I wanted to convert a few videos to web compatible formats (webm and mp4). I run the recipe ffmpeg -i input.mp4 -c vp9 -b:v 0 -crf 41 output.webm, but kept getting the error Invalid encoder type 'vp9' even after manually building ffmpeg on my machine.

Running ffmpeg -codecs | grep vp9 revealed that I did have the right codecs installed:

DEV.L. vp9                  Google VP9 (decoders: vp9 libvpx-vp9 ) (encoders: libvpx-vp9 )

Finally, I've found that there's a more robust way to specify codec: -codec:v libvpx-vp9.

So the correct recipe would be like this:

  ffmpeg -i input.mp4 -codec:v vp9 -b:v 0 -crf 41 output.webm