[FFmpeg-user] Low-quality transcode

[Ted Park]

> For one particular talk, the quality of the transcode from MP4 to WebM
> is very low. I use VBR encoding, but in most cases I end up with a
> transcoded file that is about 60-70% of the size of the original file.

> The command line that was originally used is this one:
> 
> 'ffmpeg' '-loglevel' 'warning' '-y' '-f' 'concat' '-safe' '0' '-i'
> '/tmp/transSnsDON/concat.txt' '-c:v' 'libvpx-vp9' '-b:v' '1024k'
> '-minrate' '512k' '-maxrate' '1484.8k' '-r:v' '25/1' '-crf' '32'
> '-speed' '4' '-pass' '1' '-passlogfile'
> '/srv/sreview/output/2020/AW1.125/ada_rad.webm-multipass' '-c:a'
> 'libopus' '-b:a' '128k' '-ar' '48000' '-t' '1331.934' '-pix_fmt'
> 'yuv420p' '-metadata' 'title=On Rapid Application Development in Ada'
> '-metadata' 'event=FOSDEM 2020' '-metadata' 'speakers=Tomasz Maluszycki'
> '-metadata' 'date=2020-02-01'
> '/srv/sreview/output/2020/AW1.125/ada_rad.webm'
> 
> 'ffmpeg' '-loglevel' 'warning' '-y' '-f' 'concat' '-safe' '0' '-i'
> '/tmp/transSnsDON/concat.txt' '-c:v' 'libvpx-vp9' '-b:v' '1024k'
> '-minrate' '512k' '-maxrate' '1484.8k' '-r:v' '25/1' '-crf' '32'
> '-speed' '2' '-pass' '2' '-passlogfile'
> '/srv/sreview/output/2020/AW1.125/ada_rad.webm-multipass' '-c:a'
> 'libopus' '-b:a' '128k' '-ar' '48000' '-t' '1331.934' '-pix_fmt'
> 'yuv420p' '-metadata' 'title=On Rapid Application Development in Ada'
> '-metadata' 'event=FOSDEM 2020' '-metadata' 'speakers=Tomasz Maluszycki'
> '-metadata' 'date=2020-02-01'
> '/srv/sreview/output/2020/AW1.125/ada_rad.webm’


Those settings seem like it would set up the encoder for constrained quality mode rather than VBR as you are expecting, setting a maximum quality. So as long as you kept the crf same in all your attempts not seeing an increase in quality is expected.

Check out this article on vp9 rc. It uses ffmpeg as an example.
https://developers.google.com/media/vp9/bitrate-modes <https://developers.google.com/media/vp9/bitrate-modes>