vp9 conversion

Parameters

Parameter	Function
`$INPUT`	the input video file
`$CRF`	the target quality (33 is a good value, the smaller the better the video quality)
`$THREADS`	the number of threads to use converting the video
`$GOP`	the number of frames after which another inter frame is forced. You can seek between inter frames, so if you want to seek make this something like 5 * `$FPS`
`$OUTFILE`	the target video file name
`$WIDTH`	the target width when scaling. When setting the height to `-1` the aspect ratio is kept

Other regulations:

The webm container only supports webvtt subtitles, so if you have embedded subtitle in the video -c:s webvtt tries to convert them into webvtt.

Converting a single file

With one-pass:

ffmpeg -i $INPUT -c:v libvpx-vp9 -b:v 0 -crf $CRF -threads $THREADS -g $GOP -tile-columns 6 -frame-parallel 1 -c:s webvtt -speed 1 -c:a libopus -b:a 64k -f webm $OUTFILE

When doing a two-pass, the speed value can be changed and the audio conversion can be omitted during 1st pass:

ffmpeg -i $INPUT -pass 1 -passlogfile $PREFIX -c:v libvpx-vp9 -b:v 0 -crf $CRF -threads $THREADS -g $GOP -tile-columns 6 -frame-parallel 1 -speed 4 -an -f webm /dev/null
ffmpeg -i $INPUT -pass 2 -passlogfile $PREFIX -c:v libvpx-vp9 -b:v 0 -crf $CRF -threads $THREADS -g $GOP -tile-columns 6 -frame-parallel 1 -c:s webvtt -speed 1 -c:a libopus -b:a 64k -f webm $OUTFILE

When scaling the video just add the following parameter:

-vf scale=$WIDTH:-1

Merging several files into one

To merge several files into one a "complex filter" has to be used. When this is used, no normal filter can be used (e.g. for scaling). To simply merge two files use the following parameters:

-i $INPUT1 -i $INPUT2 -filter_complex "[0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]" -map '[v]' -map '[a]'

This takes the first video and audio streams from the first file and the first video and audio streams from the second file. It then concats the two (n=2) streams in one video (v=1) and audio stream (a=1) and exports them as [v] and [a]. Later it maps them to the output file.

When trying to scale this video you can chain complex filters. This would look like this:

-i $INPUT1 -i $INPUT2 -filter_complex "[0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]; [v]scale=$WIDTH:-1[v2]" -map '[v2]' -map '[a]'

Known Errors and Workarounds

Invalid channel layout 5.1(side) for specified mapping family -1

Either add an audio filter

-af "channelmap=channel_layout=5.1"

or enhance the complex filter with

;[a] channelmap=channel_layout=5.1 [a2]