[FFmpeg-devel] [PATCH] avformat/movenc: Remove dts delay from duration.

[Martin Storsjö]

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 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?

// Martin