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

[Pierre-Anthony Lemieux]

On Fri, Jul 26, 2024 at 2:29 PM Michael Niedermayer
<michael at niedermayer.cc> wrote:
>
> On Thu, Jul 25, 2024 at 05:06:04PM -0700, Pierre-Anthony Lemieux wrote:
> > On Thu, Jul 25, 2024 at 2:17 AM Tomas Härdin <git at haerdin.se> wrote:
> > >
> > > sön 2024-07-21 klockan 14:07 +0900 skrev Pierre-Anthony Lemieux:
> > > > On Sat, Jul 20, 2024 at 5:12 PM Tomas Härdin <git at haerdin.se> wrote:
> > > > >
> > > > > tor 2024-07-18 klockan 23:10 +0900 skrev Pierre-Anthony Lemieux:
> > > > > > On Mon, Jul 15, 2024 at 10:33 PM Tomas Härdin <git at haerdin.se>
> > > > > > wrote:
> > > > > > >
> > > > > > > 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.
> > > > > >
> > > > > > Per the spec, the decoder uses the COD information in tile-parts
> > > > > > over
> > > > > > the COD information in the header.
> > > > > >
> > > > > > The issue here is that a decoder, upon seeing HOMOGENEOUS, simply
> > > > > > does
> > > > > > not bother with looking for COD information in tile-parts,
> > > > > > thereby
> > > > > > missing critical information.
> > > > >
> > > > > So it is actually perfectly legal? Then it seems this patch is
> > > > > wrong
> > > >
> > > > What is not "illegal": the HOMOGENEOUS flag being equal to true *and*
> > > > having COD marker segments in tile-parts.
> > > >
> > > > This is what the patch detects.
> > > >
> > > > FFMPEG can decode such illegal codestream. Other decoders might not.
> > > >
> > > > The question is: what should FFMPEG do? Should FFMPEG exit or warn
> > > > and continue.
> > >
> > > If the spec allows it but it's perhaps unadviced then warning about it
> > > seems reasonable
> >
> > (I totally messed up my double negative. Repeat below. Sorry for the confusion.)
> >
> > What is "illegal": the HOMOGENEOUS flag being equal to true *and*
> > having COD marker segments in tile-parts.
> >
> > This is what the patch detects.
> >
> > FFMPEG can decode such illegal codestream. Other decoders might not.
> >
> > The question is: what should FFMPEG do? Should FFMPEG exit or warn and continue.
>
> Does such a codestream actually exist ? I mean is this just a hypothetical case
> or something existing ?

Such a codestream exists: a codestream with the HOMOGENEOUS flag equal
to true *and* COD marker segments in tile-parts is "illegal" per the
specification. Such a codestream may cause certain decoders (esp.
hardware decoders) to fail because, based on the value of the
HOMOGENEOUS flag, they will not expect COD marker segments in
tile-parts. FFMPEG will not fail, because it does not use the value of
the HOMOGENEOUS flag to configure itself.

>
> thx
>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> Rewriting code that is poorly written but fully understood is good.
> Rewriting code that one doesnt understand is a sign that one is less smart
> than the original author, trying to rewrite it will not make it better.
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