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Using ffmpeg to cut up video

Solutons:


You can use the -ss option to specify a start timestamp, and the -t option to specify the encoding duration. The timestamps need to be in HH:MM:SS.xxx format or in seconds (s.msec).

The following would clip the first 30 seconds, and then clip everything that is 10 seconds after that:

ffmpeg -ss 00:00:30.0 -i input.wmv -c copy -t 00:00:10.0 output.wmv
ffmpeg -ss 30 -i input.wmv -c copy -t 10 output.wmv

Note that -t is an output option and always needs to be specified after -i.

Some tips:

  • For older ffmpeg versions, if you use -ss after -i, you get more accurate seeking at the expense of a slower execution altogether. See also: Seeking with FFmpeg
  • You can use -to instead of -t to specify the timestamp to which you want to cut. So, instead of -i <input> -ss 30 -t 10 you could also do -i <input> -ss 30 -to 40 to achieve the same thing.

  • If your ffmpeg does not support -c, or -to, it is likely very outdated. Compile a new version yourself or download a static build from their homepage. It’s really not complicated.

As other people mentioned, putting -ss before (much faster) or after (more accurate) the -i makes a big difference. The section “Fast And Accurate Seeking” on the ffmpeg seek page tells you how to get both, and I have used it, and it makes a big difference. Basically you put -ss before AND after the -i, just make sure to leave enough time before where you want to start cutting to have another key frame. Example:
If you want to make a 1-minute clip, from 9min0sec to 10min 0sec in Video.mp4, you could do it both quickly and accurately using:

ffmpeg -ss 00:08:00 -i Video.mp4 -ss 00:01:00 -t 00:01:00 -c copy VideoClip.mp4

The first -ss seeks fast to (approximately) 8min0sec, and then the second -ss seeks accurately to 9min0sec, and the -t 00:01:00 takes out a 1min0sec clip.

Also note this important point from that page: “If you use -ss with -c:v copy, the resulting bitstream might end up being choppy, not playable, or out of sync with the audio stream, since ffmpeg is forced to only use/split on i-frames.”

This means you need to re-encode the video, even if you want to just copy it, or risk it being choppy and out of sync. You could try just -c copy first, but if the video sucks you’ll need to re-do it.

I found that -ss combined with -c copy resulted in a half-second chop at the start.

To avoid that, you have to remove the -c copy (which admittedly will do a transcode).

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