Change speed of clip based on a curve?

[cnelsonsic]

Does anyone have any ideas as to how to change the speed of a clip gradually over a bezier curve?

Is there any demand for this feature?
Am I the only one who wants this?

[Zulko]

I am not sure that I understand exactly what you want, but you can change the timeline of a clip very freely with clip.fl_time(some_time_warping_function). Here is an example in which the timeline is replaced by t+sin(t), so that the clip will gradually be sped up and slowed down, with a period of 2Pi seconds (this applies to the audio and the video):

clip = VideoFileClip("my_video.mp4")
clip2 = clip.fl_time( lambda t: t+ sin(t) )
clip2.to_videofile("my_time_warped_video.mp4")

To slow down a clip progressively a clip of N seconds you would write something like

clip.fl_time(lambda t: N*t/(N+t)).set_duration(2*N)

Now if you have a profile of the speed over time, you could integrate this profile to get the profile of a timeline, which you can tranform into a function timeline(t) and then clip.fl_time(timeline) would do the trick.

I don't know if there is a proper Bezier curve library in Python. That would certainly be a job for Scipy.

[cnelsonsic]

That sounds like what I'm after. I'll give it a shot.
Thanks!

[Zulko]

Cool. Note that 'going back in time' (i.e. negative speed) is possible for the video but not fully tested with the audio (you might run into a bug).

[SreeniASU]

Hi Zulko,
I tried this and it seems that the speed of the video clip changes, whereas the accompanying audio doesn't speed up. The file format is mp4. Can you please help?

[Zulko]

Try this, with apply_to=['audio'] in fl_time:

clip = VideoFileClip("my_video.mp4")
clip2 = clip.fl_time( lambda t: t+ sin(t), apply_to=['audio'] )
# maybe indicate a new duration for the clip:
clip2 = clip2.set_duration(some_new_duration)
clip2.write_videofile("my_time_warped_video.mp4")

[SreeniASU]

Thanks Zulko for your quick response. It worked like a charm. But, I have one more concern. On changing the playback rate of the video the voice of the person in it also changes. That is, if I double or halve the speed of video, the dialogues are clear, but the voice becomes annoyingly unpleasant. This doesn't happen if I play the original video in a video player with 2x or 0.5x speed. Is there any way to avoid that?

[isdito]

hello,
I'm looking for a way to slow down a point in the video but then x time later it returns to normal speed.

Imagine recording it at 50 fps or 100 fps per second while its playback is constant at 1/25, when the car is close then use those frames. Typical of after effects programs is a retime curve.

Slow down a point of the video and then continue normal

any help please

[keikoro]

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