[FFmpeg-user] Frame Accurate and Fast Clip Trimming

[Robert Nagy]

I need to frame accurately trim the start on a long clip in a fast way.

I am aware that for frame accurate seeking one needs to decode and
re-encode a clip. However, that would be to slow for my usage.

So what I was think of was seeking to the first frame before the seek point
and decode to the first frame after the seek point. Re-encode that part and
then concat the new start with the clip from the first key after the seek
point.

I'm not quite sure how to achieve this. In particular, how I would make
sure to seek to the keyframe after and not before for the second concat
part?