Using ffmpeg to change framerate

Question

I am trying to convert a video clip (MP4, yuv420p) from 30 fps to 24 fps. The number of frames is correct so my output should change from 20 minutes at 30fps to 25 minutes at 24fps. Everything else should remain the same.

Try as I might everything I try with ffmpeg converts the frame rate but changes the number of frames to keep the same duration or changes the duration without altering the framerate.

So I have been typically trying things like;

ffmpeg -y -r 30 -i seeing_noaudio.mp4 -r 24 seeing.mp4

(I'm doing this on windows but normally would be on linux). That converts the framerate but drops frames so the total duration is unaltered.

Or I have tried

ffmpeg -y -i seeing_noaudio.mp4 -filter:v "setpts=1.25*PTS" seeing.mp4

Which changes the duration but not the framerate.

Surely I should be able to do this with a single ffmpeg command without having to reencode or even as some people suggested going back to the original raw frames.

Help please

Answer 1

With re-encoding:

ffmpeg -y -i seeing_noaudio.mp4 -vf "setpts=1.25*PTS" -r 24 seeing.mp4

Without re-encoding:

First step - extract video to raw bitstream

ffmpeg -y -i seeing_noaudio.mp4 -c copy -f h264 seeing_noaudio.h264

Remux with new framerate

ffmpeg -y -r 24 -i seeing_noaudio.h264 -c copy seeing.mp4