[FFmpeg-user] Is a video generated with transitions different from a normal video?

[Rob Hallam]

On Sun, 20 Jun 2021 at 13:28, <iburman at compuscience.com> wrote:

> Now I want to create a new video by extracting the part that has the
> transition, with an additional second on each side, so I want to cut from 6
> to 12 and I use the following:
>
> Ffmpeg -ss 6 -i input.mp4 -to 6 -c copy output.mp4
>
> I expect a final video with 6 seconds, showing the transition starting at 1
> and ending at 5. Instead I get a video with 9 seconds and the transition
> seems to start at 5.

I'm no expert but have done similar things and this feels like it
might be a 'seeking while doing a codec copy' issue.

>From the Wiki:

> Using -ss as input option together with -c:v copy might not be accurate since ffmpeg is forced to only use/split on i-frames. Though it will—if possible—adjust the start time of the stream to a negative value to compensate for that. Basically, if you specify "second 157" and there is no key frame until second 159, it will include two seconds of audio (with no video) at the start, then will start from the first key frame. So be careful when splitting and doing codec copy.

https://trac.ffmpeg.org/wiki/Seeking#Seekingwhiledoingacodeccopy

I am happy to be corrected if off-base.

Incidentally, it is good practice to post full, uncut console output
when seeking help with ffmpeg usage here.

Cheers,
Rob