I'm trying to use ffmpeg to cut video files at precise times. The ffmpeg help shows an option -timecode_frame_start to specify the starting frame but I am unable to get this command to work. The resulting video always starts at the beginning of the original video. Here's an example of the command I'm running:
ffmpeg -i input.mpg -acodec copy -vcodec copy -timecode_frame_start 200 -vframes 210 -n ouput.mpgI've moved the timecode_frame_start option before and after the other options with no change in results. Is there an additional option I need to specify? I've tried various file formats, mkv, avi, mp4, and it doesn't appear the problem is codec related. Here is one file I've tried:
Am I missing something?
34 Answers
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.mp4Note 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.mp4Note 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.mp4This, however, is slower than the -ss option shown above, since the entire video will be decoded.
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.
3LosslessCut 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.
1I 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:
- 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.
- 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.
- a middle part (y, z) which contains complete encoded blocks, where it can be copied as is.
- 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.
1
