ffmpeg mp4 to webm/ogv

ffmpeg

ffmpeg -i local-video.mp4 -c:v libvpx -b:v 1M -c:a libvorbis output.webm
ffmpeg -i local-video.mp4 -c:v libtheora -q:v 7 -c:a libvorbis -q:a 4 output.ogv

https://superuser.com/questions/1096841/how-do-i-convert-mp4-to-ogv-while-still-retaining-the-same-quality-using-ffmpeg
https://askubuntu.com/questions/351593/bulk-batch-convert-mp4-and-m4v-files-to-webm-via-command-line

ffmpeg -i inputfile.mkv -vf "select=eq(n,0)" -q:v 31 output_image.png

https://stackoverflow.com/questions/4425413/how-to-extract-the-1st-frame-and-restore-as-an-image-with-ffmpeg