[FFmpeg-devel] [patch 3/3] Make timing calculations less dependant on start_time being defined.

[Neil Brown]

On Thursday August 23, michaelni at gmx.at wrote:
> > Index: libavformat/asf.c
> > ===================================================================
> > --- libavformat/asf.c	(revision 10189)
> > +++ libavformat/asf.c	(working copy)
> > @@ -793,7 +793,7 @@
> >              /* new packet */
> >              av_new_packet(&asf_st->pkt, asf->packet_obj_size);
> >              asf_st->seq = asf->packet_seq;
> > -            asf_st->pkt.pts = asf->packet_frag_timestamp;
> > +            asf_st->pkt.pts = asf->packet_frag_timestamp - asf->hdr.preroll;
> >              asf_st->pkt.stream_index = asf->stream_index;
> 
> 1.
> asf.c does
> asf->packet_frag_timestamp= AV_NOPTS_VALUE;
> at some point, so your code might fail fatally as 
> AV_NOPTS_VALUE - asf->hdr.preroll is not valid

So maybe
  if (asf->packet_frag_timestamp == AV_NOPTS_VALUE)
      asf_st->pkt.pts = AV_NOPTS_VALUE;
  else
      asf_st->pkt.pts = asf->packet_frag_timestamp - asf->hdr.preroll;

??

> 
> 2.
> the av_find_stream_info() and related code will buffer packets which
> have no timestamp until the first timestamp and then correct the
> previous packets before passing them to te user app
> so ffmpeg.c should not see audio packets without timestmamps as you
> described (see code related to first_dts for the stuff which fixes up
> the initial timestamps)

I looked more deeply.  The audio packets do have time stamps.  But
do_audio_out and do_video_out in ffmpeg.c take different approaches to
dealing with a difference between expected and actual timestamp.
At the beginning we expect 0 and get 1579ms.  do_video_out makes 44
copies of the first frame.  do_audio_out doesn't seem to do anything
useful unless I set "-async 1".  But then I get 1.5 seconds of nothing
at the start of the clip, which shouldn't be there.
So I could
   ffmpeg -ss 0:0:1.15  .... -async 1

And get a correctly decoded file, but that really shouldn't be needed.

So: would you accept this?

Thanks,
NeilBrown

-------------------
According to http://msdn2.microsoft.com/en-us/library/bb643323.aspx,
preroll should be subtracted from all timestamps.

Index: libavformat/asf.c
===================================================================
--- libavformat/asf.c	(revision 10189)
+++ libavformat/asf.c	(working copy)
@@ -793,7 +793,10 @@
             /* new packet */
             av_new_packet(&asf_st->pkt, asf->packet_obj_size);
             asf_st->seq = asf->packet_seq;
-            asf_st->pkt.pts = asf->packet_frag_timestamp;
+            if (asf->packet_frag_timestamp == AV_NOPTS_VALUE)
+                asf_st->pkt.pts = AV_NOPTS_VALUE;
+            else
+                asf_st->pkt.pts = asf->packet_frag_timestamp - asf->hdr.preroll;
             asf_st->pkt.stream_index = asf->stream_index;
             asf_st->pkt.pos =
             asf_st->packet_pos= asf->packet_pos;