How to create a slow motion version of an MTS / AVCHD movie on OS X?

I have an MTS file recorded at 60pfs.

I want to save a version of it that's playing at 1/2 speed with 30fps, which I can e.g. upload to Youtube for such slow motion playback there.

How can I do that with some OS X tool(s)?

If you want to install FFmpeg, do so by installing Homebrew first, then brew install ffmpeg.

All you have to do is:

ffmpeg -i input.mp4 -vf "setpts=(1/speed)*PTS" output.mp4

… where speed is the speedup factor, e.g. 2 for doubling, or 0.5 for slow motion at half speed. FFmpeg will change the presentation timestamp of the individual frames instead of the actual framerate.

This often works better than just setting a different framerate. If you were to set a lower framerate, all FFmpeg would is drop the frames in between. You'd end up with a video of the same duration, but no real slow motion.

If you want to use mencoder for OS X instead, then use Homebrew to brew install mplayer. It will ship with an mencoder binary. Now, you could try one of these:

mencoder -fps 12 -nosound -ovc copy in.mp4 -o out.mp4

… where 12 would be the result frame rate. Or:

mencoder -speed 1/2 -nosound -ovc copy in.mp4 -o out.mp4

… where you can set the speed factor manually.