Changing frame rate of QuickTime screen recordings.

I recently tried to make a 18fps test movie when asking the service to digitize Super 8 films. It was surprisingly hard because almost no app supports such old framesrates (DaVinci Resolve v14.2 added support for 16 and 18fps, also After Effects and Scratch support old frame rates like 18fps). In the end I let the service use their routine workflow to digitize unique frames 25fps that can be slowed down to 72% for normal motion (in fact they used 71.85% because that syncs possible audio).

Anyways I could also losslessly convert 25fps H.264 to 18fps with this tip:

Change frame rate without re-encoding. It requires remuxing the file (mp4, m4v) to a different containter format MKV and then remuxing it back into an mp4. Here is an example that changes a video to 18fps:

mkvmerge --default-duration 0:18fps --fix-bitstream-timing-information 0 original-video.mp4 -o temp-video.mkv

ffmpeg -i temp-video.mkv -c:v copy slow-video.mp4

...install instructions in macOS 10.15:

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

brew install ffmpeg

brew install mkvtoolnix

https://superuser.com/questions/320045/change-the-frame-rate-of-an-mp4-video-with-ffmpeg

Those fps conversion Terminal commands seem to work properly on video-only files -- an audio track seems to make the video stutter.