The most basic command is:
ffmpeg -i input.mp4 -vcodec libvpx-vp9 -strict experimental output.webm
FFmpeg currently considers this encoder to be experimental. The default settings will probably not provide a great looking output. This is currently a very slow encoder but optimization development is continuing.
Following the recommendations in the FFmpeg VPx Encoding Guide produces a more acceptable output, but I believe the guide was written for VP8 in mind and I'm unsure how much VP8 differs from VP9 in terms of options, etc.
Default audio encoder will be
libvorbis
. Refer to the Vorbis specific information on the FFmpeg Theora and Vorbis Encoding Guide for more info.For a list of currently supported encoders refer to
ffmpeg -encoders
:$ ffmpeg -encoders | grep -i vp … V..... libvpx libvpx VP8 (codec vp8) V..... libvpx-vp9 libvpx VP9 (codec vp9)