[CSS Blending] Wrong color result for basic blend modes

• Component: CSS
• Status: RESOLVED WORKSFORME

[Ion Rosca]

Created attachment 227417 [details]
Sample for 'difference' mode results 

According to the spec[1], the 'difference' blend mode should be computed using this formula: B(Cb, Cs) = | Cb - Cs |.
For example: Fuchsia difference Aqua = Yellow

The result in webkit is different than Yellow. On retina display it's a pale yellow rgb(204, 214, 0), but on non-retina displays, the result is a pale green rgb(0, 190, 0).

The behaviour is the same for both accelerated and non-accelerated blending. I did not test for other blend modes than difference (TBD).

[1] http://dev.w3.org/fxtf/compositing-1/#blendingdifference

[Mihai Tica]

(In reply to comment #0)
> Created an attachment (id=227417) [details]
> Sample for 'difference' mode results 
> 
> According to the spec[1], the 'difference' blend mode should be computed using this formula: B(Cb, Cs) = | Cb - Cs |.
> For example: Fuchsia difference Aqua = Yellow
> 
> The result in webkit is different than Yellow. On retina display it's a pale yellow rgb(204, 214, 0), but on non-retina displays, the result is a pale green rgb(0, 190, 0).
> 
> The behaviour is the same for both accelerated and non-accelerated blending. I did not test for other blend modes than difference (TBD).
> 
> [1] http://dev.w3.org/fxtf/compositing-1/#blendingdifference

There are also problems with other blend modes. The biggest differences are with soft-light.
In fact, when validating canvas blend modes, the tests submitted https://bugs.webkit.org/show_bug.cgi?id=119082 entirely skip soft-light since the differences between the formula and the result are consistent.

Imho, this is rather a CG/CA bug.

[Radar WebKit Bug Importer]

<rdar://problem/16626421>

[Alexey Proskuryakov]

This doesn't appear to be failing on bots any more, will remove the expectation.