Bug 33361 – WebGL canvas paints background color twice

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Simon Fraser (smfr)]

A WebGL canvas with a CSS background color with alpha will render that background color twice. If the canvas has rounded corners, one layer of background color will show through at the corners.

[Simon Fraser (smfr)]

Created attachment 46157 [details]
Testcase

[Simon Fraser (smfr)]

Reopening, since this will be an issue once the patch in bug 33361 is landed.

The issue is that, for a directly composited WebGL layer, HTMLCanvasElement::paint() may never be called, so the beginPaint()/reshape()/endPaint() calls will never be called on the WebGLRenderingContext.

Now, some style change can happen that requires that the canvas's layer gets backing store and paints (say, a border is added). Then this method will call WebGLRenderingContext::reshape(), which causes the canvas to be cleared.

[Simon Fraser (smfr)]

That last comment was meant for bug 33392.

[Simon Fraser (smfr)]

Created attachment 46185 [details]
Patch

Comment on attachment 46185 [details]
Patch

> +        * rendering/RenderLayerBacking.h:
> +        * rendering/RenderLayerBacking.cpp:
> +        (WebCore::is3DCanvas):
> +        (WebCore::RenderLayerBacking::RenderLayerBacking):
> +        (WebCore::RenderLayerBacking::updateGraphicsLayerConfiguration):
> +        (WebCore::RenderLayerBacking::updateGraphicsLayerGeometry):
> +        (WebCore::hasBoxDecorationsOrBackgroundImage):
> +        (WebCore::RenderLayerBacking::isSimpleContainerCompositingLayer):
> +        (WebCore::RenderLayerBacking::hasNonCompositingContent):
> +        (WebCore::RenderLayerBacking::hasPaintedContent):
> +        (WebCore::RenderLayerBacking::isDirectlyCompositedImage):
> +        (WebCore::RenderLayerBacking::rendererContentChanged):

Per-function change log comments are encouraged.

>  static bool hasBoxDecorations(const RenderStyle*);
> -static bool hasBoxDecorationsWithBackgroundImage(const RenderStyle*);
> +static bool hasBoxDecorationsOrBackgroundImage(const RenderStyle*);

The naming here is really unfortunate, because it gives “box decoration” conflicting meanings. In hasBoxDecorations(), a background color counts as “box decoration”, but in hasBoxDecorationsOrBackgroundImage() it doesn’t. Your rename doesn’t make things better, just different.

> +// An image can be directly compositing if it's the sole content fo the layer, and has no box decorations

Typo: “fo”

> +    // Returns true if this layer has content that needs to be rendered by painting into the backing store.
> +    bool hasPaintedContent() const;

Not a fan of this name (sounds like this function tells you whether the layer has painted its content already), but I don’t have an alternative suggestion.

r=me

[Simon Fraser (smfr)]

Landed in http://trac.webkit.org/changeset/53034 with naming cleanup.