Re-mux WebM to MP4
If you want to stream copy (re-mux) and avoid re-encoding:
ffmpeg -i input.webm -c copy output.mp4
This will copy the VP9/VP8 video and Opus/Vorbis audio from WebM to MP4. This is like a “copy and paste”. No re-encoding occurs, so no quality is lost and the process is very fast.
I was able to convert by
ffmpeg -i video.webm -strict experimental video.mp4.
ffmpeg -i vid.webm vid-1.mp4 Ref Takes 17.8 sec. Output file is 1.5 MB.
ffmpeg -i vid.webm -crf 1 -c:v libx264 vid-2.mp4 Takes 18 sec. Output file is 7.6 MB.
ffmpeg -i vid.webm -crf 0 -c:v libx264 vid-3.mp4 Ref Takes 21 sec. Output file is 11.9 MB.
ffmpeg -fflags +genpts -i vid.webm -r 24 vid-4.mp4 Ref Takes 0.16 sec. Output file is 1.5 MB.
ffmpeg -i vid.webm -c copy vid-5.mp4 Ref Takes 2.8 sec. Output file is 64.6 MB.
ffmpeg -i vid.webm -strict experimental vid-6.mp4 Ref Takes 18 sec. Output file is 1.5 MB.
ffmpeg -i vid.webm -c copy -strict experimental vid-7.mp4 Ref Takes 0.16 sec. Output file is 64.6 MB.
ffmpeg -i vid.webm -c:v copy -strict experimental vid-8.mp4 Also Ref Takes 0.69 sec. Output file is 64.5 MB.