Fixing color bleeding when the canvas is stretched.

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 305 matching lines @@
 
 void HTMLCanvasElement::didFinalizeFrame() {
   notifyListenersCanvasChanged();
 
   if (m_dirtyRect.isEmpty())
     return;
 
   // Propagate the m_dirtyRect accumulated so far to the compositor
   // before restarting with a blank dirty rect.
   FloatRect srcRect(0, 0, size().width(), size().height());
-  m_dirtyRect.intersect(srcRect);
+
   LayoutBox* ro = layoutBox();
   // Canvas content updates do not need to be propagated as
   // paint invalidations if the canvas is accelerated, since
   // the canvas contents are sent separately through a texture layer.
   if (ro && (!m_context || !m_context->isAccelerated())) {
+    // If ro->contentBoxRect() is larger than srcRect the canvas's image is
+    // being stretched, so we need to account for color bleeding caused by the
+    // interpollation filter.
+    if (ro->contentBoxRect().width() > srcRect.width() ||
+        ro->contentBoxRect().height() > srcRect.height()) {
+      m_dirtyRect.inflate(0.25);

[Justin Novosad, 2016/10/25 14:30:15]

Are you sure 0.25 is always enough? Even at very large scale factors? 0.5 seems
more logical to me.

+    }
+
+    m_dirtyRect.intersect(srcRect);
     LayoutRect mappedDirtyRect(enclosingIntRect(
         mapRect(m_dirtyRect, srcRect, FloatRect(ro->contentBoxRect()))));
     // For querying PaintLayer::compositingState()
     // FIXME: is this invalidation using the correct compositing state?
     DisableCompositingQueryAsserts disabler;
     ro->invalidatePaintRectangle(mappedDirtyRect);
   }
   m_dirtyRect = FloatRect();
 
   m_numFramesSinceLastRenderingModeSwitch++;
@@ skipping 1003 matching lines @@
   mojom::blink::OffscreenCanvasSurfacePtr service;
   Platform::current()->interfaceProvider()->getInterface(
       mojo::GetProxy(&service));
   m_surfaceLayerBridge =
       wrapUnique(new CanvasSurfaceLayerBridge(std::move(service)));
   return m_surfaceLayerBridge->createSurfaceLayer(this->width(),
                                                   this->height());
 }
 
 }  // namespace blink