[FFmpeg-devel] [PATCH] Helpful message when page_id != described in PMT

[Michael Niedermayer]

On Sat, Jun 28, 2014 at 01:00:39PM +0530, Anshul Maheshwari wrote:
> On Sat, Jun 28, 2014 at 8:03 AM, Michael Niedermayer <michaelni at gmx.at>
> wrote:
> 
> > On Sat, Jun 28, 2014 at 03:36:28AM +0530, Anshul Maheshwari wrote:
> > > Please find patch attached, 1 line change not worth explaining.
> > > for details on Ticket 2161 please go through ticket i have added comment
> > > there.
> > >
> > >
> > > -Anshul
> >
> > >  dvbsubdec.c |    2 ++
> > >  1 file changed, 2 insertions(+)
> > > c8c7026b675e32bc3a3e908899febcea55cfa3b8
> >  0001-Helpful-message-when-page_id-described-in-PMT.patch
> > > From 6befb5b74a2d40afe326579dce1264cbd8f970d9 Mon Sep 17 00:00:00 2001
> > > From: Anshul Maheshwari <anshul.ffmpeg at gmail.com>
> > > Date: Sat, 28 Jun 2014 03:29:05 +0530
> > > Subject: [PATCH] Helpful message when page_id != described in PMT
> > >
> > > Related to Ticket 2161
> > > ---
> > >  libavcodec/dvbsubdec.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/libavcodec/dvbsubdec.c b/libavcodec/dvbsubdec.c
> > > index e958ae1..f227573 100644
> > > --- a/libavcodec/dvbsubdec.c
> > > +++ b/libavcodec/dvbsubdec.c
> > > @@ -1561,6 +1561,8 @@ static int dvbsub_decode(AVCodecContext *avctx,
> > >                          segment_type, page_id, segment_length);
> > >                  break;
> > >              }
> > > +        } else {
> > > +            av_log(avctx, AV_LOG_WARNING,"Invalid Page segment\n");
> >
> > why are they invalid ?
> > are they not just "not supported" ?
> >
> 
> They are invalid because they does not match with any composition id and
> ancillary id.
> 
> since in mpegts.c in function ff_parse_mpeg2_descriptor we have already
> specified
> avpriv_request_sample(fc, "DVB subtitles with multiple languages");
> so if we have multi compostion or ancillary id, then we have already warned
> them.
> Though I have one such video and i working to support multi language.

so its "not-supported" not invalid


>
> According to spec
> page_id: The page_id identifies the subtitle service of the data contained
> in this subtitling_segment. Segments with a
> page_id value signalled in the subtitling descriptor as the composition
> page id, carry subtitling data specific for one
> subtitle service. Accordingly, segments with the page_id signalled in the
> subtitling descriptor as the ancillary page id,
> carry data that may be shared by multiple subtitle services.

nothing in the quoted text says anything about page_id values which are
not listed in the subtitling descriptor, thus from this alone you
cannot draw any conclusions about such cases.
its a bit like that from
if a == 5 then X
if a == 7 then Y

you cannot draw the conclusion that a can only be 5 or 7


>
> So if any one honoring ancillary and compostion page id(not equal to -1),
> then they should/must
> keep page_id equals to either ancillary or composition id.
>

> I think there is no any way to support this dangling page_id.

this may or may not be, but
even from "its impossible to support" you cannot draw the conclusion
that its "invalid"


> 
> If you have idea about how and why to support then please share :)

my complaint is about the terminology used by the warning not that
there is a warning

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

During times of universal deceit, telling the truth becomes a
revolutionary act. -- George Orwell
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