Re: Fwd: Puzzling of PNG Error Handling

Thank you for this feedback, which has been raised as an issue on the 
PNG Working Group GitHub

https://github.com/w3c/PNG-spec/issues/320

Your feeling is correct, and the specification is clear: it explicitly 
states that an unknown ancillary chunk is never an error, (even if it 
contains a syntax error.)

On 2023-07-01 17:55, Chris Lilley wrote:
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>
>
>
> -------- Forwarded Message --------
> Subject:  [Moderator Action] Puzzling of PNG Error Handling
> Date:  Thu, 29 Jun 2023 19:30:37 +0000
> From:  DLiang Fun <dliangfun@gmail.com>
> To:  public-png@w3.org
>
>
>
> Dear PNG Working Group,
>
> I hope this email finds you well. I wanted to bring to your attention 
> a minor matter regarding the PNG specification on error handling.
>
> 1) In regards to chunk types, the specification clearly states that 
> each byte in the chunk type should be limited to ISO 646 letters (A-Z 
> and a-z). Therefore, it appears reasonable to report errors if this 
> condition is not satisfied.
>
> 2) The Error Handling section explicitly states that encountering an 
> unknown ancillary chunk is never considered an error.
>
> Now, here's the silly question that arises: If a chunk type contains 
> non-ISO 646 letters but is still an unknown ancillary chunk, should we 
> report an error?
>
> While reading the specification, I do have the feeling that we should 
> not report an error in such a case. However, the specification does 
> not explicitly indicate that, in case of conflicts, we should follow 
> the Error Handling philosophy first.
>
> I have observed divergent implementations regarding this matter. 
> Specifically, while libspng and lodepng choose to ignore the chunk in 
> question, libpng halts the parsing process and reports a fatal error. 
> Hence, I believe it would be beneficial to explicitly clarify this 
> aspect in the specification.
>
> I kindly request your consideration and input on this topic. Your 
> guidance in ensuring consistency and clarity within the specification 
> would be greatly appreciated.
>
> Thank you for your time and attention.. I am looking forward to 
> hearing your insights on this matter.
>
> Best regards,
> Dliangfun

-- 
Chris Lilley
@svgeesus
Technical Director @ W3C
W3C Strategy Team, Core Web Design
W3C Architecture & Technology Team, Core Web & Media