[FFmpeg-user] Selecting/cutting small sections of videos with ffmpeg

[Andy Civil]

The problem is that a MP4 file contains "I Frames" which are self contained, and 
other frames which are only differences, they're (usually) calculated from 
previous frames. A MP4 file may be entirely I frames, or at the extreme, may 
have only one I frame at the start, and then P frames (difference frames) from 
there on.

If you want to copy a file without recoding it, then you can ONLY cut it to 
start at an I frame. (You can finish anywhere.)

The behaviour of FFmpeg is that if you ask to cut at a certain time with -ss 
then it will scan through the file to that point, then continue to scan forward 
until it finds an I frame, and then copy from that point on. Unless you 
carefully calculate your start point to coincide with an I frame, then you will 
lose some video.

Your clip is so short that I suspect that there isn't even a single I frame 
between your start point (3 seconds) and your end point (2 seconds later). In 
this case you will get no video at all, which is exactly what your error says.

Thank you for posting your output, I guess the other respondant didn't notice 
the link.

--
Andy


On 2012-09-05 6:46 AM, Chris Racey wrote:
> I'm very much a novice user of ffmpeg, and I'm using it with the windows
> command prompt. I have 7 second long, 25fps, mp4 videos and I'm tyring to
> cut out small 2 second sections of them using ffmpeg.
>
> The problem I'm having is I cant get it to cut properly if I specify a
> start point other than the beginning of the video.
>
> Originally I was running this command:-
>
> Code: ffmpeg -i full.mp4 -ss 3 -t 2 -vcodec copy -acodec copy cut.mp4
>
> This fails with errors saying I should (check -ss / -t / -frames parameters
> if used) (paste bin of the error: http://pastebin.com/RWJ6wFZ1).
>
> If i set -ss to 0 this cuts 2 seconds from the start and works fine.
>
> Following some advice from IRC I tried this:-
>
> Code: ffmpeg -i full.mp4 -vf select='gte(t\,3)*lte(t\,5)' -vcodec copy
> -acodec copy test.mp4
>
> This ignores the specified window and copies over the whole 7 seconds.
>
> Could anyone tell me where I'm going wrong?
>
> Many thanks,
> Chris
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