First I asked this question at superuser which required me to upgrade away from Ubuntu's ancient ffmpeg. Then I asked this question at Ask Ubuntu whose answer requires Ubuntu's ancient version of ffmpeg.
Is there a reasonable way to move the question to superuser, or should I just delete it here and re-ask it there.
Also, it seems like a reasonable question to ask "How do I do this on Ubuntu?" so if possible, I'd like to leave it asked here.