[FFmpeg-user] Towards better trims & concatenations

[Rob Hallam]

On Mon, 8 Jan 2024 at 23:02, Mark Filipak <markfilipak.imdb at gmail.com> wrote:

> [explanation snipped]
> Oh, I think I see why your difficulty, Rob.

Thank you for taking the time to write the explanations, they are much
appreciated.

My difficulty, as you guessed, is I don't know about the internals of
video containers.

I thought it might work like being handed pages from a book- annoying
if they're in the wrong order, but fixable to get the right order
since there are page numbers. From what you say, it sounds like in
this analogy not only are the pages (packets) in the wrong order, the
page numbers (timestamps) are wrong- and not even consistently wrong?

> Trimming errors are wrecking concatenations ...
> [snip]
> trimming has to take PTS into account so that the cut happens in the right spot with no leftover
> packets that shouldn't be there, but that apparently isn't happening and I have the proof.

That certainly sounds consistent with behaviour I saw in the past when
I tried to re-join trimmed clips.

> To be frank, Rob, if you want to help yourself, you may want to help me. I published my procedure.
> Duplicate it and apply it to some of the videos you've had problems with. Learn how to use
> '-framecrc' and '-showinfo'. It will take you awhile, but it will be time well spent. It will
> demystify a lot for you. I'll be here to help if you like.

Thank you. The videos I've worked with in the past were simple
h264-in-mkv/mp4, but at the time having to do a full re-encode was
irksome. If I get a chance to replicate your procedure I'll post the
results.

Regardless, good luck!

Cheers,
Rob