Re: XMP in PNG

As Chris said, XMP in PNG has been around for at least 15 years and is implemented by all major image processing tools (commercial and open source, from ImageMagick to GIMP) and search engines (both desktop and web).

Leonard

From: Chris Lilley <chris@w3.org>
Date: Tuesday, November 30, 2021 at 5:15 PM
To: Chris Blume (ProgramMax) <programmax@gmail.com>, Leonard Rosenthol <lrosenth@adobe.com>
Cc: public-png@w3.org <public-png@w3.org>
Subject: Re: XMP in PNG

On 2021-11-30 23:43, Chris Blume (ProgramMax) wrote:
> it sounds like this tone mapping metadata perhaps shouldn't be in XMP
> anyway. I think we might want to create a new chunk for it so we can
> place datastream position requirements on the chunk. It should go
> before the frame it affects.
Yes, that sounds reasonable.
> That said, do we still feel like there is a need for XMP in PNG?
> (I'm not doubting it. I just want to make sure we check ourselves here.)

It is already being used, at least in Adobe applications, so the need is
already demonstrated and it is just a matter of registering this usage.

It is also being done incorrectly by other applications, indicating a
benefit to standardization:

https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fnewsgroup.xnview.com%2Fviewtopic.php%3Ft%3D31356&amp;data=04%7C01%7Clrosenth%40adobe.com%7C6daf123658d94eb8a84c08d9b44ede1a%7Cfa7b1b5a7b34438794aed2c178decee1%7C0%7C0%7C637739073105324301%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=BGMp5SCdgADXnIEH7H7r%2BOK9R%2Bxl3MoA1DyFggb35MU%3D&amp;reserved=0

 > XnViewMP writes XMP metadata in PNG files in the incorrect chunk, in
the tEXt instead of iTXt, causing bad interaction with other
applications which might lead to file corruption or loss of metadata.

(Incorrect because tEXt is Latin-1 only, and because the convention is
to use iTXt)

> Exif has plenty of existing users and is a noticeable gap in PNG
> compared to other image formats. It would be easy to justify why we
> feel it should be included.
> Does XMP cross the threshold of enough demand & users & clearly should
> be part of the standard?

It wouldn't be a new chunk; just a new registered keyword plus some
pointers to the existing spec on what to put in the iTXt chunk.

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