ffmpeg之AVPacket

1. AVPacket简介

   AVPacket用于存放压缩后的数据。
    通常是解复用的输出，解码的输入；或者是编码的输出，复用的输入。
    对于视频数据，AVPacket通常包含一帧视频压缩数据；对于音频数据数据，AVPacket则通常包含了数帧的音频压缩数据。
    其大小是public ABI的一部分，因此可以在栈上分配。
    其数据有效期取决于“buf”或“destruct”，如果字段设置了，数据包的管理由ffmpeg负责，数据包在avt_free_packet（内部调用avt_buffer_unref）调用前都有效；如果字段未设置，数据包由调用者管理。

2. AVPacket定义

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1. typedef struct AVPacket {
   
2.     /**
   
3.      * A reference to the reference-counted buffer where the packet data is
   
4.      * stored.
   
5.      * May be NULL, then the packet data is not reference-counted.
   
6.      */
   
7.     AVBufferRef* buf;
   
8.     /**
   
9.      * Presentation timestamp in AVStream->time_base units; the time at which
   
10.      * the decompressed packet will be presented to the user.
    
11.      * Can be AV_NOPTS_VALUE if it is not stored in the file.
    
12.      * pts MUST be larger or equal to dts as presentation cannot happen before
    
13.      * decompression, unless one wants to view hex dumps. Some formats misuse
    
14.      * the terms dts and pts/cts to mean something different. Such timestamps
    
15.      * must be converted to true pts/dts before they are stored in AVPacket.
    
16.      */
    
17.     int64_t pts;
    
18.     /**
    
19.      * Decompression timestamp in AVStream->time_base units; the time at which
    
20.      * the packet is decompressed.
    
21.      * Can be AV_NOPTS_VALUE if it is not stored in the file.
    
22.      */
    
23.     int64_t dts;
    
24.     uint8_t* data;
    
25.     int size;
    
26.     int stream_index;
    
27.     /**
    
28.      * A combination of AV_PKT_FLAG values
    
29.      */
    
30.     int flags;
    
31.     /**
    
32.      * Additional packet data that can be provided by the container.
    
33.      * Packet can contain several types of side information.
    
34.      */
    
35.     AVPacketSideData* side_data;
    
36.     int side_data_elems;
    
37. 
    
38.     /**
    
39.      * Duration of this packet in AVStream->time_base units, 0 if unknown.
    
40.      * Equals next_pts - this_pts in presentation order.
    
41.      */
    
42.     int duration;
    
43. #if FF_API_DESTRUCT_PACKET
    
44.     attribute_deprecated
    
45.     void (*destruct)(struct AVPacket *);
    
46.     attribute_deprecated
    
47.     void* priv;
    
48. #endif
    
49.     int64_t pos; ///< byte position in stream, -1 if unknown
    
50. 
    
51.     /**
    
52.      * Time difference in AVStream->time_base units from the pts of this
    
53.      * packet to the point at which the output from the decoder has converged
    
54.      * independent from the availability of previous frames. That is, the
    
55.      * frames are virtually identical no matter if decoding started from
    
56.      * the very first frame or from this keyframe.
    
57.      * Is AV_NOPTS_VALUE if unknown.
    
58.      * This field is not the display duration of the current packet.
    
59.      * This field has no meaning if the packet does not have AV_PKT_FLAG_KEY
    
60.      * set.
    
61.      *
    
62.      * The purpose of this field is to allow seeking in streams that have no
    
63.      * keyframes in the conventional sense. It corresponds to the
    
64.      * recovery point SEI in H.264 and match_time_delta in NUT. It is also
    
65.      * essential for some types of subtitle streams to ensure that all
    
66.      * subtitles are correctly displayed after seeking.
    
67.      */
    
68.     int64_t convergence_duration;
    
69. } AVPacket;

3. AVPacket字段说明

    AVBufferRef* buf ： data的引用计数结构体，可能为NULL。不为NULL时，数据管理由ffmpeg负责，为NULL时由调用者自己负责。
    int64_t pts ： 压缩数据显示时间戳。比较关键的数据，在做seek和播放进度的时候都要用到它，pts只是一个数量，对应于AVStream->time_base，要根据time_base才能转换为具体的时间,音频和视频一般有不同的time_base，所以在做音视频同步一定要做转换，不能直接拿pts做。
    int64_t dts ： 压缩数据解码时间戳。
    uint8_t* data ：压缩数据。
    int   size ：压缩数据大小。
    int   stream_index ：所属媒体流的索引。用来区分音频，视频，和字幕数据。
    int   flags ： 标识域。其中为1表示该数据是一个关键帧，AV_PKT_FLAG_KEY 0x0001 关键帧
    AVPacketSideData* side_data ：附加数据。
    int side_data_elems ：附加数据大小。
    int   duration ：压缩数据时长。
    void  (*destruct)(struct AVPacket *) ：压缩数据释放函数指针。
    void* priv  ：
    int64_t pos ：压缩数据在媒体流中的偏移量，未知为-1。
    int64_t convergence_duration ：