[cairo] GraphicsContextCairo::FillRect with a solid colour does not respect operator

[Rob Bradford]

In the GraphicsContext function:

void GraphicsContext::fillRect(const FloatRect& rect, const Color& color, ColorSpace styleColorSpace, CompositeOperator op)

(which is not overridden in the GraphicsContextCairo)

It sets the operator and then proceeds to call the fillRect(const FloatRect& rect, const Color& color, ColorSpace styleColorSpace) (i.e. the version without the operator)

The problem is that the fillRectWithColor in GraphicsContextCairo.cpp always uses the OVER operator.

Without respecting the operator I think we would see incorrect results from code that triggers

"
        if (baseColor.alpha()) {
            if (bgColor.alpha())
                baseColor = baseColor.blend(bgColor);

            context->fillRect(backgroundRect, baseColor, style()->colorSpace(), CompositeCopy);
        } else if (bgColor.alpha()) {
            CompositeOperator operation = shouldClearBackground ? CompositeCopy : context->compositeOperation();
            context->fillRect(backgroundRect, bgColor, style()->colorSpace(), operation);
        } else if (shouldClearBackground)
            context->clearRect(backgroundRect);

" from RenderBoxModelObject::paintFillLayerExtended

[Rob Bradford]

Created attachment 105968 [details]
Proposed patch

[Rob Bradford]

Created attachment 105969 [details]
Fix gramatical errors in the ChangeLog entry on the original patch

[Martin Robinson]

Do you have a test case where this fails? In what cases would this lead to degraded performance?

[Rob Bradford]

(In reply to comment #3)
> Do you have a test case where this fails? In what cases would this lead to degraded performance?

I observed this bug by observation of the code. I will try and figure out a test case that triggers a visual issue because of this bug.

In terms of performance painting with OVER is more costly than SOURCE since SOURCE can be optimised to a blit by Cairo in many situations (since it doesn't depend on the value that is currently in the destination.)

[Martin Robinson]

(In reply to comment #4)

> In terms of performance painting with OVER is more costly than SOURCE since SOURCE can be optimised to a blit by Cairo in many situations (since it doesn't depend on the value that is currently in the destination.)

Thanks. This is very likely a bug!

[Eric Seidel (no email)]

Comment on attachment 105969 [details]
Fix gramatical errors in the ChangeLog entry on the original patch

View in context: https://bugs.webkit.org/attachment.cgi?id=105969&action=review

How do we test this?

> Source/WebCore/ChangeLog:5
> +

Please don't remove the REviewed by line, otherwise the tools can't fill it in fo ryou.

[Martin Robinson]

*** Bug 84848 has been marked as a duplicate of this bug. ***

[Martin Robinson]

*** This bug has been marked as a duplicate of bug 84848 ***