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

["zhilizhao(赵志立)"]

> On May 18, 2022, at 4:39 AM, Derek Buitenhuis <derek.buitenhuis at gmail.com> wrote:
> 
> 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 | 24 ++++++++++++++++--------
> 1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index d7be593a86..fe3b2b15ab 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -1270,12 +1270,12 @@ 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, int track_id)
> {

I have an idea which needs more refactor:

1. Remove track_id argument, the `if (track_id >= 0)` code path, and the related
part in search_frag_timestamp().

Setting track_id in search_frag_timestamp() is defect. sc->has_sidx is checked
there, while tfra can be used here.

track_id means to set a higher priority on a stream, while now we have dst_st,
which has more information than track_id.


2. Give dst_st a higher priority, and search all streams.


>     MOVFragmentStreamInfo * frag_stream_info;
>     int64_t timestamp;
> -    int i;
> +    int i, j;
> 
>     if (track_id >= 0) {
>         frag_stream_info = get_frag_stream_info(frag_index, index, track_id);
> @@ -1287,15 +1287,23 @@ 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];
>         timestamp = get_stream_info_time(frag_stream_info);
> -        if (timestamp != AV_NOPTS_VALUE)
> -            return timestamp;
> +        if (timestamp != AV_NOPTS_VALUE) {
> +            if (frag_stream)
> +                return av_rescale_q(timestamp, frag_stream->time_base, dst_st->time_base);
> +            else
> +                return timestamp;
> +        }
>     }
>     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;
> @@ -1317,7 +1325,7 @@ 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, id)) == AV_NOPTS_VALUE)
>             m++;
> 
>         if (m < b && frag_time <= timestamp)
> @@ -8782,7 +8790,7 @@ 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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