Hi,
On Fri, 11 Dec 2020, Josh Allmann wrote:
On Fri, 11 Dec 2020 at 14:07, Martin Storsjö <mar...@martin.st> wrote:
On Fri, 11 Dec 2020, Josh Allmann wrote:
> A negative start_dts value (eg, delay from edit lists) typically yields
> a duration larger than end_pts. During edit list processing, the
> delay is removed again, yielding the correct duration within the elst.
>
> However, other duration-carrying atoms (tkhd, mvhd, mdhd) still have
> the delay incorporated into their durations. This is incorrect.
>
> Fix this by withholding delay from the duration if edit lists are used.
> This also simplifies edit-list processing a bit, since the delay
> does not need to be removed from the calculated duration again.
> ---
>
> The mov spec says that the tkhd duration is "derived from the track's
> edits" [1] and the duratons of the other atoms (mvhd, mdhd) are in turn
> taken from the longest track. So it seems that incorporating the delay
> into the track duration is a bug in itself when the edit list has the
> correct duration, and this propagates out tothe other top-level durations.
>
> Unsure of how this change interacts with other modes that may expect
> negative timestamps such as CMAF, so the patch errs on the side of
> caution and only takes effect if edit lists are used. Can loosen that
> up if necessary.
>
> [1]
https://developer.apple.com/library/archive/documentation/QuickTime/QTFF/QTFFChap2/qtff2.html#//apple_ref/doc/uid/TP40000939-CH204-BBCEIDFA
>
> libavformat/movenc.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> index 7db2e28840..31441a9f6c 100644
> --- a/libavformat/movenc.c
> +++ b/libavformat/movenc.c
> @@ -2831,7 +2831,14 @@ static int64_t calc_pts_duration(MOVMuxContext *mov,
MOVTrack *track)
> if (track->end_pts != AV_NOPTS_VALUE &&
> track->start_dts != AV_NOPTS_VALUE &&
> track->start_cts != AV_NOPTS_VALUE) {
> - return track->end_pts - (track->start_dts + track->start_cts);
> + int64_t dur = track->end_pts, delay = track->start_dts +
track->start_cts;
> + /* Note, this delay is calculated from the pts of the first sample,
> + * ensuring that we don't reduce the duration for cases with
> + * dts<0 pts=0. */
If you have a stream starting with dts<0 pts=0, you'll have start_pts =
start_dts + start_cts = 0. That gives delay=0 after your modification. But
the comment says "don't reduce the duration for cases with pts=0" - where
the delay variable would be zero anyway?
I'm not quite sure what you mean - that the comment is outdated?
Or that this modification would perhaps not behave as expected?
Yeah, the comment seems wrong here - it looks like it's been moved along
with the code, but it doesn't really make sense here and/or for the case
you're describing, I think.
For what it's worth, the cases I'm concerned with have start_pts < 0.
I don't manage to follow the reasoning and explanation in the commit
message. To be able to concretely reason about this issue at all, we need
to look at a concrete example. Can you provide a sample input file and a
reproducible command, and point out which exact field in the muxer output
of that case that you consider wrong?
Had to create a trac to find somewhere to host the sample. Tried to put
some details there but the formatting seems messed up and I can't figure
out how to edit, apologies. So here is some more info -
Input sample:
https://trac.ffmpeg.org/raw-attachment/ticket/9028/test-timecode.mp4
Run the following for a transmuxed clip from 3s for a 5s duration:
ffmpeg -ss 3 -i test-timecode.mp4 -t 5 -c copy out.mp4
Note that the actual cut location is mid-GOP, so there's a 1s pts delay
at the beginning of the output file with negative pts.
ffprobe shows:
ffprobe -show_streams -show_format out.mp4 2>&1 | grep duration=
duration=5.166992 # stream duration - correct
duration=6.167000 # format duration - incorrect
mp4dump'ing out.mp4 gives this:
# incorrect: duration should be sum of elst durations
[tkhd] size=12+80, flags=3
duration = 6167
Thanks, I've reproduced this. I'll look closer into it and the suggested
patch and/or other ways of solving it, soon, but please bear with me, I*m
a bit swamped...
// Martin
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