[FFmpeg-user] trouble with -vcodec copy

[Carl Eugen Hoyos]

Arlo Leach <lists <at> arlomedia.com> writes:

> ffmpeg -i band_formatted.mp4 -f mp4 -vcodec copy -acodec copy -ss 30 -t 30 -y
band_formatted_sample.mp4
> 
> to 
> 
> ffmpeg -ss 30 -t 30 -i band_formatted.mp4 -f mp4 -vcodec copy -acodec copy -y
band_formatted_sample.mp4
> 
> I started getting more than 30 seconds of video, but no blank video. It looks
> like the start of the extraction moves forward to the next earlier keyframe
> and starts from there instead of the requested location.

Yes, but I suspect you don't see the "blank video" with MPlayer (or ffplay), but
QuickTime does not want to display the initial non-key frames that are copied
with the first of your command lines (that asks ffmpeg to seek to exactly 30
seconds, no matter if there is a key frame or not). The second command line asks
to seek to approximately 30 seconds (depending on where a keyframe - or whatever
FFmpeg believes is a keyframe - can be found).

> So I might get a 34-second sample instead of a 30-second sample.
> 
> I also found that if I added the -g flag in my original conversion command, I
> could write more keyframes into the full video and increase the accuracy of
> my sample timing. That only created a slight increase in the
> file size of my full video, which in my application is a good tradeoff.

I don't think -g has any effect with -vcodec copy. (But if you mean you first
encode the sample and cut it later, than more keyframes of course increase the
seeking accuracy.)

Carl Eugen