by Jernej Virag
http://www.virag.si/2012/01/web-video-encoding-tutorial-with-ffmpeg-0-9/
Web is full of articles about encoding videos with FFmpeg, howevermost of them are obsolete and use old non-working FFmpeg parameters. Sohere’s my guide for encoding web videos using recent FFMpeg versions for Flash and HTML5.
Step 1: Get a new build of FFmpegMost distribution builds are very old, buggy and have numerous issues. Don’t use them.
Ubuntu users: UbuntuForums has a great guide for compiling newest FFMpeg build. I strongly suggest you use a stable FFmpeg build (that’s 0.9 at the moment) instead of git master. Don’t forget libx264 for H.264 and libvpx for VP8/WEBM if youwant that support.
Windows users: Zeranoe has great static builds of FFmpeg for Windows with libx264 and libvpx included. Use those for encoding.
OS X users: MacPorts should be able to compile andinstall FFmpeg with libvpx and libx264. As I don’t use any machines with OS X you’ll have to check for yourself.
If you already know what “bitrates, profiles etc.” are and just want the command lines skip to step 4 or sample command lines.
Now you’ll have to decide on two things – the resolution, bitrate and profile you want those videos in.
Resolution gives “sharpness” to the overall image.If you choose a low resolution, the video will be small and it will beblurry when users will put it full-screen. Typically people use 360p(that’s a shorthand naming made popular by HD televison, meaning picture has height of 360 with width corresponding to wanted aspect ratio),480p, 720p and 1080p resolutions, as those correspond to common screensizes, which avoids excessive blurriness.
Contrary to popular belief, resolution does not affect file size.
Bitrate tells the encoder about how many bits should each second of video have. It directly determines file size of thevideo along with the quality. Set too low, it will cause the video tolook very blocky (especially in fast-moving scenes) and set too highwill make your files excessively large.
When choosing bitrate you need to remember, it is directly connected toresolution – storing pictures of certain resolution requires more bits,so if you want higher resolution videos, you’ll have to choose higherbitrate for them to not look like garbage.
A rule of thumb to calculate file size from bitrate is:
filesize (in MB) = (bitrate in Mbit/s * 8) * (video length in seconds)
Some general resolution/bitrate guidelines that we’ve found to work well at Viidea:
| Resolution | Bitrate | Approximate size of 10 min video |
|---|---|---|
| 320p (for mobile phones) | 180 kbit/s | ~13 MB |
| 360p | 300 kbit/s | ~22 MB |
| 480p | 500 kbit/s | ~37 MB |
| 576p (SD) | 850 kbit/s | ~63 MB |
| 720p (HD) | 1000 kbit/s | ~75 MB |
The values in the table were optimized for lecture-type contentrecorded with SD cameras, so if you want to encode something moredynamic (like Transformers ;) ) I suggest you bump the bitrate up alittle.
Profile constrains H.264 to a subset of features –higher profiles require more CPU power to decode and are able togenerate better looking videos at same bitrate. You should always choose the best profile your target devices support.
Basic support matrix for devices:
| Device | Max supported profile |
|---|---|
| Desktop browsers iPhone 4S, iPad 2 Tegra Android tablets Xbox 360, Playstation 3 | High profile |
| iPhone 3GS, iPhone 4, iPad high-end Android phones | Main profile |
| iPhone, iPhone 3G medium/low-end Android devices other embedded players | Baseline profile |
Remember, most devices also have a maximum resolution and bitrate they can handle. That is usually expressed in as a H.264 level and can be set in FFmpeg with -level parameter (this will make FFmpegabort encoding of videos which couldn’t be played on the device).
Step 3: Choose audio format and bitrateFor audio, the choice is a lot simpler – if you want to encode for mobile devices I strongly suggest you use AAC+, which has a very noticeable improvement of audio quality at the same bitrate. Sadly, libaacplus inFFmpeg, doesn’t support encoding to higher bitrates than 64 kbps. Withthat in mind, AAC+ is perfect for podcasts, lectures and other speechcontent, since it retains quality even at low bitrates (differencebetween “normal” LC-AAC at 96 kbit/s vs. AAC+ at 64kbit/s ispractically not noticeable).
Otherwise, rule of thumb is to take around 128 kbit/s for standard quality material and 192kbit/s for something more complex.
So, you made all the hard choices and now it’s time to encode yourvideo. FFmpeg command line to encode a standard web video looks likethis:
ffmpeg -i input_file.avi -vcodec libx264 -vprofile high -preset slow -b:v 500k -maxrate 500k -bufsize 1000k -vf scale=-1:480 -threads 0 -acodec libvo_aacenc -b:a 128k output_file.mp4
You can also encode the video in two-passes, which gives the addedbenefit of quality increase and more accurate file size for givenbitrate:
1st pass:
ffmpeg -i input_file.avi -vcodec libx264 -vprofile high -preset slow -b:v 500k -maxrate 500k -bufsize 1000k -vf scale=-1:480 -threads 0 -pass 1 -an -f mp4 /dev/null
2nd pass:
ffmpeg -i input_file.avi -vcodec libx264 -vprofile high -preset slow -b:v 500k -maxrate 500k -bufsize 1000k -vf scale=-1:480 -threads 0 -pass 2 -acodec libvo_aacenc -b:a 128k -f mp4 output_file.mp4
Scary isn’t it?
Warning: ffmpeg command line arguments are positionsensitive – make sure you don’t mix up the order. Good rule of thumb toprevent mistakes is to keep the order of
ffmpeg [input options] -i [input filename] -vcodec [video options] -acodec [audio options] [output file options] [output filename]
Let’s break down all those parameters:
-i [input file] - this specifies the name of input file
-vcodec libx264 – tells FFmpeg to encode video to H.264 using libx264 library
-vprofile high – sets H.264 profile to “High” as per Step 2. Other valid options are baseline, main
-preset slow – sets encoding preset for x264 – slowerpresets give more quality at same bitrate, but need more time to encode. “Slow” is a good balance between encoding time and quality.
Other valid options are: ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow, placebo (never use this one)
-b:v - sets video bitrate in bits/s
-maxrate and -bufsize – forces libx264 to build video in a way, that it could be streamedover 500kbit/s line considering device buffer of 1000kbits. Very usefulfor web – setting this to bitrate and 2x bitrate gives good results.
-vf scale – applies “scale” filter, which resizes video to desired resolution. “720:480″ would resize video to 720×480, “-1″means “resize so the aspect ratio is same.”
Usually you set only height of the video, so for 380p you set “scale=-1:380″, for 720p “scale=-1:720″ etc.
-threads 0 – tells libx264 to choose optimal number ofthreads to encode, which will make sure all your processor cores in thecomputer are used
-acodec libvo_aacenc – tells FFmpeg to encode video to AAC using libvo_aacenc library
-b:a - sets audio bitrate in bits/s
-pass [1|2] – tells FFmpeg to process video in multiple passes and sets the current pass
-an – disables audio, audio processing has no effect on first pass so it’s best to disable it to not waste CPU
That’s basically all there is to it.
“Standard” web video (480p at 500kbit/s):
ffmpeg -i input_file.avi -vcodec libx264 -vprofile high -presetslow -b:v 500k -maxrate 500k -bufsize 1000k -vf scale=-1:480 -threads 0-acodec libvo_aacenc -b:a 128k output_file.mp4
360p video for older mobile phones (360p at 250kbit/s in baseline profile):
ffmpeg -i inputfile.avi -vcodec libx264 -vprofile baseline -preset slow -b:v 250k -maxrate 250k -bufsize 500k -vf scale=-1:360 -threads 0-acodec libvo_aacenc -ab 96k output.mp4
480p video for iPads and tablets (480p at 400kbit/s in main profile):
ffmpeg -i inputfile.avi -vcodec libx264 -vprofile main -presetslow -b:v 400k -maxrate 400k -bufsize 800k -vf scale=-1:480 -threads 0-acodec libvo_aacenc -ab 128k output.mp4
High-quality SD video for archive/storage (PAL at 1Mbit/s in high profile):
ffmpeg -i inputfile.avi -vcodec libx264 -vprofile high -presetslower -b:v 1000k -vf scale=-1:576 -threads 0 -acodec libvo_aacenc -ab196k output.mp4
If you’re interested in encoding WebM instead of H.264, read the next article :)