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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.mpg

I'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:

http://www.seaotter.com/marine/movies/hermit-long-01.mpg

Am I missing something?

pseyfert
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curmil
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4 Answers4

80

Using -ss to seek

One 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

This will re-encode the video, and you will consequently lose some quality. You can also set -c copy and copy over the data:

ffmpeg -ss 5.32 -i input.mp4 -c copy out.mp4

Note that cutting on exact frames with bitstream copy may result in output that is not playable on all platforms/players.

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

Using -filter:v

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

ffmpeg -i input.mp4 -filter:v '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.

slhck
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21

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.

Tim Krief
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Nuo Chen
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8

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.

mwfearnley
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Expectator
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0

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.

M.Hamdy.Ghanem
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