Bug 113732 – [Texmap] Hierarchy of layers with opacity may result in wrong blending.

• Component: Layout and Rendering
• Status: RESOLVED FIXED

[Bruno Abinader (history only)]

Created attachment 196038 [details]
Test

When working on a layout test for bug 50418, I have noticed that composited layers with a mixed set of layout positioning methods (fixed, absolute, relative, float) causes some visual artifacts with layout and rendering (see attached test). In Chrome it behaves correctly, while on both EFL and Qt running MiniBrowser I observed the same issues.

[Bruno Abinader (history only)]

Created attachment 196039 [details]
Expected result

Expected result as obtained from Chrome.

[Bruno Abinader (history only)]

Created attachment 196040 [details]
Actual result

Actual (buggy) result from both Qt and EFL builds (might also be happening on other AC-enabled ports).

[Bruno Abinader (history only)]

I found out yesterday two things:

1. If we change both location and size of the tileRect passed to paintWithIntermediateSurface to be IntPoint(0,0) and maxTextureSize, respectively, the texture layers are properly rendered but with messy opacity blending.

2. When using "-webkit-transform-style: preserve-3d;" on all composited layers, the issue disappears. My guess is that this makes shouldBlend() return false, thus forcing codepath to not enter paintUsingOverlapRegions().

[Noam Rosenthal]

Yes, the bug is probably in the overlap region calculation then.

[Noam Rosenthal]

Created attachment 196637 [details]
Much simplified test case

[Bruno Abinader (history only)]

(In reply to comment #5)
> Created an attachment (id=196637) [details]
> Much simplified test case

No'am, I haven't noticed any issues with your testcase, at least not on EFL port. I have a fix for the surface positioning issue (see below), however opacity blending results are still wrong.

diff --git a/Source/WebCore/platform/graphics/texmap/TextureMapperLayer.cpp b/Source/WebCore/platform/graphics/texmap/TextureMapperLayer.cpp
index 5911513..70c98be 100644
--- a/Source/WebCore/platform/graphics/texmap/TextureMapperLayer.cpp
+++ b/Source/WebCore/platform/graphics/texmap/TextureMapperLayer.cpp
@@ -412,7 +412,10 @@ void TextureMapperLayer::paintWithIntermediateSurface(const TextureMapperPaintOp
         mainSurface = replicaSurface;
     }
 
-    commitSurface(options, mainSurface, rect, options.opacity);
+    // Offset needs to be reset when committing to surface.
+    TextureMapperPaintOptions surfaceOptions(options);
+    surfaceOptions.offset = IntSize();
+    commitSurface(surfaceOptions, mainSurface, rect, options.opacity);
 }
 
 void TextureMapperLayer::paintRecursive(const TextureMapperPaintOptions& options)

Can you please test this on your environment to see if it works? I'll attach it as a patch to make it easy to apply.

[Bruno Abinader (history only)]

Created attachment 197035 [details]
Exploratory patch

When committing the surface, resetting the offset value makes the painted surfaces being correctly positioned. However, the opacity blending still have incorrect values when compared to non-composited result.

[Bruno Abinader (history only)]

Comment on attachment 197035 [details]
Exploratory patch

As Noam commented on IRC, this fix is wrong (only affects the provided test-case).

[Noam Rosenthal]

Created attachment 197136 [details]
Patch

[Allan Sandfeld Jensen]

Comment on attachment 197136 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=197136&action=review

> Source/WebCore/platform/graphics/texmap/TextureMapperGL.h:100
> +        enum MirrorMode {
> +            ShouldNotMirror,
> +            ShouldMirror
> +        };

Could you add a comment about what MirrorMode represents?

[Noam Rosenthal]

Created attachment 197141 [details]
Patch

[Build Bot]

Comment on attachment 197141 [details]
Patch

Attachment 197141 [details] did not pass mac-wk2-ews (mac-wk2):
Output: http://webkit-commit-queue.appspot.com/results/17690016

New failing tests:
compositing/overlap-blending/nested-overlap.html

[Build Bot]

Created attachment 197174 [details]
Archive of layout-test-results from webkit-ews-12 for mac-mountainlion-wk2

The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews.
Bot: webkit-ews-12  Port: mac-mountainlion-wk2  Platform: Mac OS X 10.8.2

[Noam Rosenthal]

Created attachment 197220 [details]
Patch

[Noam Rosenthal]

Created attachment 197221 [details]
Patch

[WebKit Commit Bot]

Comment on attachment 197221 [details]
Patch

Clearing flags on attachment: 197221

Committed r148090: <http://trac.webkit.org/changeset/148090>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.