Question about speed change


I have a question about shotcut’s behaviour when the source video has a higher FPS then the target/export file.
let’s say a 50fps video is converted to 25fps
or, another example (but same problem), a 25 fps, 2minutes long video speed up twice to become a 1 minute video, still 25 fps.

shotcut will need to “skip” 50% of the frame (1/2).

but how does it do this? just skip? or time blending with the previous / next one? resulting in “motion blur” ? is there any option to choose this?

of course this example is easy, with small numbers… but what if a 72 hours time lapse is speedup to 3 minutes?

and same question again if a video is slowed down 4x… and shotcut needs to “create” 4 intermediate frames… between 2 original A, B

frame 0 = A
frame 1 = 75% A + 25% B
frame 2 = 50% A + 50% B
frame 3 = 25% A + 75% B
frame 4 = B

just curious about how it works now…

thank you.

You have mixed up two different concepts in your question:

  • Frame rate conversion
  • Speed change

In your example, you are only converting the frame rate. If the source clip frame rate is 50fps and the Shotcut Video Mode is 25fps, then Shotcut will skip every other frame.

Now you are talking about speed change. This depends on how you changed the speed.

If you set the speed parameter in the clip Properties panel, then Shotcut uses simple frame skipping and repeating. Frames will be dropped or duplicated as needed to achieve the speed.

If you used one of the time filters:

They have an image mode with options “nearest” and “blend”. If you choose blend, then the filter will blend images to make new intermediate images. I would not describe this as a motion effect. But it is more like the ratio blending that you described in your question.

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perfect and clear answer to my question!

thank you a lot :slight_smile:

Except when the source frame rate is higher than the project frame rate. Then, it does a more precise frame selection from the source than a naive approach. For example a 50 fps video at half speed in a 25 fps project plays smoothly rather than repeating frames.

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Thank you, it seems that you already master this topic! :+1: :+1:

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