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

[WATANABE Osamu <owatanab@es.takushoku-u.ac.jp>]

> @Osamu Watanabe Can you update the patch to make FFMPEG exit when
> encountering an illegal codestream, even if FFMPEG can theoretically
> decode it?

I will update the patch in a few days.


> On Jul 27, 2024, at 6:11, Pierre-Anthony Lemieux <pal@sandflow.com> wrote:
> 
> On Fri, Jul 26, 2024 at 1:04?AM Tomas Hardin <git@haerdin.se> wrote:
>> 
>> tor 2024-07-25 klockan 17:06 -0700 skrev Pierre-Anthony Lemieux:
>>> On Thu, Jul 25, 2024 at 2:17?AM Tomas Hardin <git@haerdin.se> wrote:
>>>> 
>>>> son 2024-07-21 klockan 14:07 +0900 skrev Pierre-Anthony Lemieux:
>>>>> On Sat, Jul 20, 2024 at 5:12?PM Tomas Hardin <git@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 Hardin
>>>>>>> <git@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 Hardin
>>>>>>>>> <git@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.
>> 
>> Since it's illegal then we should complain and bail out. Users that
>> *really* want it to keep going can change the code.
> 
> @Osamu Watanabe Can you update the patch to make FFMPEG exit when
> encountering an illegal codestream, even if FFMPEG can theoretically
> decode it?
> 
>> 
>> /Tomas
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