[Webkit-unassigned] [Bug 34578] New: [Qt] Rendering semitransparent layers is slow with Qt

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=34578

           Summary: [Qt] Rendering semitransparent layers is slow with Qt
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: Performance, Qt
          Severity: Normal
          Priority: P2
         Component: WebKit Qt
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: benjamin.poulain at nokia.com
                CC: benjamin.poulain at nokia.com


Currently, Qt is abnormally slow to render semi-transparent layers.

In GraphicsContext::beginTransparencyLayer(), we create a new transparent
Pixmap and start a painter on it. In GraphicsContext::endTransparencyLayer() we
paint this pixmap on top of the existing canvas.

Creating the pixmaps, the painter is relatively slow. The big bottleneck is
really the blending of the pixmap on top of what is already painted. For a site
using transparent layer (like Facebook alerts), blending take more than half of
the painting time.

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