Clipping Video Using FFMPEG and KeyFrames - Need More KeyFrames?

[Alan2106]

Hi All,

I am trying to use ffmpeg (running on Ubuntu 18.04LTS) to clip a video. The general command I am using is:

ffmpeg -i Input.mkv -ss 00:00:00 -to 00:00:09.500 -vcodec copy -acodec copy -scodec copy Output.mkv

The current clip was itself clipped from a much larger video file (graduation ceremony), and ffmpeg reports the total clip length to be 10.64 seconds. If it matters, ffmpeg also reports a 'start' of minus 0.007 seconds:

Duration: 00:00:10.64, start: -0.007000

If I run ffprobe on the clip it reports that there are only five keyframes (every two or three seconds or so):

[FRAME]
pkt_pts_time=0.473000
pict_type=I
[/FRAME]
[FRAME]
pkt_pts_time=2.606000
pict_type=I
[/FRAME]
[FRAME]
pkt_pts_time=5.473000
pict_type=I
[/FRAME]
[FRAME]
pkt_pts_time=7.873000
pict_type=I
[/FRAME]
[FRAME]
pkt_pts_time=10.473000
pict_type=I
[/FRAME]

If possible, I would like to snip the end off (I am guessing at about 9.5 seconds, but I would likely try a few different times around that), but (if I understand correctly) I need to do that at a keyframe. If I could increase the number of keyframes from five to, say, twenty, then I would be able to be more precise about the start and end of the clip (I think).

Q1) Is that correct about keyframes or have I misunderstood?

Q2) If I do need to start and stop at a keyframe, how can I increase the number of them 'in' the file so that I can cut more precisely?


I am pretty much a newb at this stuff, so apologies if I have not given some info that is important - just let me know what is required and how to find it, and I will post more details.


Thanks,

Alan.

[jagabo]

The only way to create more keyframes is to reencode the video. If you're going to reencode the video you might as well just reencode the part you want (0 to 9.5 seconds). There are "smart encoders" that only reencode cut GOPs -- like VideoRedo. Or you could do a smart reencode your self by saving the first part, 0 to 7.873 seconds, with "-vcodec copy", reencoding from 7.873 seconds to 9.5 seconds, then appending the two.

If you only need to cut a bit off the end of the file, AviDemux will allow you to mark out on a P frame. That might be close enough for you.