[csswg-drafts] EOTF correction / standard for alpha? (#12492)

gregbenz has just created a new issue for https://github.com/w3c/csswg-drafts:

== EOTF correction / standard for alpha? ==
ETOF affects alpha / transparency of image compositing. This may be particularly important for compositing of HDR assets which may be edited in linear and exported in PQ / HLG and mixed next to SDR assets more commonly encoded with an EOTF typically near a 1.8-2.2 gamma.

For example, the attached images of stripes on a transparent background will look very different when matted to #F00 red in Photoshop vs Chrome (which can only show the AVIF result) vs Safari Tech Preview 223. Chrome shows a completely different result from either. The opacity of the image is lower in both browser, causing colorimetry errors (should see yellow mixing along the entire edge of the vertical green line), squares darker, and thinner lines. 

Color mixing is consistent in areas where red exists in the image as that color blending would have been done by Photoshop when exporting as a single JXL layer. However matting to red in the browser is very different. 32-bit Photoshop will show yellow where green and red mix along the entire vertical axis (mixed in linear light), whereas Safari shows a dark edge as it seems to be mixed in something other than linear (per jxlinfo CLI tool, this JXL has a linear transfer).

[stripes (alpha test).zip](https://github.com/user-attachments/files/21300814/stripes.alpha.test.zip)

The Photoshop results are generally correct and as expected. Note that if you open the JXL in Photoshop and mat to red, the results are a match to the source TIF. The AVIF is reasonably close for colorimetry if you change to 32-bit mode (to use the same linear EOTF for alpha compositing as in the 32-bit TIF / JXL) - the AVIF is brighter and I'm not sure if that's a bug in in one of my conversion steps (detailed in the attachment for anyone who wishes to recreate from the original TIF source).

I'm not entirely sure which standards body is best to raise this issue, please let me know if this is not the ideal forum to discuss how opacity should be managed for images on the web (or if this is already being managed elsewhere).

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/12492 using your GitHub account


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Received on Thursday, 17 July 2025 16:05:10 UTC