Commands for ffmpeg to speed up or slow down framerate

http://blog.grio.com/2012/01/fast-and-slow-motion-video-with-ffmpeg.html

That link points to the right and proper way to do speed ups and slow motion with ffmpeg.

I might actually go a little further and post another explanation of the command.

ffmpeg -i input.mp4 -vf “setpts=(1/X)*PTS” output.mp4

details:

ffmpeg        - calls ffmpeg program 
-i input.mp4  - file input name of the
-vf           - command required for the
"setpts=      - command sets the type.
(1/x)         - speeds up the  video
(x/1)         - slows down the video
*PTS"         - closes command.

output.mp4    - file name of output video


There are some refinements to this command, though. 

You can set the framerate stamped on the output video with 
-r   to stop ffmpeg dropping frames, with a bit of math. 

What good is all this without a few examples.

My GoPro Hero3 will output 240fps in WVGA mode. 
To convert that to 25fps slomo, I use the following command:
ffmpeg -i input.mp4 -r 25 -vf "setpts=(9.6/1)*PTS" output.mp4


That's maybe a little extreme... say you want to take some 960p footage down from 100fps to 25fps slowmo:

ffmpeg -i input.mp4 -r 25 -vf "setpts=(4/1)*PTS" output.mp4


Experiment with it a little. I already have two scripts to make conversion easy for the two commonly available GoPro Hero3 output framerates available in Europe. Americans might have to do a little more math to get a clean conversion with no frames dropped or duplicated, as far as slomo goes. 


For some reason I am unable to reply to comments. So, in response to the comment below:

Nope, forget completely about duration.
Old fps is 6, new fps is 24. 24/6 = 4, so you want your video to play 4 times as fast.

You would use something like this command:

ffmpeg -i input.mp4 -r 25 -vf "setpts=(1/4)*PTS" output.mp4

Good luck!
P.S. Sorry for the long time to reply. I had to figure out how....