Blend background with existing content

Source/core/rendering/RenderBoxModelObject.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 688 matching lines @@
     // Paint the color first underneath all images, culled if background image occludes it.
     // FIXME: In the bgLayer->hasFiniteBounds() case, we could improve the culling test
     // by verifying whether the background image covers the entire layout rect.
     if (!bgLayer->next()) {
         IntRect backgroundRect(pixelSnappedIntRect(scrolledPaintRect));
         bool boxShadowShouldBeAppliedToBackground = this->boxShadowShouldBeAppliedToBackground(bleedAvoidance, box);
         if (boxShadowShouldBeAppliedToBackground || !shouldPaintBackgroundImage || !bgLayer->hasOpaqueImage(this) || !bgLayer->hasRepeatXY()) {
             if (!boxShadowShouldBeAppliedToBackground)
                 backgroundRect.intersect(paintInfo.rect);
 
-            // If we have an alpha and we are painting the root element, go ahead and blend with the base background color.
-            Color baseColor;
-            bool shouldClearBackground = false;
-            if (isOpaqueRoot) {
-                baseColor = view()->frameView()->baseBackgroundColor();
-                if (!baseColor.alpha())
-                    shouldClearBackground = true;
-            }
-
             GraphicsContextStateSaver shadowStateSaver(*context, boxShadowShouldBeAppliedToBackground);
             if (boxShadowShouldBeAppliedToBackground)
                 applyBoxShadowForBackground(context, this);
 
-            if (baseColor.alpha()) {
-                if (bgColor.alpha())
-                    baseColor = baseColor.blend(bgColor);
+            if (isOpaqueRoot) {
+                // If we have an alpha and we are painting the root element, go ahead and blend with the base background color.
+                Color baseColor = view()->frameView()->baseBackgroundColor();
+                bool shouldBlendWithExistingContent = document().settings() && document().settings()->shouldBlendWithExistingContent();

[enne (OOO), 2013/09/24 18:35:48]

bikeshed: shouldBlendWithExistingContent is a little bit ambiguous in the
context of a document setting.  (What's being blended? What content?) Maybe
something like shouldClearDocumentBackground? Or, maybe there's some other
better name?

+                CompositeOperator operation = shouldBlendWithExistingContent ? context->compositeOperation() : CompositeCopy;
 
-                context->fillRect(backgroundRect, baseColor, CompositeCopy);
+                if (baseColor.alpha()) {
+                    if (bgColor.alpha())
+                        baseColor = baseColor.blend(bgColor);
+                    context->fillRect(backgroundRect, baseColor, operation);
+                } else if (bgColor.alpha()) {
+                    context->fillRect(backgroundRect, bgColor, operation);
+                } else if (!shouldBlendWithExistingContent) {
+                    context->clearRect(backgroundRect);
+                }
             } else if (bgColor.alpha()) {
-                CompositeOperator operation = shouldClearBackground ? CompositeCopy : context->compositeOperation();
-                context->fillRect(backgroundRect, bgColor, operation);
-            } else if (shouldClearBackground)
-                context->clearRect(backgroundRect);
+                context->fillRect(backgroundRect, bgColor, context->compositeOperation());
+            }
         }
     }
 
     // no progressive loading of the background image
     if (shouldPaintBackgroundImage) {
         BackgroundImageGeometry geometry;
         calculateBackgroundImageGeometry(bgLayer, scrolledPaintRect, geometry, backgroundObject);
         geometry.clip(paintInfo.rect);
         if (!geometry.destRect().isEmpty()) {
             CompositeOperator compositeOp = op == CompositeSourceOver ? bgLayer->composite() : op;
@@ skipping 2043 matching lines @@
     ASSERT(!beforeChild || toBoxModelObject == beforeChild->parent());
     for (RenderObject* child = startChild; child && child != endChild; ) {
         // Save our next sibling as moveChildTo will clear it.
         RenderObject* nextSibling = child->nextSibling();
         moveChildTo(toBoxModelObject, child, beforeChild, fullRemoveInsert);
         child = nextSibling;
     }
 }
 
 } // namespace WebCore