[Webkit-unassigned] [Bug 135483] New: TextureMapperGL stencil bug

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=135483

           Summary: TextureMapperGL stencil bug
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: abaldeva at ea.com


Hi,

Going through the TextureMapperGL implementation, I see that BeginClip advances the bit plane to the next 

clipStack().setStencilIndex(stencilIndex * 2);

However, EndClip does not revert it back before calling clipStack().applyIfNeeded(m_context3D.get());

The applyIfNeeded disables stencil test only if stencilIndex is 1.

if (clipState.stencilIndex == 1)
        context->disable(GraphicsContext3D::STENCIL_TEST);
    else
        context->enable(GraphicsContext3D::STENCIL_TEST); 

There is no place in the code that is attempting to reverse the impact of BeginClip. Isn’t a right bit shift required in the EndClip call before the applyIfNeeded call? Otherwise, I don’t see how EndClip will disable stenciling test ever. The problem is probably more obvious if you have nested clipping. 

You can also see the problem on classing falling leaves demo. The leaves are visible outside the center container.You'll first need to disable the scissor based clipping otherwise the stencil path would be optimized and never hit. 

Previous related changes - http://trac.webkit.org/changeset/108696 

--Arpit

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