Support transparency in JPEG-XL #2040

What it says on the tin. We have transparency layer support but don't seem to handle it correctly. Possibly an issue with alpha premultiplication?
We may need to look at how we do this for our PNG support.

What it says on the tin. We have transparency layer support but don't seem to handle it correctly. Possibly an issue with alpha premultiplication? We may need to look at how we do this for our PNG support.

Firefox and Palemoon both show the 3 colors on the following test page:
https://asozial.org/.well-known/transparency.html

The image should be fully transparent, but Firefox and Palemoon seem to ignore that if there are color values set.

Firefox and Palemoon both show the 3 colors on the following test page: https://asozial.org/.well-known/transparency.html The image should be fully transparent, but Firefox and Palemoon seem to ignore that if there are color values set.

https://jpegxl.info/test-page/

We already have transparency support in the surface; the only problem is that our rendering pipe for it expects premultiplied alpha. I can slot this in easily enough along with the RB channel swapping we're doing.

(I still strongly disagree with alpha premultiplication practice. It is literally losing color data; see also our non-premul gradients vs. premul gradients in other browsers; test here)

https://jpegxl.info/test-page/ We already have transparency support in the surface; the only problem is that our rendering pipe for it expects premultiplied alpha. I can slot this in easily enough along with the RB channel swapping we're doing. (I still strongly disagree with alpha premultiplication practice. It is literally losing color data; see also our non-premul gradients vs. premul gradients in other browsers; test [here](http://testserver.palemoon.org/gradients/gradient-transparent-test.html))

Some fast integer math did the trick!

Image1.jpg

155 KiB

Can we uplift this for 31.4.2? The workaround does seem to work properly and I doubt it will break anything. Transparency is a pretty important feature as JPEG-XL is supposed to replace PNG too, and IMO it wouldn't be a good look both for Pale Moon and JPEG-XL if users see that their converted PNGs which have an alpha channel don't look right in the browser.

Yeah I planned to cherry pick this for the sec release before the holidays. I just didn't get around to marking it on the repo yet (and I'll be travelling the next 4 days so won't be around). I'll mark it here with the appropriate label and milestone to make sure I don't forget.

reopen for uplift

Uplifted in [49a86db5f3]

Uplifted in [49a86db5f3]

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Support transparency in JPEG-XL #2040