Re: XMP in PNG

On 2021-11-30 14:02, Chris Blume (ProgramMax) wrote:
> During the meeting, Chris Lilley mentioned that XMP in PNG could go 
> inside the "iTXt" chunk 
> <https://www.w3.org/TR/2003/REC-PNG-20031110/#11iTXt> [1] with an 
> unregistered keyword "XML:com.adobe.xmp". This makes sense to me.
To be clear - this was not a proposal from me, but a report on how it is 
currently done, for example in Adobe Photoshop, when you save as PNG 
(export as PNG strips all non-essential information).
> Other keywords represent other known metadata such as copyright, 
> author, title, creation time...
> Perhaps we could add an official keyword for XMP.
We could - or we could register the one already in use, which is a path 
of least resistance.
>
> However, the datastream placement of "iTXt" and similar chunks has no 
> restriction, according to PNG spec part 5.6, Chunk ordering 
> <https://www.w3.org/TR/2003/REC-PNG-20031110/#5ChunkOrdering> [2]. If 
> the XMP data itself could affect how the image is decoded, we would 
> want to enforce that it appears before the "IDAT" chunk in the 
> datastream. Similarly, perhaps XMP would require a different 
> (un)safe-to-copy property.
Good point. Does it?

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