Refactoring source image usage in CanvasRenderingContext2D

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 581 matching lines @@
         }
     }
 }
 
 void HTMLCanvasElement::didMoveToNewDocument(Document& oldDocument)
 {
     setObservedDocument(document());
     HTMLElement::didMoveToNewDocument(oldDocument);
 }
 
+PassRefPtr<Image> HTMLCanvasElement::getSourceImageForCanvas(HTMLCanvasElement* canvas, ExceptionState& exceptionState, CanvasImageSourceUsage usage, bool* isVolatile) const
+{
+    if (isVolatile)
+        *isVolatile = false;
+    if (!width() || !height()) {
+        if (usage == PatternCanvasImageSourceUsage)
+            exceptionState.throwDOMException(InvalidStateError, String::format("The canvas %s is 0.", canvas->width() ? "height" : "width"));
+        return nullptr;
+    }
+
+    if (usage == PatternCanvasImageSourceUsage || canvas == this)
+        return copiedImage();

[Stephen White, 2014/02/27 18:53:41]

You might want to check the canvas-to-canvas self-copying for performance
regressions. I remember it was too easy to trigger a CPU readback in this case
inadvertently.

+
+    if (m_context && m_context->is3d()) {
+        m_context->paintRenderingResultsToCanvas();
+        *isVolatile = true;

[Stephen White, 2014/02/27 18:53:41]

Please add a comment here indicating that this only works because
canvas-to-canvas copies currently do a flush at the deferred canvas level. We
may have to revisit this if we later implement deferral for canvas-to-canvas
copies (or maybe not, but it seems good to document).

+    }
+    return buffer() ? m_imageBuffer->copyImage(DontCopyBackingStore, Unscaled) : nullptr;
 }
+
+bool HTMLCanvasElement::wouldTaintOrigin(CanvasRenderingContext* dst) const
+{
+    return !originClean();
+}
+
+FloatSize HTMLCanvasElement::sourceSize() const
+{
+    return FloatSize(width(), height());
+}
+
+}