[FFmpeg-devel] [PATCH v4 1/3] avcodec/jpeg2000dec: Add support for CAP and CPF markers

[Tomas Härdin]

fre 2024-07-12 klockan 12:51 -0700 skrev Pierre-Anthony Lemieux:
> On Thu, Jul 11, 2024 at 10:28 PM Tomas Härdin <git at haerdin.se> wrote:
> > 
> > > +            if (s->in_tile_headers == 1 && s->isHT && (!s-
> > > > Ccap15_b11))
> > > +                av_log(s->avctx, AV_LOG_WARNING, "COD marker is
> > > found in HOMOGENEOUS HT set\n");
> > 
> > How bad is this and the other markers being present in this case?
> 
> At the very least, it means that signaling is inconsistent within the
> codestream since the standard states that:
> """
> The HOMOGENEOUS set is the set of HTJ2K codestreams where:
> • none of the functional marker segments, e.g., COD, COC, RGN, QCD,
> QCC, and POC, are present in any
> tile-part header; and
> • no PPT marker segment is present.
> """
> 
> The point of signalling that a codestream is "HOMOGENEOUS" is to
> allow
> decoders to configure themselves solely based on information
> retrieved
> entirely from the main header.
> 
> Since, AFAIK, FFMPEG does not rely on the HOMOGENEOUS to short-
> circuit
> configuration, incorrect HOMOGENEOUS signalling will likely not
> impact
> FFMPEG.

It could happen that information in tile headers contradict information
in the main header, right? In such a case it sounds like we can't be
sure which decode is the correct one.

Much like with broken MXF files I think we should error out because of
the potential ambiguity, to discourage broken encoders from
proliferating. Else we'll have to deal with them perpetually

/Tomas