[FFmpeg-devel] [PATCH]concatdec: measure duration when it is not available

[Nicolas George]

Le nonidi 29 pluviôse, an CCXXIV, Bodecs Bela a écrit :
> I have checked the code where and when duration filled and made some
> reasoning about it.
> Duration value for input files has been populated some time after opening.
> (estimate_timings function in utils.c) And never again corrected or called
> this function..
> So, if early in processing there is no duration info, it remains empty.
> When input is not a seekable file but a not-seekable stream, the only chance
> to get a duration info  when the stream itself contains info about this
> value. (eg. flv metadata, mp4-moov)
> But mpegts format does not contain this info. So the only moment when
> duration calculatable is when we finish the reading. I think contoniously
> updating the duration value after each packet would not be a good idea.
> (cur_dts is updated is AVStream->info struct)
> 
> So reading a file via pipe or reading hls stream via http it is normal that
> duration value remains empty.
> 
> thus, I think to handle the cases when duration value is not populated, this
> is not a bug fix but a reasonable solution for these cases.
> 
> I agree with you that we should not "measure" our-own the pts values but I
> could not find any existing/available data member to have this info.

Sorry for the late reply, I was distracted.

If I summarize correctly your findings:

For formats without a reliable duration header with seekable backing, the
duration is evaluated at the opening by peeking at the last timestamps.

With unseekable backing, the duration is not evaluated at all.

In particular, the duration is not updated once the end of the file is
reached.

Well, I thought it was and wrote the code in consequence, thanks for
correcting me.

Still, the logic for tracking the duration can be a bit tricky, possibly
trickier than your original patch if there are B-frames and timestamps
resets. I would rather have it in a common part of the code than in the
concat demuxer.

I suspect we could consider adding AVStream.current_duration that tracks the
maximum recently seen PTS for each stream.

(We could also have a function that peeks in AVStream.pts_buffer, but that
looks tricky; and the corresponding duration is not available.)

I hope other can give advice about this issue.

Regards,

-- 
  Nicolas George
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