[FFmpeg-user] Smooth FPS change via ffmpeg

[RazrFalcon]

Hi!

I have a 240FPS video made by GoPro and I'm looking for a way to gradually
change a video framerate. If I want to simply slow down a video I can use:

ffmpeg -i raw.mp4 -filter_complex "[v:0]setpts=8*PTS" -r 30 out.mp4

and it works fine. But when I'm trying to use a "variable" framerate like
this:

ffmpeg -i raw.mp4 -filter_complex "[v:0]setpts='lerp(2,8,T/5)*PTS'" -r 30
out.mp4

(5 is a video duration in seconds)

the resulting video stutters a lot.

Is there are a correct way to smoothly change the framerate using some
curve/function?

Thanks.