On Mon, Nov 02, 2020 at 12:42:19PM +0100, 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;
> +
> 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
>
Ping.
--
Matthieu B.
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