[FFmpeg-devel] [PATCH v3] mov: Compare frag times in correct time base when seeking a stream without a corresponding sidx

[Zhao Zhili]

> -----Original Message-----
> From: ffmpeg-devel-bounces at ffmpeg.org <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Derek Buitenhuis
> Sent: 2022年8月16日 22:49
> To: ffmpeg-devel at ffmpeg.org
> Subject: [FFmpeg-devel] [PATCH v3] mov: Compare frag times in correct time base when seeking a stream without a corresponding sidx
> 
> Some muxers, such as GPAC, create files with only one sidx, but two streams
> muxed into the same fragments pointed to by this sidx.
> 
> Prevously, in such a case, when we seeked in such files, we fell back
> to, for example, using the sidx associated with the video stream, to
> seek the audio stream, leaving the seekhead in the wrong place.
> 
> We can still do this, but we need to take care to compare timestamps
> in the same time base.
> 
> Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
> ---
>  libavformat/mov.c | 38 ++++++++++++++++++++------------------
>  1 file changed, 20 insertions(+), 18 deletions(-)
> 
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 31f3249ca6..1d8c5b2904 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -1271,15 +1271,18 @@ static int64_t get_stream_info_time(MOVFragmentStreamInfo * frag_stream_info)
>      return frag_stream_info->tfdt_dts;
>  }
> 
> -static int64_t get_frag_time(MOVFragmentIndex *frag_index,
> -                             int index, int track_id)
> +static int64_t get_frag_time(AVFormatContext *s, AVStream *dst_st,
> +                             MOVFragmentIndex *frag_index, int index)
>  {
>      MOVFragmentStreamInfo * frag_stream_info;
> +    MOVStreamContext *sc = dst_st->priv_data;
>      int64_t timestamp;
> -    int i;
> +    int i, j;
> 
> -    if (track_id >= 0) {
> -        frag_stream_info = get_frag_stream_info(frag_index, index, track_id);
> +    // If the stream is referenced by any sidx, limit the search
> +    // to fragments that referenced this stream in the sidx
> +    if (sc->has_sidx) {
> +        frag_stream_info = get_frag_stream_info(frag_index, index, dst_st->id);
>          if (frag_stream_info->sidx_pts != AV_NOPTS_VALUE)
>              return frag_stream_info->sidx_pts;
>          if (frag_stream_info->first_tfra_pts != AV_NOPTS_VALUE)

get_frag_time() can be called with an mp4 file which has no sidx at all. In that case,
dst_st should have a higher priority than other streams, even if sc->has_sidx is false.
And first_tfra_pts might be used here, which makes the check of sc->has_sidx unnatural.
So in my opinion, the check on sc->has_sidx should be removed.

Secondly, this part can be simplified with get_stream_info_time(), which is:

+    frag_stream_info = get_frag_stream_info(frag_index, index, dst_st->id);
+    timestamp = get_stream_info_time(frag_stream_info);
+    if (timestamp != AV_NOPTS_VALUE)
+        return timestamp;

Sorry I missed this part with patch v2, because I can't remember what did myself
mean in the replying to patch v1 😊

> @@ -1288,28 +1291,27 @@ static int64_t get_frag_time(MOVFragmentIndex *frag_index,
>      }
> 
>      for (i = 0; i < frag_index->item[index].nb_stream_info; i++) {
> +        AVStream *frag_stream = NULL;
>          frag_stream_info = &frag_index->item[index].stream_info[i];
> +        for (j = 0; j < s->nb_streams; j++)
> +            if (s->streams[j]->id == frag_stream_info->id)
> +                frag_stream = s->streams[j];
> +        if (!frag_stream) {
> +            av_log(s, AV_LOG_WARNING, "No stream matching sidx ID found.\n");

Neither frag_stream_info->id nor s->streams[j]->id comes from sidx ID.

> +            continue;
> +        }
>          timestamp = get_stream_info_time(frag_stream_info);
>          if (timestamp != AV_NOPTS_VALUE)
> -            return timestamp;
> +            return av_rescale_q(timestamp, frag_stream->time_base, dst_st->time_base);
>      }
>      return AV_NOPTS_VALUE;
>  }
> 
> -static int search_frag_timestamp(MOVFragmentIndex *frag_index,
> +static int search_frag_timestamp(AVFormatContext *s, MOVFragmentIndex *frag_index,
>                                   AVStream *st, int64_t timestamp)
>  {
>      int a, b, m, m0;
>      int64_t frag_time;
> -    int id = -1;
> -
> -    if (st) {
> -        // If the stream is referenced by any sidx, limit the search
> -        // to fragments that referenced this stream in the sidx
> -        MOVStreamContext *sc = st->priv_data;
> -        if (sc->has_sidx)
> -            id = st->id;
> -    }
> 
>      a = -1;
>      b = frag_index->nb_items;
> @@ -1318,7 +1320,7 @@ static int search_frag_timestamp(MOVFragmentIndex *frag_index,
>          m0 = m = (a + b) >> 1;
> 
>          while (m < b &&
> -               (frag_time = get_frag_time(frag_index, m, id)) == AV_NOPTS_VALUE)
> +               (frag_time = get_frag_time(s, st, frag_index, m)) == AV_NOPTS_VALUE)
>              m++;
> 
>          if (m < b && frag_time <= timestamp)
> @@ -8862,7 +8864,7 @@ static int mov_seek_fragment(AVFormatContext *s, AVStream *st, int64_t timestamp
>      if (!mov->frag_index.complete)
>          return 0;
> 
> -    index = search_frag_timestamp(&mov->frag_index, st, timestamp);
> +    index = search_frag_timestamp(s, &mov->frag_index, st, timestamp);
>      if (index < 0)
>          index = 0;
>      if (!mov->frag_index.item[index].headers_read)
> --
> 2.36.1
> 
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