Bug 24571 – Element is visible even if backface-visibility is 'hidden' and the back is facing towards the viewer.

• Status: RESOLVED
• Resolution: WONTFIX
• Component: Layout and Rendering

[Kim Grönholm]

Element is visible even if backface-visibility is 'hidden' and the back is facing towards the viewer.

[Kim Grönholm]

Created attachment 28574 [details]
Test case for backface visibility

A simple test case for -webkit-backface-visibility property

[Kim Grönholm]

Created attachment 28575 [details]
Patch to enable backface visibility property to have effect

This small patch enables -webkit-backface-visibility CSS property to have effect and fixes the test case.

[Simon Fraser (smfr)]

backface-visibility isn't supported unless you have rendering that supports 3d transforms.

The fix is also incorrect, because you need to compute a transform relative to the root, and not just the container.

[Simon Fraser (smfr)]

Comment on attachment 28575 [details]
Patch to enable backface visibility property to have effect

> Index: WebCore/rendering/RenderLayer.cpp
> ===================================================================
> --- WebCore/rendering/RenderLayer.cpp	(revision 41663)
> +++ WebCore/rendering/RenderLayer.cpp	(working copy)
> @@ -1955,6 +1955,12 @@ RenderLayer::paintLayer(RenderLayer* roo
>          TransformationMatrix transform;
>          transform.translate(x, y);
>          transform = *m_transform * transform;
> +
> +        // Don't paint anything if backface-visibility is 'hidden' and the back is facing towards the viewer.
> +        if (renderer()->style()->backfaceVisibility() == BackfaceVisibilityHidden) {
> +            if (transform.inverse().m33() < 0)
> +            return;
> +        }

This is wrong; backface-visibility has to take into account the transform to the root, not just the parent layer. It really needs to be implemented by the GraphicsLayer machinery.

[Simon Fraser (smfr)]

backface-visibility is only supported when ACCELERATED_COMPOSITING is enabled, and in that situation it works.

If you have a 3d rendering path that does not rely on ACCELERATED_COMPOSITING, then it presumably requires a bunch of extra changes which would also need to be committed.

[Simon Fraser (smfr)]

*** Bug 37460 has been marked as a duplicate of this bug. ***