1253937 - color-burn rendering is wrong with CompositorOGL

Status: RESOLVED INVALID

(Core :: Graphics: Layers, defect)

Description

[Markus Stange [:mstange]]

Attachment: testcase with mix-blend-mode: color-burn

The mix-blend-mode testcase should match the reference. On OS X it does not.

Comment 1

[Markus Stange [:mstange]]

Attachment: testcase with background-blend-mode: color-burn

Comment 2

[Markus Stange [:mstange]]

Attachment: reference

Comment 3

[Markus Stange [:mstange]]

So for R and G we have Cs = 0, Cb = 0.5, alpha_s = 0.5, alpha_b = 1.0.
The expected result after the final compositing step is 0.
I don't really see how this is supposed work since the final compositing step is using OVER on a 0.5 backdrop, using the original source alpha value of 0.5. In order to overwrite 0.5 with 0 you'd have to use an alpha value of 1.

Comment 4

[Markus Stange [:mstange]]

Huh. This bug might be invalid.
I got the reference by looking at the rendering of Release Firefox on OS X and of Safari. Both are using CoreGraphics. And now I think they're wrong.

Firefox with the Skia content backend agrees with our new rendering. And unsurprisingly, so does Chrome.

Rik, do you know anything about CG's discrepancy with respect to color-burn?

Comment 5

[Markus Stange [:mstange]]

And here's a tweet complaining about differences in color-burn rendering on iOS between CoreGraphics and CoreImage: https://twitter.com/tinrocket/status/297885629495246848

Comment 6

[Markus Stange [:mstange]]

Yup, just a CG bug. They are ignoring the alpha component of the source and treating the source as opaque.

I'll file a bug with Apple.

Comment 7

[Rik Cabanier]

Yes, we've logged the same bug with Apple several times.
They are aware of the issue but for some reason they can't/won't fix it.

Comment 8

[Markus Stange [:mstange]]

Thanks Rik. Are you aware of any other blend modes that are buggy in CG or is it just color-burn?

Comment 9

[Rik Cabanier]

Yes, I believe color-dodge has some glitches as well. However, it only happens in the CA layer. CG is fine.