Extract the frame rate1:
ffmpeg -i *.webm 2>&1 | sed -n "s/.*, \(.*\) fp.*/\1/p"
29.84
This should tell you how many frames you’ll get.
Extract the frames:
ffmpeg -i video.webm thumb%04d.jpg -hide_banner
- https://askubuntu.com/questions/110264/how-to-find-frames-per-second-of-any-video-file [↩]