On 07/25/2020 06:08 PM, Mohammed Hamdy wrote:
Hi,
I have an idea to make ffmpeg do a fast and precise cut, by re-ncoding just
shorts parts around the cut points, and copy the rest of the desired segment
without re-encoding. I wish it can be done. Details:
suppose I want to generate a new video that doesn't contain the chuck starts at
t1 to t2. I want ffmpeg to do this to divide the segment (t1, t2) to 3 parts:
1. a part (t1-x, t1+y), which is a complete encoded block that should be
re-encoded to be able to copy the part (t1, y) precisely.
It's impossible to re-encode precisely. There's always some loss. But perhaps you don't mean
"re-encode", eh? Perhaps you mean to copy the packets without decoding. That's theoretically
possible if tl-x and tl+y are precisely at key frames and there are no predictive frames that cross
the boundary (though they could be dropped) and there has been no prior editing that already has
damaged B-frame & P-frame references (which does happen, even in commercially mastered material).
2. a part (t2-z, t3+w), which is a complete encoded block that should be
re-encoded to be able to copy the part (z, t2) precisely.
3. a middle part (y, z) which contains complete encoded blocks, where it
can be copied as is.
Then join the 3 parts resulted from the above steps. Note that the first two
parts are expected to be small (and one of them or both can be zero length),
so, the re-encoding process will be fast. This will make us able to have exact
cuts with slightly slower operation but still super faster than re-encoding the
full video. It can be even faster if we can do multiple cuts with one command,
so we traverse the frames once.
So, can I do that today with ffmpeg? Or, can ffmpeg support it soon?
Not likely, but I'm just another Bozo on this bus.
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