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

[Matthieu Bouron]

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