[FFmpeg-devel] BUG in use of extradata and extradata_size with dvb subtitles and teletext

[Andriy Lysnevych]

Hi,

Thanks for your comments.

1) As I pointed before there are two different AVPacket.data formats of DVB
subtitles in FFmpeg. It is not obvious and causes developers to do bugs.
Because of this copy subtitles from TS demuxer to TS muxer does not work.

>the reason it was failing in copy was due to the not copying of the ids
from the input stream to the output stream. >Thats all that was needed.

We investigated the issue. Subtitles in TS stream should go in format "0x20
0x00 [subtitles data] 0xFF". Because TS demuxer generates AVPacket.data in
different format than TS muxer expects on output you get TS stream with
wrong subtitles in format: "0x20 0x00 [subtitles data]" (i.e. without 0xFF
at the end). That is why copy does not work. Making all DVB subtitles
internally to be in one format fixes the issue.

> In addition there is another issue: as far as I can tell this patch
breaks backward-compatibility.
> So an application that uses its own MPEG-TS demuxer together with our DVB
subtitle decoder will break after this change.

Yes our goal is to make all DVB subtitles across all FFmpeg components to
be in one format. It is not possible to do without breaking
backward-compatibility. Otherwise it will cause more bugs in future.

2) Regarding matroska, nut and wtv:

>I checked: matroska, nut and wtv all support DVB subtitles in FFmpeg.
>So I would expect that your patch misses changes for all 3 of those
>(though I have a suspicion that the exact formats those use differs
>slightly so that at least stream copy from those into MPEG-TS might not
>work well either way).

As I see the same DVB subtitles parser is used for all matroska, nut, TS
and wtv. The parser expects DVB subtitles to be in format "0x20 0x00
[subtitles data] 0xFF":

if (buf_size < 2 || buf[0] != 0x20 || buf[1] != 0x00) {
    av_dlog(avctx, "Bad packet header\n");
    return -1;
}

...

else if (*p == 0xff)

...

>From other hand I see that DVB subtitles encoder generates subtitles in
format "0x00 [subtitles data] 0xFF" (without leading 0x20).

Only MPEG-TS muxer has "hack" that adds leading 0x20 before each DVB
subtitle payload (actually it incorrectly adds 0x20 to teletext also and
that is why muxing teletext is broken too). Nut and matroska do not add
0x20 before payload to DVB subtitles encoder (can't find this in code). As
the result muxing DVB subtitles into matroska and nut is currently broken.
At least data expected by parser and data generated by muxers is different.
Please correct me if I am wrong.

I can't find documentation how DVB subtitles should be stored in matroska
or nut. Also I can't find real samples to check it. Please share
documentation or samples with us if you have them.

Our patch fixes subtitle muxing in matroska and nut. As the result all
subtitles will be in format: "0x20 0x00 [subtitles data] 0xFF" even in nut
and matroska.

About metadata: as a developer I need ability to specify DVB subtitles
parameters for TS muxer via FFmpeg API to correctly generate PMT of TS
stream. Also I need ability to read/copy the same parameters from TS
demuxer.
If metadata is not the place to store these values please advice better way
to do it.

Regards,
Andriy Lysnevych
30 дек. 2013 г. 21:12 пользователь "Reimar Döffinger" <
Reimar.Doeffinger at gmx.de> написал:

> On Mon, Dec 30, 2013 at 07:07:37PM +0100, Reimar Döffinger wrote:
> > On 30.12.2013, at 17:47, Andriy Lysnevych <andriy.lysnevych at gmail.com>
> wrote:
> >
> > > Hi,
> > >
> > > Copy does not work because of different formats AVPacket payload for
> DVB
> > > subtitles. I described the issue here:
> https://trac.ffmpeg.org/ticket/2989
> > >
> > > Patch for DVB subtitles attached. Please review it. After accepting it
> we
> > > will create patch to fix Teletext issues.
> > >
> > > The patch does the following:
> > > 1) Uses the same AVPacket payload for DVB subtitles across all FFmpeg
> > > (Fixed: DVB subtitles parser, DVB subtitles encoder, DVB subtitles
> encoder,
> > > MPEGTS muxer)
> >
> > Obviously not all, DVB subtitles should be possible inside MKV and MXF
> (as far as I remember), too, at least.
> > Would have to check if FFmpeg really misses support for that...
>
> I checked: matroska, nut and wtv all support DVB subtitles in FFmpeg.
> So I would expect that your patch misses changes for all 3 of those
> (though I have a suspicion that the exact formats those use differs
> slightly so that at least stream copy from those into MPEG-TS might not
> work well either way).
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