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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)?

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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.

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thanks, but when I try this although the framerate indeed decreases, the entire video seems to get drastically degraded in terms of quality. Is there any way to do this in a lossless way or with very little loss? – GJ. Nov 4 '12 at 17:18
FFmpeg will use libx264 for encoding and choose the default quality factor (CRF) of 23 here. Use a lower factor for higher quality, e.g. 18: ffmpeg -i … -vf … -crf 18 output.mp4 – slhck Nov 4 '12 at 17:44
do you mean to say that there's no way to losslessly slow it down? it would seem very odd since even such modifications as to rotate/flip the video can be performed losslessly. – GJ. Nov 4 '12 at 21:27
Hm. Probably not with FFmpeg, at least no way I know of. mencoder can do it, but I unfortunately haven't found a working version for OS X yet. (See update to answer) – slhck Nov 4 '12 at 21:42
Whoops, turns out there is an mencoder binary from Homebrew. – slhck Nov 4 '12 at 21:48

AviDemux seems to be the way to go:

You can change the Framerate by going to Video > Framerate and setting it as 30fps.

Here's a blog describing how to export it for YouTube on Windows, but the process should be similar on a OS X.

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