[FFmpeg-devel] [PATCH] mov: fix DTS calculation for samples with negative stts duration

[Michael Niedermayer]

On Thu, May 28, 2015 at 12:11:00AM +0200, Andreas Cadhalpun wrote:
> A negative sample duration is invalid according to the spec, but there
> are samples that use it for the DTS calculation, e.g.:
> http://files.1f0.de/samples/mp4-negative-stts-problem.mp4
> 
> These currently get out of A/V sync.
> 
> Also change the logging type to AV_LOG_WARNING, because decoding the
> sample can continue.
> 
> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> ---
>  libavformat/mov.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 51cdd21..730f097 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -2234,12 +2234,6 @@ static int mov_read_stts(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>          sample_count=avio_rb32(pb);
>          sample_duration = avio_rb32(pb);
>  
> -        /* sample_duration < 0 is invalid based on the spec */
> -        if (sample_duration < 0) {
> -            av_log(c->fc, AV_LOG_ERROR, "Invalid SampleDelta %d in STTS, at %d st:%d\n",
> -                   sample_duration, i, c->fc->nb_streams-1);
> -            sample_duration = 1;
> -        }
>          if (sample_count < 0) {
>              av_log(c->fc, AV_LOG_ERROR, "Invalid sample_count=%d\n", sample_count);
>              return AVERROR_INVALIDDATA;
> @@ -2439,6 +2433,7 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
>          unsigned int distance = 0;
>          unsigned int rap_group_index = 0;
>          unsigned int rap_group_sample = 0;
> +        int64_t dts_correction = 0;
>          int rap_group_present = sc->rap_group_count && sc->rap_group;
>          int key_off = (sc->keyframe_count && sc->keyframes[0] > 0) || (sc->stps_count && sc->stps_data[0] > 0);
>  
> @@ -2522,6 +2517,19 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
>  
>                  current_offset += sample_size;
>                  stream_size += sample_size;
> +                current_dts += dts_correction;

i think this could depending on the next stts value end up making
DTS non monotone which seems wrong/unintended


> +                /* A negative sample duration is invalid based on the spec,
> +                 * but some samples need it to correct the DTS. */
> +                if (sc->stts_data[stts_index].duration < 0) {
> +                    av_log(mov->fc, AV_LOG_WARNING,
> +                           "Invalid SampleDelta %d in STTS, at %d st:%d\n",
> +                           sc->stts_data[stts_index].duration, stts_index,
> +                           st->index);
> +                    dts_correction = sc->stts_data[stts_index].duration - 1;
> +                    sc->stts_data[stts_index].duration = 1;
> +                } else {
> +                    dts_correction = 0;
> +                }
>                  current_dts += sc->stts_data[stts_index].duration;
>                  distance++;
>                  stts_sample++;
> -- 
> 2.1.4
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-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.
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