[Webkit-unassigned] [Bug 48391] New: qtwebkit graphics CompositionMode_Source mode optimization for raster engine

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=48391

           Summary: qtwebkit graphics CompositionMode_Source mode
                    optimization for raster engine
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: WebKit Qt
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: chunronglai at gmail.com


In WebCore/platform/graphics/qt/ImageQt.cpp there is 
       if (!image->hasAlpha() && painter->compositionMode() == QPainter::CompositionMode_SourceOver)
         compositionMode = QPainter::CompositionMode_Source;
 which potentially means that a memcopy can be used to compose source over of an opaque pixel instead of the alpha blending.

 However in the underground raster engine, this even leads to worse performance,
    void QRasterPaintEnginePrivate::recalculateFastImages()
    {
    ......
    s->flags.fast_images = !(s->renderHints & QPainter::SmoothPixmapTransform)
                && rasterBuffer->compositionMode == QPainter::CompositionMode_SourceOver
                && s->matrix.type() <= QTransform::TxShear;
    ......
    }

 As the discussion in https://lists.webkit.org/pipermail/webkit-qt/2010-October/000929.html, we might extend fast_images to include CompositionMode_Source as well.

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