How to cut at exact frames using ffmpeg?

LosslessCut is a cross-platform GUI tool that uses FFmpeg as a backend and losslessly cuts your video. You can choose to cut at a keyframe or any frame.

I have a solution, but I don't know how to do it using current ffmpeg commands (my trials to copy at keyframes didn't come accurate too. I want to know how ffmpeg decides the cutpoints).

I suggested suggest this algorithm, to divide the segment (t1, t2) that we want to copy to 3 parts:

1. a part (t1-x, t1+y), which is a complete encoded block that should be re-encoded to be able to copy the part (t1, y) precisely.
2. a part (t2-z, t3+w), which is a complete encoded block that should be re-encoded to be able to copy the part (z, t2) precisely.
3. a middle part (y, z) which contains complete encoded blocks, where it can be copied as is.
4. Join the 3 parts resulted from the above steps.

Note that the first two parts are expected to be small (and one of them or both can be zero length), so, the re-encoding process will be fast. This will make us able to have exact cuts with slightly slower operation but still super faster than re-encoding the full video. It can be even faster if we can do multiple cuts with one command, so we traverse the frames once. I hope if someone can apply this, and tell us how, or mention some of the ffmpeg team, or deliver it to them anyhow.

The option

-vf select="between(n,start_frame_num,end_frame_num),setpts=PTS-STARTPTS"

e.g.,

-vf select="between(n,200,300),setpts=PTS-STARTPTS"

cuts video from(includes) 200th to(includes) 300th frame, the sequence counting starts from 0.

timecode_frame_start does not work like this.

Seeking based on frame numbers is not possible. The only way to start at specific frames is to convert a number of frames to ss.ms syntax, or hh:mm:ss.ms. So, if your video is at 25 fps, and you want to start at 133 frames, you would need to first calculate the timestamp:

133 / 25 = 5.32

Then run:

ffmpeg -ss 5.32 -i input.mp4 -c:v libx264 -c:a aac out.mp4

Note that cutting on exact frames with bitstream copy (-c:v copy) is not possible, since not all frames are intra-coded ("keyframes"). A video must begin with a keyframe to be decoded properly. You will therefore have to re-encode the video, e.g. to H.264 using -c:v libx264 as shown above. You can also choose a lossless codec like -c:v ffv1 which preserves the quality of the input video.

To summarize, -ss will always be frame-accurate when performing re-encoding.

If you further want to encode a specific number of frames, use -frames:v, for example:

ffmpeg -ss 5.32 -i input.mp4 -c:v libx264 -c:a aac -frames:v 60 out.mp4

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Note that you you also have the choice to use the select/aselect filters to select frames/audio samples.

ffmpeg -i input.mp4 -vf 'select=gte(n,100)' -c:v libx264 -c:a aac out.mp4

This, however, is slower than the -ss option shown above, since the entire video will be decoded.