Don't anti-alias backgrounds of images.

Source/core/paint/ImagePainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/paint/ImagePainter.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/Element.h"
 #include "core/editing/FrameSelection.h"
@@ skipping 182 matching lines @@
     InspectorInstrumentation::willPaintImage(&m_renderImage);
 
     InterpolationQuality previousInterpolationQuality = context->imageInterpolationQuality();
     context->setImageInterpolationQuality(interpolationQuality);
     context->drawImage(image.get(), alignedRect, CompositeSourceOver, m_renderImage.shouldRespectImageOrientation());
     context->setImageInterpolationQuality(previousInterpolationQuality);
 
     InspectorInstrumentation::didPaintImage(&m_renderImage);
 }
 
+void ImagePainter::paintBoxDecorationBackground(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
+{
+    // Don't anti-alias the background of an image. See crbug.com/423834 for the reason why.
+    // However, don't turn off anti-aliasing for subclasses such as video.
+    bool shouldAntialias = !m_renderImage.isRenderImage();

[chrishtr, 2014/11/11 19:34:21]

For the record, I also spent a while this morning trying out a generalization of
this patch, by extending the bleed avoidance logic in BoxDecorationData.
That code contains various special cases that are similar in concept to what
this CL is attempting.

Unfortunately, it's hard to get this type of bleed avoidance cleanly into
BoxDecorationData, without adding a new bitfield member to represent "will
eventually have an obscured background". This is because
a) images load asynchronously, so there is a difference between "obscure now"
and "will end up obscured"
b) images may be partially transparent, so we don't even know that the
background will be obscured after load
c) even if we new after load, animated gifs can change transparency between
frames (or at least that's what the code assumes)

All this makes me more comfortable with this targeted approach.

+
+    bool oldShouldAntialias = paintInfo.context->shouldAntialias();
+
+    paintInfo.context->setShouldAntialias(shouldAntialias);
+    BoxPainter(m_renderImage).paintBoxDecorationBackground(paintInfo, paintOffset);
+    paintInfo.context->setShouldAntialias(oldShouldAntialias);
+}
+
 } // namespace blink