[FFmpeg-devel] [PATCH 4/4] lavf/mov: Add support for edit list parsing.

[Michael Niedermayer]

On Sat, Aug 27, 2016 at 03:30:24PM -0700, Sasi Inguva wrote:
> On Fri, Aug 26, 2016 at 5:55 PM, Michael Niedermayer <michael at niedermayer.cc
> > wrote:
> 
> > On Fri, Aug 26, 2016 at 12:49:19PM -0700, Sasi Inguva wrote:
> > > I think there is some bug in mp3 decoder which is making skip
> > > samples -1431655766 for ~/tickets/5528/fire.mp3 . For now I have removed
> > > the assert from the 3rd commit.
> > > For the file one.mov , I think the audio has edit list with start time
> > > correspending to the second sample - (which should be media time 1024 if
> > I
> > > guess correctly). This indicates that the first sample be used for
> > encoder
> > > delay.
> > >  Previously without edit  list parsing , we used to offset the start_dts
> > by
> > > -1024 to make the second sample timestamp start from zero.
> > >              sc->time_offset = start_time - empty_duration;
> > > -            current_dts = -sc->time_offset;
> > >              if (sc->ctts_count>0 && sc->stts_count>0 &&
> > >
> > > But now edit list parsing handles the rebasing of timestamps to zero,
> > > because it will  assign increasing timestamps  starting from zero, to
> > > samples present in the edit list.
> >
> > > Because the first sample is not in the
> > > edit list, we mark it as DISCARD, which flag av_decode_audio4 will look
> > at
> > > and decode-and-discard that frame. So it wouldn't matter what the first
> > > sample timestamp should be assigned because it is anyway discarded after
> > > decode.
> >
> > current applications using libavformat have no knowledge of the
> > discard flag you can add the DISCARD flag, you can set it on packets but
> > applications written or built for libavformat 57 dont have to know
> > this flag and can treat the packets like any other packet.
> >
> 
> Yes. they can treat the packet like any other packet. They don't have to
> know about the discard flag.
> 
> Adding this feature without a major version bump requires things to
> > still work reasonable with the old semantics that apps are build
> > around. That should be possible unless iam missing something
> 
> 
> As I have pointed out earlier this code will change the timestamps of the
> packets. In the case of multiple edit lists, the code will also repeat some
> packets, and might make the timestamps non-monotonous. I don't know if the
> last behavior is not  an acceptable expectation from av_read_frame.

if timestamps repeat then it will not be possible to seek in the file
by timestamp (to all positions) and i suspect also not by byte position.
How would one seek then ?
or do i misunderstand ?


> However as I've pointed out, we are already showing the wrong packets for
> videos with multiple edit lists by not parsing the edit lists currently,
> which will introduce AVSync issues. So this patch wont make things any
> worse for those cases in my view.

Is it difficult to adjust the timestamps of discarded packets to avoid
timestamp discontinuities ? (for the cases where this is possible of
course only)
the timestamps for discarded packets i would assume are meaningless in
the new semenatics but they matter for the old semantics 
again, please correct me if iam wrong

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

If you drop bombs on a foreign country and kill a hundred thousand
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