4L2_CID_MPEG_CX2341X_BASE+4)
enum v4l2_mpeg_cx2341x_video_temporal_filter_mode {
V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_MANUAL = 0,
V4L2_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER_MODE_AUTO = 1,
};
#define V4L2_CID_MPEG_CX2341X_VIDEO_TEMPORAL_FILTER (V4L2_CID_MPEG_CX2341X_BASE+5)
#define V4L2_CID_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE (V4L2_CID_MPEG_CX2341X_BASE+6)
enum v4l2_mpeg_cx2341x_video_median_filter_type {
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_OFF = 0,
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR = 1,
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_VERT = 2,
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_HOR_VERT = 3,
V4L2_MPEG_CX2341X_VIDEO_MEDIAN_FILTER_TYPE_DIAG = 4,
};
#define V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_BOTTOM (V4L2_CID_MPEG_CX2341X_BASE+7)
#define V4L2_CID_MPEG_CX2341X_VIDEO_LUMA_MEDIAN_FILTER_TOP (V4L2_CID_MPEG_CX2341X_BASE+8)
#define V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_BOTTOM (V4L2_CID_MPEG_CX2341X_BASE+9)
#define V4L2_CID_MPEG_CX2341X_VIDEO_CHROMA_MEDIAN_FILTER_TOP (V4L2_CID_MPEG_CX2341X_BASE+10)
#define V4L2_CID_MPEG_CX2341X_STREAM_INSERT_NAV_PACKETS (V4L2_CID_MPEG_CX2341X_BASE+11) /* Camera class control IDs */
#define V4L2_CID_CAMERA_CLASS_BASE (V4L2_CTRL_CLASS_CAMERA | 0x900)
#define V4L2_CID_CAMERA_CLASS (V4L2_CTRL_CLASS_CAMERA | 1) #define V4L2_CID_EXPOSURE_AUTO (V4L2_CID_CAMERA_CLASS_BASE+1)
enum v4l2_exposure_auto_type {
V4L2_EXPOSURE_AUTO = 0,
V4L2_EXPOSURE_MANUAL = 1,
V4L2_EXPOSURE_SHUTTER_PRIORITY = 2,
V4L2_EXPOSURE_APERTURE_PRIORITY = 3
};
#define V4L2_CID_EXPOSURE_ABSOLUTE (V4L2_CID_CAMERA_CLASS_BASE+2)
#define V4L2_CID_EXPOSURE_AUTO_PRIORITY (V4L2_CID_CAMERA_CLASS_BASE+3) #define V4L2_CID_PAN_RELATIVE (V4L2_CID_CAMERA_CLASS_BASE+4)
#define V4L2_CID_TILT_RELATIVE (V4L2_CID_CAMERA_CLASS_BASE+5)
#define V4L2_CID_PAN_RESET (V4L2_CID_CAMERA_CLASS_BASE+6)
#define V4L2_CID_TILT_RESET (V4L2_CID_CAMERA_CLASS_BASE+7) #define V4L2_CID_PAN_ABSOLUTE (V4L2_CID_CAMERA_CLASS_BASE+8)
#define V4L2_CID_TILT_ABSOLUTE (V4L2_CID_CAMERA_CLASS_BASE+9) #define V4L2_CID_FOCUS_ABSOLUTE (V4L2_CID_CAMERA_CLASS_BASE+10)
#define V4L2_CID_FOCUS_RELATIVE (V4L2_CID_CAMERA_CLASS_BASE+11)
#define V4L2_CID_FOCUS_AUTO (V4L2_CID_CAMERA_CLASS_BASE+12) /*
* T U N I N G
*/
struct v4l2_tuner
{
__u32 index;
__u8 name[32];
enum v4l2_tuner_type type;
__u32 capability;
__u32 rangelow;
__u32 rangehigh;
__u32 rxsubchans;
__u32 audmode;
__s32 signal;
__s32 afc;
__u32 reserved[4];
}; struct v4l2_modulator
{
__u32 index;
__u8 name[32];
__u32 capability;
__u32 rangelow;
__u32 rangehigh;
__u32 txsubchans;
__u32 reserved[4];
}; /* Flags for the 'capability' field */
#define V4L2_TUNER_CAP_LOW 0x0001
#define V4L2_TUNER_CAP_NORM 0x0002
#define V4L2_TUNER_CAP_STEREO 0x0010
#define V4L2_TUNER_CAP_LANG2 0x0020
#define V4L2_TUNER_CAP_SAP 0x0020
#define V4L2_TUNER_CAP_LANG1 0x0040 /* Flags for the 'rxsubchans' field */
#define V4L2_TUNER_SUB_MONO 0x0001
#define V4L2_TUNER_SUB_STEREO 0x0002
#define V4L2_TUNER_SUB_LANG2 0x0004
#define V4L2_TUNER_SUB_SAP 0x0004
#define V4L2_TUNER_SUB_LANG1 0x0008 /* Values for the 'audmode' field */
#define V4L2_TUNER_MODE_MONO 0x0000
#define V4L2_TUNER_MODE_STEREO 0x0001
#define V4L2_TUNER_MODE_LANG2 0x0002
#define V4L2_TUNER_MODE_SAP 0x0002
#define V4L2_TUNER_MODE_LANG1 0x0003
#define V4L2_TUNER_MODE_LANG1_LANG2 0x0004 struct v4l2_frequency
{
__u32 tuner;
enum v4l2_tuner_type type;
__u32 frequency;
__u32 reserved[8];
}; /*
* A U D I O
*/
struct v4l2_audio
{
__u32 index;
__u8 name[32];
__u32 capability;
__u32 mode;
__u32 reserved[2];
}; /* Flags for the 'capability' field */
#define V4L2_AUDCAP_STEREO 0x00001
#define V4L2_AUDCAP_AVL 0x00002 /* Flags for the 'mode' field */
#define V4L2_AUDMODE_AVL 0x00001 struct v4l2_audioout
{
__u32 index;
__u8 name[32];
__u32 capability;
__u32 mode;
__u32 reserved[2];
}; /*
* M P E G S E R V I C E S
*
* NOTE: EXPERIMENTAL API
*/
#if 1 /*KEEP*/
#define V4L2_ENC_IDX_FRAME_I (0)
#define V4L2_ENC_IDX_FRAME_P (1)
#define V4L2_ENC_IDX_FRAME_B (2)
#define V4L2_ENC_IDX_FRAME_MASK (0xf) struct v4l2_enc_idx_entry {
__u64 offset;
__u64 pts;
__u32 length;
__u32 flags;
__u32 reserved[2];
}; #define V4L2_ENC_IDX_ENTRIES (64)
struct v4l2_enc_idx {
__u32 entries;
__u32 entries_cap;
__u32 reserved[4];
struct v4l2_enc_idx_entry entry[V4L2_ENC_IDX_ENTRIES];
}; #define V4L2_ENC_CMD_START (0)
#define V4L2_ENC_CMD_STOP (1)
#define V4L2_ENC_CMD_PAUSE (2)
#define V4L2_ENC_CMD_RESUME (3) /* Flags for V4L2_ENC_CMD_STOP */
#define V4L2_ENC_CMD_STOP_AT_GOP_END (1 << 0) struct v4l2_encoder_cmd {
__u32 cmd;
__u32 flags;
union {
struct {
__u32 data[8];
} raw;
};
}; #endif /*
* D A T A S E R V I C E S ( V B I )
*
* Data services API by Michael Schimek
*/ /* Raw VBI */
struct v4l2_vbi_format
{
__u32 sampling_rate; /* in 1 Hz */
__u32 offset;
__u32 samples_per_line;
__u32 sample_format; /* V4L2_PIX_FMT_* */
__s32 start[2];
__u32 count[2];
__u32 flags; /* V4L2_VBI_* */
__u32 reserved[2]; /* must be zero */
}; /* VBI flags */
#define V4L2_VBI_UNSYNC (1<< 0)
#define V4L2_VBI_INTERLACED (1<< 1) /* Sliced VBI
*
* This implements is a proposal V4L2 API to allow SLICED VBI
* required for some hardware encoders. It should change without
* notice in the definitive implementation.
*/ struct v4l2_sliced_vbi_format
{
__u16 service_set;
/* service_lines[0][...] specifies lines 0-23 (1-23 used) of the first field
service_lines[1][...] specifies lines 0-23 (1-23 used) of the second field
(equals frame lines 313-336 for 625 line video
standards, 263-286 for 525 line standards) */
__u16 service_lines[2][24];
__u32 io_size;
__u32 reserved[2]; /* must be zero */
}; /* Teletext World System Teletext
(WST), defined on ITU-R BT.653-2 */
#define V4L2_SLICED_TELETEXT_B (0x0001)
/* Video Program System, defined on ETS 300 231*/
#define V4L2_SLICED_VPS (0x0400)
/* Closed Caption, defined on EIA-608 */
#define V4L2_SLICED_CAPTION_525 (0x1000)
/* Wide Screen System, defined on ITU-R BT1119.1 */
#define V4L2_SLICED_WSS_625 (0x4000) #define V4L2_SLICED_VBI_525 (V4L2_SLICED_CAPTION_525)
#define V4L2_SLICED_VBI_625 (V4L2_SLICED_TELETEXT_B | V4L2_SLICED_VPS | V4L2_SLICED_WSS_625) struct v4l2_sliced_vbi_cap
{
__u16 service_set;
/* service_lines[0][...] specifies lines 0-23 (1-23 used) of the first field
service_lines[1][...] specifies lines 0-23 (1-23 used) of the second field
(equals frame lines 313-336 for 625 line video
standards, 263-286 for 525 line standards) */
__u16 service_lines[2][24];
enum v4l2_buf_type type;
__u32 reserved[3]; /* must be 0 */
}; struct v4l2_sliced_vbi_data
{
__u32 id;
__u32 field; /* 0: first field, 1: second field */
__u32 line; /* 1-23 */
__u32 reserved; /* must be 0 */
__u8 data[48];
}; /*
* A G G R E G A T E S T R U C T U R E S
*/ /* Stream data format
*/
struct v4l2_format
{
enum v4l2_buf_type type;
union
{
struct v4l2_pix_format pix; // V4L2_BUF_TYPE_VIDEO_CAPTURE
struct v4l2_window win; // V4L2_BUF_TYPE_VIDEO_OVERLAY
struct v4l2_vbi_format vbi; // V4L2_BUF_TYPE_VBI_CAPTURE
struct v4l2_sliced_vbi_format sliced; // V4L2_BUF_TYPE_SLICED_VBI_CAPTURE
__u8 raw_data[200]; // user-defined
} fmt;
}; /* Stream type-dependent parameters
*/
struct v4l2_streamparm
{
enum v4l2_buf_type type;
union
{
struct v4l2_captureparm capture;
struct v4l2_outputparm output;
__u8 raw_data[200]; /* user-defined */
} parm;
}; /*
* A D V A N C E D D E B U G G I N G
*
* NOTE: EXPERIMENTAL API
*/ /* VIDIOC_DBG_G_REGISTER and VIDIOC_DBG_S_REGISTER */ #define V4L2_CHIP_MATCH_HOST 0 /* Match against chip ID on host (0 for the host) */
#define V4L2_CHIP_MATCH_I2C_DRIVER 1 /* Match against I2C driver ID */
#define V4L2_CHIP_MATCH_I2C_ADDR 2 /* Match against I2C 7-bit address */ struct v4l2_register {
__u32 match_type; /* Match type */
__u32 match_chip; /* Match this chip, meaning determined by match_type */
__u64 reg;
__u64 val;
}; /* VIDIOC_G_CHIP_IDENT */
struct v4l2_chip_ident {
__u32 match_type; /* Match type */
__u32 match_chip; /* Match this chip, meaning determined by match_type */
__u32 ident; /* chip identifier as specified in <media/v4l2-chip-ident.h> */
__u32 revision; /* chip revision, chip specific */
}; /*
* I O C T L C O D E S F O R V I D E O D E V I C E S
*
*/
#define VIDIOC_QUERYCAP _IOR ('V', 0, struct v4l2_capability)
#define VIDIOC_RESERVED _IO ('V', 1)
#define VIDIOC_ENUM_FMT _IOWR ('V', 2, struct v4l2_fmtdesc)
#define VIDIOC_G_FMT _IOWR ('V', 4, struct v4l2_format)
#define VIDIOC_S_FMT _IOWR ('V', 5, struct v4l2_format)
#define VIDIOC_REQBUFS _IOWR ('V', 8, struct v4l2_requestbuffers)
#define VIDIOC_QUERYBUF _IOWR ('V', 9, struct v4l2_buffer)
#define VIDIOC_G_FBUF _IOR ('V', 10, struct v4l2_framebuffer)
#define VIDIOC_S_FBUF _IOW ('V', 11, struct v4l2_framebuffer)
#define VIDIOC_OVERLAY _IOW ('V', 14, int)
#define VIDIOC_QBUF _IOWR ('V', 15, struct v4l2_buffer)
#define VIDIOC_DQBUF _IOWR ('V', 17, struct v4l2_buffer)
#define VIDIOC_STREAMON _IOW ('V', 18, int)
#define VIDIOC_STREAMOFF _IOW ('V', 19, int)
#define VIDIOC_G_PARM _IOWR ('V', 21, struct v4l2_streamparm)
#define VIDIOC_S_PARM _IOWR ('V', 22, struct v4l2_streamparm)
#define VIDIOC_G_STD _IOR ('V', 23, v4l2_std_id)
#define VIDIOC_S_STD _IOW ('V', 24, v4l2_std_id)
#define VIDIOC_ENUMSTD _IOWR ('V', 25, struct v4l2_standard)
#define VIDIOC_ENUMINPUT _IOWR ('V', 26, struct v4l2_input)
#define VIDIOC_G_CTRL _IOWR ('V', 27, struct v4l2_control)
#define VIDIOC_S_CTRL _IOWR ('V', 28, struct v4l2_control)
#define VIDIOC_G_TUNER _IOWR ('V', 29, struct v4l2_tuner)
#define VIDIOC_S_TUNER _IOW ('V', 30, struct v4l2_tuner)
#define VIDIOC_G_AUDIO _IOR ('V', 33, struct v4l2_audio)
#define VIDIOC_S_AUDIO _IOW ('V', 34, struct v4l2_audio)
#define VIDIOC_QUERYCTRL _IOWR ('V', 36, struct v4l2_queryctrl)
#define VIDIOC_QUERYMENU _IOWR ('V', 37, struct v4l2_querymenu)
#define VIDIOC_G_INPUT _IOR ('V', 38, int)
#define VIDIOC_S_INPUT _IOWR ('V', 39, int)
#define VIDIOC_G_OUTPUT _IOR ('V', 46, int)
#define VIDIOC_S_OUTPUT _IOWR ('V', 47, int)
#define VIDIOC_ENUMOUTPUT _IOWR ('V', 48, struct v4l2_output)
#define VIDIOC_G_AUDOUT _IOR ('V', 49, struct v4l2_audioout)
#define VIDIOC_S_AUDOUT _IOW ('V', 50, struct v4l2_audioout)
#define VIDIOC_G_MODULATOR _IOWR ('V', 54, struct v4l2_modulator)
#define VIDIOC_S_MODULATOR _IOW ('V', 55, struct v4l2_modulator)
#define VIDIOC_G_FREQUENCY _IOWR ('V', 56, struct v4l2_frequency)
#define VIDIOC_S_FREQUENCY _IOW ('V', 57, struct v4l2_frequency)
#define VIDIOC_CROPCAP _IOWR ('V', 58, struct v4l2_cropcap)
#define VIDIOC_G_CROP _IOWR ('V', 59, struct v4l2_crop)
#define VIDIOC_S_CROP _IOW ('V', 60, struct v4l2_crop)
#define VIDIOC_G_JPEGCOMP _IOR ('V', 61, struct v4l2_jpegcompression)
#define VIDIOC_S_JPEGCOMP _IOW ('V', 62, struct v4l2_jpegcompression)
#define VIDIOC_QUERYSTD _IOR ('V', 63, v4l2_std_id)
#define VIDIOC_TRY_FMT _IOWR ('V', 64, struct v4l2_format)
#define VIDIOC_ENUMAUDIO _IOWR ('V', 65, struct v4l2_audio)
#define VIDIOC_ENUMAUDOUT _IOWR ('V', 66, struct v4l2_audioout)
#define VIDIOC_G_PRIORITY _IOR ('V', 67, enum v4l2_priority)
#define VIDIOC_S_PRIORITY _IOW ('V', 68, enum v4l2_priority)
#define VIDIOC_G_SLICED_VBI_CAP _IOWR ('V', 69, struct v4l2_sliced_vbi_cap)
#define VIDIOC_LOG_STATUS _IO ('V', 70)
#define VIDIOC_G_EXT_CTRLS _IOWR ('V', 71, struct v4l2_ext_controls)
#define VIDIOC_S_EXT_CTRLS _IOWR ('V', 72, struct v4l2_ext_controls)
#define VIDIOC_TRY_EXT_CTRLS _IOWR ('V', 73, struct v4l2_ext_controls)
#if 1 /*KEEP*/
#define VIDIOC_ENUM_FRAMESIZES _IOWR ('V', 74, struct v4l2_frmsizeenum)
#define VIDIOC_ENUM_FRAMEINTERVALS _IOWR ('V', 75, struct v4l2_frmivalenum)
#define VIDIOC_G_ENC_INDEX _IOR ('V', 76, struct v4l2_enc_idx)
#define VIDIOC_ENCODER_CMD _IOWR ('V', 77, struct v4l2_encoder_cmd)
#define VIDIOC_TRY_ENCODER_CMD _IOWR ('V', 78, struct v4l2_encoder_cmd) /* Experimental, only implemented if CONFIG_VIDEO_ADV_DEBUG is defined */
#define VIDIOC_DBG_S_REGISTER _IOW ('V', 79, struct v4l2_register)
#define VIDIOC_DBG_G_REGISTER _IOWR ('V', 80, struct v4l2_register) #define VIDIOC_G_CHIP_IDENT _IOWR ('V', 81, struct v4l2_chip_ident)
#endif #ifdef __OLD_VIDIOC_
/* for compatibility, will go away some day */
#define VIDIOC_OVERLAY_OLD _IOWR ('V', 14, int)
#define VIDIOC_S_PARM_OLD _IOW ('V', 22, struct v4l2_streamparm)
#define VIDIOC_S_CTRL_OLD _IOW ('V', 28, struct v4l2_control)
#define VIDIOC_G_AUDIO_OLD _IOWR ('V', 33, struct v4l2_audio)
#define VIDIOC_G_AUDOUT_OLD _IOWR ('V', 49, struct v4l2_audioout)
#define VIDIOC_CROPCAP_OLD _IOR ('V', 58, struct v4l2_cropcap)
#endif #define BASE_VIDIOC_PRIVATE 192 /* 192-255 are private */ #endif /* __LINUX_VIDEODEV2_H */ /*
* Local variables:
* c-basic-offset: 8
* End:
*/
Appendix B. Video Capture Example
/*
* V4L2 video capture example
*
* This program can be used and distributed without restrictions.
*/ #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h> #include <getopt.h> /* getopt_long() */ #include <fcntl.h> /* low-level i/o */
#include <unistd.h>
#include <errno.h>
#include <malloc.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h> #include <asm/types.h> /* for videodev2.h */ #include <linux/videodev2.h> #define CLEAR(x) memset (&(x), 0, sizeof (x)) typedef enum {
IO_METHOD_READ,
IO_METHOD_MMAP,
IO_METHOD_USERPTR,
} io_method; struct buffer {
void * start;
size_t length;
}; static char * dev_name = NULL;
static io_method io = IO_METHOD_MMAP;
static int fd = -1;
struct buffer * buffers = NULL;
static unsigned int n_buffers = 0; static void
errno_exit (const char * s)
{
fprintf (stderr, "%s error %d, %s\n",
s, errno, strerror (errno)); exit (EXIT_FAILURE);
} static int
xioctl (int fd,
int request,
void * arg)
{
int r; do r = ioctl (fd, request, arg);
while (-1 == r && EINTR == errno); return r;
} static void
process_image (const void * p)
{
fputc ('.', stdout);
fflush (stdout);
} static int
read_frame (void)
{
struct v4l2_buffer buf;
unsigned int i; switch (io) {
case IO_METHOD_READ:
if (-1 == read (fd, buffers[0].start, buffers[0].length)) {
switch (errno) {
case EAGAIN:
return 0; case EIO:
/* Could ignore EIO, see spec. */ /* fall through */ default:
errno_exit ("read");
}
} process_image (buffers[0].start); break; case IO_METHOD_MMAP:
CLEAR (buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP; if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {
switch (errno) {
case EAGAIN:
return 0; case EIO:
/* Could ignore EIO, see spec. */ /* fall through */ default:
errno_exit ("VIDIOC_DQBUF");
}
} assert (buf.index < n_buffers); process_image (buffers[buf.index].start); if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF"); break; case IO_METHOD_USERPTR:
CLEAR (buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_USERPTR; if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {
switch (errno) {
case EAGAIN:
return 0; case EIO:
/* Could ignore EIO, see spec. */ /* fall through */ default:
errno_exit ("VIDIOC_DQBUF");
}
} for (i = 0; i < n_buffers; ++i)
if (buf.m.userptr == (unsigned long) buffers[i].start
&& buf.length == buffers[i].length)
break; assert (i < n_buffers); process_image ((void *) buf.m.userptr); if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF"); break;
} return 1;
} static void
mainloop (void)
{
unsigned int count; count = 100; while (count-- > 0) {
for (;;) {
fd_set fds;
struct timeval tv;
int r; FD_ZERO (&fds);
FD_SET (fd, &fds); /* Timeout. */
tv.tv_sec = 2;
tv.tv_usec = 0; r = select (fd + 1, &fds, NULL, NULL, &tv); if (-1 == r) {
if (EINTR == errno)
continue; errno_exit ("select");
} if (0 == r) {
fprintf (stderr, "select timeout\n");
exit (EXIT_FAILURE);
} if (read_frame ())
break; /* EAGAIN - continue select loop. */
}
}
} static void
stop_capturing (void)
{
enum v4l2_buf_type type; switch (io) {
case IO_METHOD_READ:
/* Nothing to do. */
break; case IO_METHOD_MMAP:
case IO_METHOD_USERPTR:
type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
errno_exit ("VIDIOC_STREAMOFF"); break;
}
} static void
start_capturing (void)
{
unsigned int i;
enum v4l2_buf_type type; switch (io) {
case IO_METHOD_READ:
/* Nothing to do. */
break; case IO_METHOD_MMAP:
for (i = 0; i < n_buffers; ++i) {
struct v4l2_buffer buf; CLEAR (buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = i; if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
} type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
errno_exit ("VIDIOC_STREAMON"); break; case IO_METHOD_USERPTR:
for (i = 0; i < n_buffers; ++i) {
struct v4l2_buffer buf; CLEAR (buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_USERPTR;
buf.index = i;
buf.m.userptr = (unsigned long) buffers[i].start;
buf.length = buffers[i].length; if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
} type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
errno_exit ("VIDIOC_STREAMON"); break;
}
} static void
uninit_device (void)
{
unsigned int i; switch (io) {
case IO_METHOD_READ:
free (buffers[0].start);
break; case IO_METHOD_MMAP:
for (i = 0; i < n_buffers; ++i)
if (-1 == munmap (buffers[i].start, buffers[i].length))
errno_exit ("munmap");
break; case IO_METHOD_USERPTR:
for (i = 0; i < n_buffers; ++i)
free (buffers[i].start);
break;
} free (buffers);
} static void
init_read (unsigned int buffer_size)
{
buffers = calloc (1, sizeof (*buffers)); if (!buffers) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
} buffers[0].length = buffer_size;
buffers[0].start = malloc (buffer_size); if (!buffers[0].start) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
} static void
init_mmap (void)
{
struct v4l2_requestbuffers req; CLEAR (req); req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP; if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req)) {
if (EINVAL == errno) {
fprintf (stderr, "%s does not support "
"memory mapping\n", dev_name);
exit (EXIT_FAILURE);
} else {
errno_exit ("VIDIOC_REQBUFS");
}
} if (req.count < 2) {
fprintf (stderr, "Insufficient buffer memory on %s\n",
dev_name);
exit (EXIT_FAILURE);
} buffers = calloc (req.count, sizeof (*buffers)); if (!buffers) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
} for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
struct v4l2_buffer buf; CLEAR (buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers; if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))
errno_exit ("VIDIOC_QUERYBUF"); buffers[n_buffers].length = buf.length;
buffers[n_buffers].start =
mmap (NULL /* start anywhere */,
buf.length,
PROT_READ | PROT_WRITE /* required */,
MAP_SHARED /* recommended */,
fd, buf.m.offset); if (MAP_FAILED == buffers[n_buffers].start)
errno_exit ("mmap");
}
} static void
init_userp (unsigned int buffer_size)
{
struct v4l2_requestbuffers req;
unsigned int page_size; page_size = getpagesize ();
buffer_size = (buffer_size + page_size - 1) & ~(page_size - 1); CLEAR (req); req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_USERPTR; if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req)) {
if (EINVAL == errno) {
fprintf (stderr, "%s does not support "
"user pointer i/o\n", dev_name);
exit (EXIT_FAILURE);
} else {
errno_exit ("VIDIOC_REQBUFS");
}
} buffers = calloc (4, sizeof (*buffers)); if (!buffers) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
} for (n_buffers = 0; n_buffers < 4; ++n_buffers) {
buffers[n_buffers].length = buffer_size;
buffers[n_buffers].start = memalign (/* boundary */ page_size,
buffer_size); if (!buffers[n_buffers].start) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
}
} static void
init_device (void)
{
struct v4l2_capability cap;
struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
struct v4l2_format fmt;
unsigned int min; if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap)) {
if (EINVAL == errno) {
fprintf (stderr, "%s is no V4L2 device\n",
dev_name);
exit (EXIT_FAILURE);
} else {
errno_exit ("VIDIOC_QUERYCAP");
}
} if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
fprintf (stderr, "%s is no video capture device\n",
dev_name);
exit (EXIT_FAILURE);
} switch (io) {
case IO_METHOD_READ:
if (!(cap.capabilities & V4L2_CAP_READWRITE)) {
fprintf (stderr, "%s does not support read i/o\n",
dev_name);
exit (EXIT_FAILURE);
} break; case IO_METHOD_MMAP:
case IO_METHOD_USERPTR:
if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
fprintf (stderr, "%s does not support streaming i/o\n",
dev_name);
exit (EXIT_FAILURE);
} break;
} /* Select video input, video standard and tune here. */ CLEAR (cropcap); cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (0 == xioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect; /* reset to default */ if (-1 == xioctl (fd, VIDIOC_S_CROP, &crop)) {
switch (errno) {
case EINVAL:
/* Cropping not supported. */
break;
default:
/* Errors ignored. */
break;
}
}
} else {
/* Errors ignored. */
} CLEAR (fmt); fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.width = 640;
fmt.fmt.pix.height = 480;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED; if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))
errno_exit ("VIDIOC_S_FMT"); /* Note VIDIOC_S_FMT may change width and height. */ /* Buggy driver paranoia. */
min = fmt.fmt.pix.width * 2;
if (fmt.fmt.pix.bytesperline < min)
fmt.fmt.pix.bytesperline = min;
min = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height;
if (fmt.fmt.pix.sizeimage < min)
fmt.fmt.pix.sizeimage = min; switch (io) {
case IO_METHOD_READ:
init_read (fmt.fmt.pix.sizeimage);
break; case IO_METHOD_MMAP:
init_mmap ();
break; case IO_METHOD_USERPTR:
init_userp (fmt.fmt.pix.sizeimage);
break;
}
} static void
close_device (void)
{
if (-1 == close (fd))
errno_exit ("close"); fd = -1;
} static void
open_device (void)
{
struct stat st; if (-1 == stat (dev_name, &st)) {
fprintf (stderr, "Cannot identify '%s': %d, %s\n",
dev_name, errno, strerror (errno));
exit (EXIT_FAILURE);
} if (!S_ISCHR (st.st_mode)) {
fprintf (stderr, "%s is no device\n", dev_name);
exit (EXIT_FAILURE);
} fd = open (dev_name, O_RDWR /* required */ | O_NONBLOCK, 0); if (-1 == fd) {
fprintf (stderr, "Cannot open '%s': %d, %s\n",
dev_name, errno, strerror (errno));
exit (EXIT_FAILURE);
}
} static void
usage (FILE * fp,
int argc,
char ** argv)
{
fprintf (fp,
"Usage: %s [options]\n\n"
"Options:\n"
"-d | --device name Video device name [/dev/video]\n"
"-h | --help Print this message\n"
"-m | --mmap Use memory mapped buffers\n"
"-r | --read Use read() calls\n"
"-u | --userp Use application allocated buffers\n"
"",
argv[0]);
} static const char short_options [] = "d:hmru"; static const struct option
long_options [] = {
{ "device", required_argument, NULL, 'd' },
{ "help", no_argument, NULL, 'h' },
{ "mmap", no_argument, NULL, 'm' },
{ "read", no_argument, NULL, 'r' },
{ "userp", no_argument, NULL, 'u' },
{ 0, 0, 0, 0 }
}; int
main (int argc,
char ** argv)
{
dev_name = "/dev/video"; for (;;) {
int index;
int c; c = getopt_long (argc, argv,
short_options, long_options,
&index); if (-1 == c)
break; switch (c) {
case 0: /* getopt_long() flag */
break; case 'd':
dev_name = optarg;
break; case 'h':
usage (stdout, argc, argv);
exit (EXIT_SUCCESS); case 'm':
io = IO_METHOD_MMAP;
break; case 'r':
io = IO_METHOD_READ;
break; case 'u':
io = IO_METHOD_USERPTR;
break; default:
usage (stderr, argc, argv);
exit (EXIT_FAILURE);
}
} open_device (); init_device (); start_capturing (); mainloop (); stop_capturing (); uninit_device (); close_device (); exit (EXIT_SUCCESS); return 0;
}
Appendix C. GNU Free Documentation License
C.1. 0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other written document"free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
C.2. 1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The"Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as"you".
A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A "Secondary Section" is a named appendix or a front-matter section of theDocument that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.
The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that theDocument is released under this License.
The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that theDocument is released under this License.
A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not"Transparent" is called"Opaque".
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML designed for human modification. Opaque formats include PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML produced by some word processors for output purposes only.
The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.
C.3. 2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
C.4. 3. COPYING IN QUANTITY
If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
C.5. 4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in theModified Version, together with at least five of the principal authors of theDocument (all of its principal authors, if it has less than five).
C. State on the Title Page the name of the publisher of the Modified Version, as the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
F. Include, immediately after the copyright notices, a license notice giving the public permission to use theModified Version under the terms of this License, in the form shown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
H. Include an unaltered copy of this License.
I. Preserve the section entitled "History", and its title, and add to it an item stating at least the title, year, new authors, and publisher of theModified Versionas given on the Title Page. If there is no section entitled "History" in theDocument, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the"History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
K. In any section entitled "Acknowledgements" or"Dedications", preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
M. Delete any section entitled "Endorsements". Such a section may not be included in theModified Version.
N. Do not retitle any existing section as "Endorsements" or to conflict in title with any Invariant Section.
If the Modified Version includes new front-matter sections or appendices that qualify asSecondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list ofInvariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.
You may add a section entitled "Endorsements", provided it contains nothing but endorsements of yourModified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If theDocument already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of anyModified Version.
C.6. 5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the terms defined insection 4 above for modified versions, provided that you include in the combination all of theInvariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.
The combined work need only contain one copy of this License, and multiple identicalInvariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections entitled "History" in the various original documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgements", and any sections entitled"Dedications". You must delete all sections entitled"Endorsements."
C.7. 6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and dispbibute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
C.8. 7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as aModified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an"aggregate", and this License does not apply to the other self-contained works thus compiled with the Document , on account of their being thus compiled, if they are not themselves derivative works of the Document. If theCover Text requirement ofsection 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.
C.9. 8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of theDocument under the terms ofsection 4. ReplacingInvariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.
C.10. 9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
C.11. 10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.
C.12. Addendum
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have noFront-Cover Texts, write"no Front-Cover Texts" instead of "Front-Cover Texts being LIST"; likewise forBack-Cover Texts.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as theGNU General Public License, to permit their use in free software.
List of Types
- v4l2_std_id
- enum v4l2_buf_type
- enum v4l2_colorspace
- enum v4l2_ctrl_type
- enum v4l2_field
- enum v4l2_frmivaltypes
- enum v4l2_frmsizetypes
- enum v4l2_memory
- enum v4l2_priority
- enum v4l2_tuner_type
- struct v4l2_audio
- struct v4l2_audioout
- struct v4l2_buffer
- struct v4l2_capability
- struct v4l2_captureparm
- struct v4l2_chip_ident
- struct v4l2_clip
- struct v4l2_control
- struct v4l2_crop
- struct v4l2_cropcap
- struct v4l2_enc_idx
- struct v4l2_enc_idx_entry
- struct v4l2_encoder_cmd
- struct v4l2_ext_control
- struct v4l2_ext_controls
- struct v4l2_fmtdesc
- struct v4l2_format
- struct v4l2_fract
- struct v4l2_framebuffer
- struct v4l2_frequency
- struct v4l2_frmival_stepwise
- struct v4l2_frmivalenum
- struct v4l2_frmsize_discrete
- struct v4l2_frmsize_stepwise
- struct v4l2_frmsizeenum
- struct v4l2_input
- struct v4l2_jpegcompression
- struct v4l2_modulator
- struct v4l2_output
- struct v4l2_outputparm
- struct v4l2_pix_format
- struct v4l2_queryctrl
- struct v4l2_querymenu
- struct v4l2_rect
- struct v4l2_register
- struct v4l2_requestbuffers
- struct v4l2_sliced_vbi_cap
- struct v4l2_sliced_vbi_data
- struct v4l2_sliced_vbi_format
- struct v4l2_standard
- struct v4l2_streamparm
- struct v4l2_timecode
- struct v4l2_tuner
- struct v4l2_vbi_format
- struct v4l2_window
References
[EIA 608-B] Electronic Industries Alliance (http://www.eia.org),EIA 608-B "Recommended Practice for Line 21 DataService".
[EN 300 294] European Telecommunication Standards Institute(http://www.etsi.org),EN 300 294 "625-line television Wide Screen Signalling(WSS)".
[ETS 300 231] European Telecommunication Standards Institute(http://www.etsi.org),ETS 300 231 "Specification of the domestic videoProgramme Delivery Control system (PDC)".
[ETS 300 706] European Telecommunication Standards Institute(http://www.etsi.org),ETS 300 706 "Enhanced Teletext specification".
[ISO 13818-1] International Telecommunication Union (http://www.itu.ch), InternationalOrganisation for Standardisation (http://www.iso.ch),ITU-T Rec. H.222.0 | ISO/IEC 13818-1 "Informationtechnology — Generic coding of moving pictures and associatedaudio information: Systems".
[ISO 13818-2] International Telecommunication Union (http://www.itu.ch), InternationalOrganisation for Standardisation (http://www.iso.ch),ITU-T Rec. H.262 | ISO/IEC 13818-2 "Informationtechnology — Generic coding of moving pictures and associatedaudio information: Video".
[ITU BT.470] International Telecommunication Union (http://www.itu.ch),ITU-R Recommendation BT.470-6 "Conventional TelevisionSystems".
[ITU BT.601] International Telecommunication Union (http://www.itu.ch),ITU-R Recommendation BT.601-5 "Studio Encoding Parametersof Digital Television for Standard 4:3 and Wide-Screen 16:9 AspectRatios".
[ITU BT.653] International Telecommunication Union (http://www.itu.ch),ITU-R Recommendation BT.653-3 "Teletext systems".
[ITU BT.709] International Telecommunication Union (http://www.itu.ch),ITU-R Recommendation BT.709-5 "Parameter values for theHDTV standards for production and international programmeexchange".
[ITU BT.1119] International Telecommunication Union (http://www.itu.ch),ITU-R Recommendation BT.1119 "625-linetelevision Wide Screen Signalling (WSS)".
[JFIF] Independent JPEG Group (http://www.ijg.org),JPEG File Interchange Format: Version 1.02.
[SMPTE 12M] Society of Motion Picture and Television Engineers(http://www.smpte.org),SMPTE 12M-1999 "Television, Audio and Film - Time andControl Code".
[SMPTE 170M] Society of Motion Picture and Television Engineers(http://www.smpte.org),SMPTE 170M-1999 "Television - Composite Analog VideoSignal - NTSC for Studio Applications".
[SMPTE 240M] Society of Motion Picture and Television Engineers(http://www.smpte.org),SMPTE 240M-1999 "Television - Signal Parameters -1125-Line High-Definition Production".
[V4L] Alan Cox, Video4Linux API Specification.
This file is part of the Linux kernel sources underDocumentation/video4linux.
[V4LPROG] Alan Cox, Video4Linux Programming (a.k.a. The Video4LinuxBook), 2000.
About V4L driver programming. Thisbook is part of the Linux kernel DocBook documentation, for example athttp://kernelnewbies.org/documents/. SGML sources are includedin the kernel sources.
Notes
| [1] | Access permissions are associated with characterdevice special files, hence we must ensure device numbers cannotchange with the module load order. To this end minor numbers are nolonger automatically assigned by the "videodev" module as in V4L butrequested by the driver. The defaults will suffice for most peopleunless two drivers compete for the same minor numbers. |
| [2] | In earlier versions of the V4L2 API the module optionswhere named after the device special file with a "unit_" prefix, expressingthe minor number itself, not an offset. Rationale for this change is unknown.Lastly the naming and semantics are just a convention among driver writers,the point to note is that minor numbers are not supposed to be hardcodedinto drivers. |
| [3] | Given a device file name one cannot reliable findrelated devices. For once names are arbitrary and in a system withmultiple devices, where only some support VBI capturing, a/dev/video2 is not necessarily related to/dev/vbi2. The V4L |
| [4] | Drivers could recognize the |
| [5] | Actually struct v4l2_audio ought to have a |
| [6] | Some users are already confused by technical terms PAL,NTSC and SECAM. There is no point asking them to distinguish betweenB, G, D, or K when the software or hardware can do thatautomatically. |
| [7] | An alternative to the current scheme is to use pointersto indices as arguments of Indices are consistent with the rest of the APIand identify the standard unambiguously. In the present scheme ofthings an enumerated standard is looked up byv4l2_std_id. Now thestandards supported by the inputs of a device can overlap. Justassume the tuner and composite input in the example above bothexist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggestsa choice which does not exist. We cannot merge or omit sets, becauseapplications would be unable to find the standards reported by So insummary, the lookup itself is unavoidable. The difference is onlywhether the lookup is necessary to find an enumerated standard or toswitch to a standard byv4l2_std_id. |
| [8] | See Section 3.5 for a rationale. Probablyeven USB cameras follow some well known video standard. It might havebeen better to explicitly indicate elsewhere if a device cannot liveup to normal expectations, instead of this exception. |
| [9] | It will be more convenient for applications if driversmake use of the |
| [10] | Applications could call an ioctl to request events.After another process called |
| [11] | Enumerating formats an application has no a-prioriknowledge of (otherwise it could explicitely ask for them and need notenumerate) seems useless, but there are applications serving as proxybetween drivers and the actual video applications for which this isuseful. |
| [12] | It would be desirable if applications could depend ondrivers supporting all I/O interfaces, but as much as the complexmemory mapping I/O can be inadequate for some devices we have noreason to require this interface, which is most useful for simpleapplications capturing still images. |
| [13] | At the driver level |
| [14] | One could use one file descriptor and set the buffertype field accordingly when calling |
| [15] | Random enqueue order permits applications processingimages out of order (such as video codecs) to return buffers earlier,reducing the probability of data loss. Random fill order allowsdrivers to reuse buffers on a LIFO-basis, taking advantage of cachesholding scatter-gather lists and the like. |
| [16] | At the driver level |
| [17] | We expect that frequently used buffers are typically notswapped out. Anyway, the process of swapping, locking or generatingscatter-gather lists may be time consuming. The delay can be masked bythe depth of the incoming buffer queue, and perhaps by maintainingcaches assuming a buffer will be soon enqueued again. On the otherhand, to optimize memory usage drivers can limit the number of bufferslocked in advance and recycle the most recently used buffers first. Ofcourse, the pages of empty buffers in the incoming queue need not besaved to disk. Output buffers must be saved on the incoming andoutgoing queue because an application may share them with otherprocesses. |
| [18] | At the driver level |
| [19] | Since no other Linux multimediaAPI supports unadjusted time it would be foolish to introduce here. Wemust use a universally supported clock to synchronize different media,hence time of day. |
| [20] | A common application of two file descriptors is theXFree86 Xv/V4L interface driver anda V4L2 application. While the X server controls video overlay, theapplication can take advantage of memory mapping and DMA. In the opinion of the designers of this API, no driverwriter taking the efforts to support simultaneous capturing andoverlay will restrict this ability by requiring a single filedescriptor, as in V4L and earlier versions of V4L2. Making thisoptional means applications depending on two file descriptors needbackup routines to be compatible with all drivers, which isconsiderable more work than using two fds in applications which donot. Also two fd's fit the general concept of one file descriptor foreach logical stream. Hence as a complexity trade-off driversmust support two file descriptors andmay support single fd operation. |
| [21] | The X Window system defines "regions" which arevectors of struct BoxRec { short x1, y1, x2, y2; } with width = x2 -x1 and height = y2 - y1, so one cannot pass X11 clip listsdirectly. |
| [22] | ASK: Amplitude-Shift Keying. A high signallevel represents a '1' bit, a low level a '0' bit. |
| [23] | The supported standards may overlap and we need anunambiguous set to find the current standard returned by |
| [24] |
|
| [25] | For example video output requires at least two buffers,one displayed and one filled by the application. |
| [26] | The Linux kernel implements |
This is not implemented in XFree86.