Re: [PNG] Cancel upcoming meeting?

> ISO 21496-1 paywalled
The paywall is only if you want access now. Once we have the liaison set
up, we should be able to share relevant ISO TC/42 WG 18 specs in confidence
with W3C members to help with development of PNG.

Sincerely,
Fares

On Fri, Feb 28, 2025 at 10:25 AM Chris Blume (ProgramMax) <
programmax@gmail.com> wrote:

> so sleep is cancelled
>
> RIP sleep.
>
> It isn't a break but it is an incompatible new version
>
> Agreed.
> I expect that to be significantly more scrutinized, questioned, and
> tested. I guess I should file it under "maybe".
>
> ISO 22028-5 paywalled
>
> Oh. That's a problem.
> We need to loop back on if ITU-R BT.2408 is effectively the same (and not
> paywalled).
>
> cHRM chunk fix
>
> Oh! Right! I forgot about that one. Thank you.
> I think that is likely to get in. I tried playing devil's advocate and
> pretty much everyone I spoke to wanted it. Devil's advocate was the only
> pushback.
>
> a cracker can use it to defeat a decoder which uses the restart marker
>
> I didn't understand this. Do you mean "There is a restart marker at bit
> index 200" but there isn't, so the parallel and serial decodes are
> different?
>
>  Much much better to encode in blocks
>
> I agree. Pretty much every modern image compression I know operates on
> blocks. I think we'll have a very hard time getting that into PNG though.
> That would be a pretty radical change.
>
> ISO <-> W3C liaison vote
>
> Heyyyy nice!
>
> ISO 21496-1 paywalled
>
> Oh dear. I'll leave it to the liaisons to discuss this.
>
> On Thu, Feb 27, 2025 at 8:10 PM Fares Alhassen <falhassen@google.com>
> wrote:
>
>> Dear all,
>>
>> I have updates on accessing the ISO gainmap spec (ISO 21496-1),
>>
>> 1) ISO/TC 42 WG 18/23 will have a ballot on having a liaison to W3C. That
>> ballot voting should end by the end of March 2025.
>>
>> 2) ISO 21496-1 is now in the DIS stage, meaning you can access the spec
>> publicly now (https://www.iso.org/standard/86775.html). It is at bargain
>> bin ISO prices, costing 65 CHF (presumably because it is considered in a
>> "draft" stage).
>>
>> Sincerely,
>> Fares
>>
>> On Fri, Feb 28, 2025 at 9:51 AM John Bowler <
>> john.cunningham.bowler@gmail.com> wrote:
>>
>>> v4: cHRM chunk fix.  Entirely backward compatible; existing decoders
>>> (ones which actually check, which libpng no longer does) will simply reject
>>> the chunk (harmless since the colourspace can't be represented anyway if
>>> the spec is adhered to rigorousely), new decoders will handle it.
>>>
>>> Restart markers; argued about over and over again.  Fully supported at
>>> present but you can't do a complete parallel decode without knowing where
>>> the rows start and end.  That needs some information which can be cracked;
>>> i.e. a cracker can use it to defeat a decoder which uses the restart
>>> marker.  The crack is basically undetectable until all the preceding blocks
>>> have been decoded.  If you just want to do LZ77 "inflate" in parallel no
>>> biggy; doesn't require anything new.
>>>
>>> Complete parallel decode would be nice but it's so quaint and it
>>> requires a critical chunk.  Much much better to encode in blocks, not rows;
>>> much-much-much faster even though the underlying data is the same.
>>> Standard wisdom since the late '90s.
>>>
>>> John Bowler
>>>
>>>