[FFmpeg-devel] [PATCH] avformat/mov: adjust skip_samples according to seek timestamp

[Marton Balint]

On Mon, 2 Nov 2020, Matthieu Bouron wrote:

> Currently skip_samples is set to start_pad if sample_time is lesser or
> equal to 0. This can cause issues if the stream starts with packets that
> have negative pts. Calling avformat_seek_file() with ts set to 0 on such
> streams makes the mov demuxer return the right corresponding packets
> (near the 0 timestamp) but set skip_samples to start_pad which is
> incorrect as the audio decoder will discard the returned samples
> according to skip_samples from the first packet it receives (which has
> its timestamp near 0).
>
> For example, considering the following audio stream with start_pad=1344:
>
> [PKT pts=-1344] [PKT pts=-320] [PKT pts=704] [PKT pts=1728] [...]
>
> Calling avformat_seek_file() with ts=0 makes the next call to
> av_read_frame() return the packet with pts=-320 and a skip samples
> side data set to 1344 (start_pad). This makes the audio decoder
> incorrectly discard (1344 - 320) samples.
>
> This commit makes the move demuxer adjust skip_samples according to the
> stream start_pad, seek timestamp and first sample timestamp.
>
> The above example will now result in av_read_frame() still returning the
> packet with pts=-320 but with a skip samples side data set to 320
> (src_pad - (seek_timestamp - first_timestamp)). This makes the audio
> decoder only discard 320 samples (from pts=-320 to pts=0).
> ---
> libavformat/mov.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index dd0db6bca79..f99cb0df25a 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -8098,20 +8098,34 @@ static int mov_read_seek(AVFormatContext *s, int stream_index, int64_t sample_ti
>         return sample;
>
>     if (mc->seek_individually) {
> +        MOVStreamContext *sc = s->streams[stream_index]->priv_data;
> +
>         /* adjust seek timestamp to found sample timestamp */
> +        int64_t first_timestamp = st->internal->index_entries[0].timestamp;
>         int64_t seek_timestamp = st->internal->index_entries[sample].timestamp;
> 
> +        /* adjust skip samples according to stream start_pad, seek timestamp and first timestamp */
> +        int64_t skip_samples = FFMAX(sc->start_pad - (seek_timestamp - first_timestamp), 0);
> +        st->internal->skip_samples = skip_samples;
> +

Could you factorize this to a function? It seems you are doing exactly the 
same thing here and below.

Thanks,
Marton

>         for (i = 0; i < s->nb_streams; i++) {
>             int64_t timestamp;
>             MOVStreamContext *sc = s->streams[i]->priv_data;
>             st = s->streams[i];
> -            st->internal->skip_samples = (sample_time <= 0) ? sc->start_pad : 0;
>
>             if (stream_index == i)
>                 continue;
>
>             timestamp = av_rescale_q(seek_timestamp, s->streams[stream_index]->time_base, st->time_base);
> -            mov_seek_stream(s, st, timestamp, flags);
> +            sample = mov_seek_stream(s, st, timestamp, flags);
> +            if (sample >= 0) {
> +                first_timestamp = st->internal->index_entries[0].timestamp;
> +                seek_timestamp = st->internal->index_entries[sample].timestamp;
> +
> +                /* adjust skip samples according to stream start_pad, seek timestamp and first timestamp */
> +                skip_samples = FFMAX(sc->start_pad - (seek_timestamp - first_timestamp), 0);
> +                st->internal->skip_samples = skip_samples;
> +            }
>         }
>     } else {
>         for (i = 0; i < s->nb_streams; i++) {
> -- 
> 2.29.2
>
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