[FFmpeg-user] Freeze frames in timeline editing

[Nicolas George]

John Allan Farleyton (2018-07-21):
> I am trying to slow down a video to half speed beginning from frame 100
> with the setpts filter. It works, but from frame 100 I get 100 additional
> frozen dropped frames before the video continues at half speed. Is my
> command wrong or is there a problem with the setpts filter?

Your math is wrong.

> Command:
> ffmpeg -i ball.avi -vf "setpts='if(gte(N,100),2,1)'*PTS" -c:v libx264 -crf
> 20 -pix_fmt yuv420p ball_slow.avi

First, I think it is not a good idea to mix N and PTS. More reliable to
use only PTS.

For the following explanations, I will assume TB = 1 and all
computations are done as real numbers; FFmpeg works with integers and
uses TB as the base unit, you will have to take this into account.

Your example video has 30 FPS, therefore, in this particular case,
N = 30*PTS. Let us inject that into your formula:

	if(gte(30*PTS,100),2,1)*PTS

And let us translate this with more readable operators:

	(30*PTS >= 100 ? 2 : 1) * PTS

Let us swap the condition (easier to have the first case on the left)
and divide by 30:

	(PTS < 3.33 ? 1 : 2) * PTS

Well, just plot this in your favorite grapher, and you will see the
issue:

gnuplot -persist <<<'plot [0:5] (x<3.33?1:2)*x'

The frame at input PTS = 3.30 is set to output PTS 3.30;
the frame at input PTS = 3.33 is set to output PTS 6.67.
Therefore, the frame at input PTS 3.30 stays for 3.37 seconds.

To avoid jerky output, the formula in setpts needs to be continuous.

Regards,

-- 
  Nicolas George
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