[FFmpeg-devel] [PATCH 2/3] flvdec: Mark the demuxer as allowing discontinuous timestamps

[Michael Niedermayer]

On Mon, Nov 26, 2018 at 01:51:25PM +0000, Derek Buitenhuis wrote:
> On 23/11/2018 02:16, Michael Niedermayer wrote:
> > do we have some sample flv files that require this patchset / that have
> > discontinuites.
> 
> I have many. I've mailed you one privately, while I work on getting a public one.
> 
> > another thing i just realize now, why is this discontinuity issues coming up
> > now? flv support is very old. This should be a long standing issue
> 
> Probably not many codebases check the DISCONT flag. I only just added proper
> discontinuity handling to some codebases this year, and that's why *I* noticed.
> Chances are most people use the ffmpeg cli which seems to handle stuff differently,
> and doesn't necessarily care about the flag. In my case, I only try to 'fix'
> discontinuities if they appear to be in a format that allows them, during indexing.
> 
> I could add a workaround specific to FLV, or some threshold stuff, but I'd prefer
> that demuxers which may output discontinuous timestamps say they do.

The file looks like 2 files concatenated, even with FLV main header between 
them. 

the patch below should make this work with sequential demuxing assuming the
2 files dont change streams somehow with the normal demuxer.

not sure this is the best way to do it ...

also trying a random other flv related tool, FLVTool2 1.0.6 does not seem to
demux the 2nd file of the 2 correctly. Which makes me suspect more that the
file is invalid. Not that this would fundamentally change anything

thx

diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
index 4b9f46902b..c9423da31c 100644
--- a/libavformat/flvdec.c
+++ b/libavformat/flvdec.c
@@ -72,6 +72,8 @@ typedef struct FLVContext {
     int64_t *keyframe_filepositions;
     int missing_streams;
     AVRational framerate;
+    int64_t last_ts;
+    int64_t time_offset;
 } FLVContext;
 
 static int probe(AVProbeData *p, int live)
@@ -917,6 +919,17 @@ static int resync(AVFormatContext *s)
         flv->resync_buffer[j ] =
         flv->resync_buffer[j1] = avio_r8(s->pb);
 
+        if (i >= 8) {
+            uint8_t *d = flv->resync_buffer + j1 - 8;
+            if (d[0] == 'F' &&
+                d[1] == 'L' &&
+                d[2] == 'V' &&
+                d[3] < 5 && d[5] == 0) {
+                av_log(s, AV_LOG_WARNING, "Concatenated FLV detected, might fail to demux, decode and seek %Ld\n", flv->last_ts);
+                flv->time_offset = flv->last_ts + 1;
+            }
+        }
+
         if (i > 22) {
             unsigned lsize2 = AV_RB32(flv->resync_buffer + j1 - 4);
             if (lsize2 >= 11 && lsize2 + 8LL < FFMIN(i, RESYNC_BUFFER_SIZE)) {
@@ -1238,8 +1251,8 @@ retry_duration:
     ret = av_get_packet(s->pb, pkt, size);
     if (ret < 0)
         return ret;
-    pkt->dts          = dts;
-    pkt->pts          = pts == AV_NOPTS_VALUE ? dts : pts;
+    pkt->dts          = dts + flv->time_offset;
+    pkt->pts          = (pts == AV_NOPTS_VALUE ? dts : pts) +flv->time_offset ;
     pkt->stream_index = st->index;
     pkt->pos          = pos;
     if (flv->new_extradata[stream_type]) {
@@ -1282,6 +1295,10 @@ leave:
             }
         }
     }
+
+    if (ret >= 0)
+        flv->last_ts = pkt->dts;
+
     return ret;
 }
 



[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Frequently ignored answer#1 FFmpeg bugs should be sent to our bugtracker. User
questions about the command line tools should be sent to the ffmpeg-user ML.
And questions about how to use libav* should be sent to the libav-user ML.
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