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

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

[Derek Buitenhuis]

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@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)
 {
     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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Re: [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@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@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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> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> 
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