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

[Matthieu Bouron]

On Mon, Jan 04, 2021 at 09:04:05PM +0100, Marton Balint wrote:
> 
> 
> On Mon, 2 Nov 2020, 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;
> > +
> 
> Could you factorize this to a function? It seems you are doing exactly the
> same thing here and below.

New patch attached.

[...]

Thanks,

-- 
Matthieu B.
-------------- next part --------------
>From 3c95d4d5e4505edf95b9cf4b5a632be98c069123 Mon Sep 17 00:00:00 2001
From: Matthieu Bouron <matthieu.bouron at gmail.com>
Date: Fri, 30 Oct 2020 15:38:51 +0100
Subject: [PATCH] avformat/mov: adjust skip_samples according to seek timestamp

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, 15 insertions(+), 3 deletions(-)

diff --git a/libavformat/mov.c b/libavformat/mov.c
index c6a2d9c388d..e6d0de38c6f 100644
--- a/libavformat/mov.c
+++ b/libavformat/mov.c
@@ -8122,6 +8122,15 @@ static int mov_seek_stream(AVFormatContext *s, AVStream *st, int64_t timestamp,
     return sample;
 }
 
+static int64_t mov_get_skip_samples(AVStream *st, int64_t seek_ts)
+{
+    MOVStreamContext *sc = st->priv_data;
+    int64_t first_ts = st->internal->index_entries[0].timestamp;
+
+    /* compute skip samples according to stream start_pad, seek ts and first ts */
+    return FFMAX(sc->start_pad - (seek_ts - first_ts), 0);
+}
+
 static int mov_read_seek(AVFormatContext *s, int stream_index, int64_t sample_time, int flags)
 {
     MOVContext *mc = s->priv_data;
@@ -8140,18 +8149,21 @@ static int mov_read_seek(AVFormatContext *s, int stream_index, int64_t sample_ti
     if (mc->seek_individually) {
         /* adjust seek timestamp to found sample timestamp */
         int64_t seek_timestamp = st->internal->index_entries[sample].timestamp;
+        st->internal->skip_samples = mov_get_skip_samples(st, seek_timestamp);
 
         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) {
+                seek_timestamp = st->internal->index_entries[sample].timestamp;
+                st->internal->skip_samples = mov_get_skip_samples(st, seek_timestamp);
+            }
         }
     } else {
         for (i = 0; i < s->nb_streams; i++) {
-- 
2.30.0