WebGL: clarify which Front or Back buffer is used by each API.

Source/core/html/HTMLCanvasElement.cpp

Index: Source/core/html/HTMLCanvasElement.cpp
diff --git a/Source/core/html/HTMLCanvasElement.cpp b/Source/core/html/HTMLCanvasElement.cpp
index ee5bd175465dce5115e97674feaae44f0317a9c8..46163224afe2ee71e0502a50d2ade75f47efeb66 100644
--- a/Source/core/html/HTMLCanvasElement.cpp
+++ b/Source/core/html/HTMLCanvasElement.cpp
@@ -62,17 +62,26 @@ namespace blink {
 
 using namespace HTMLNames;
 
+namespace {
+
 // These values come from the WhatWG spec.
-static const int DefaultWidth = 300;
-static const int DefaultHeight = 150;
+const int DefaultWidth = 300;
+const int DefaultHeight = 150;
 
 // Firefox limits width/height to 32767 pixels, but slows down dramatically before it
 // reaches that limit. We limit by area instead, giving us larger maximum dimensions,
 // in exchange for a smaller maximum canvas size.
-static const int MaxCanvasArea = 32768 * 8192; // Maximum canvas area in CSS pixels
+const int MaxCanvasArea = 32768 * 8192; // Maximum canvas area in CSS pixels
 
 //In Skia, we will also limit width/height to 32767.
-static const int MaxSkiaDim = 32767; // Maximum width/height in CSS pixels.
+const int MaxSkiaDim = 32767; // Maximum width/height in CSS pixels.
+
+CanvasRenderingContext::SourceBuffer convertSourceBuffer(HTMLCanvasElement::SourceBuffer sourceBuffer)
+{
+    return static_cast<CanvasRenderingContext::SourceBuffer>(sourceBuffer);
+}
+
+} // namespace
 
 DEFINE_EMPTY_DESTRUCTOR_WILL_BE_REMOVED(CanvasObserver);
 
@@ -387,24 +396,24 @@ String HTMLCanvasElement::toEncodingMimeType(const String& mimeType)
 
 const AtomicString HTMLCanvasElement::imageSourceURL() const
 {
-    return AtomicString(toDataURLInternal("image/png", 0, true));
+    return AtomicString(toDataURLInternal("image/png", 0, Front));
 }
 
-String HTMLCanvasElement::toDataURLInternal(const String& mimeType, const double* quality, bool isSaving) const
+String HTMLCanvasElement::toDataURLInternal(const String& mimeType, const double* quality, SourceBuffer sourceBuffer) const
 {
-    if (m_size.isEmpty() || !buffer())
+    if (m_size.isEmpty() || !buffer() || !m_context)

[dshwang, 2014/11/22 19:32:18]

I changed a behavior of toDataURL without context. I fixed it in the next patch
set.

         return String("data:,");
 
     String encodingMimeType = toEncodingMimeType(mimeType);
 
-    // Try to get ImageData first, as that may avoid lossy conversions.
-    RefPtrWillBeRawPtr<ImageData> imageData = getImageData();

[dshwang, 2014/11/21 09:05:40]

getImageData() is implemented by only webgl, so I put this logic inside is3d if
statement.

-
-    if (imageData)
-        return ImageDataToDataURL(ImageDataBuffer(imageData->size(), imageData->data()), encodingMimeType, quality);
-
-    if (m_context && m_context->is3d()) {
-        m_context->paintRenderingResultsToCanvas(isSaving ? CanvasRenderingContext::Front : CanvasRenderingContext::Back);
+    if (m_context->is3d()) {
+        // Get pure non-premultiplied data because of inaccurate premultiplied alpha conversion of buffer()->toDataURL().
+        RefPtrWillBeRawPtr<ImageData> imageData =
+            toWebGLRenderingContext(m_context.get())->paintRenderingResultsToImageData(convertSourceBuffer(sourceBuffer));
+        // |imageData| is null when premultipliedAlpha == true.
+        if (imageData)
+            return ImageDataToDataURL(ImageDataBuffer(imageData->size(), imageData->data()), encodingMimeType, quality);
+        m_context->paintRenderingResultsToCanvas(convertSourceBuffer(sourceBuffer));
     }
 
     return buffer()->toDataURL(encodingMimeType, quality);
@@ -417,14 +426,7 @@ String HTMLCanvasElement::toDataURL(const String& mimeType, const double* qualit
         return String();
     }
 
-    return toDataURLInternal(mimeType, quality);
-}
-
-PassRefPtrWillBeRawPtr<ImageData> HTMLCanvasElement::getImageData() const
-{
-    if (!m_context || !m_context->is3d())
-        return nullptr;
-    return toWebGLRenderingContext(m_context.get())->paintRenderingResultsToImageData();
+    return toDataURLInternal(mimeType, quality, Back);
 }
 
 SecurityOrigin* HTMLCanvasElement::securityOrigin() const
@@ -672,12 +674,11 @@ void HTMLCanvasElement::ensureUnacceleratedImageBuffer()
     m_didFailToCreateImageBuffer = !m_imageBuffer;
 }
 
-Image* HTMLCanvasElement::copiedImage() const
+Image* HTMLCanvasElement::copiedImage(SourceBuffer sourceBuffer) const
 {
     if (!m_copiedImage && buffer()) {
-        if (m_context && m_context->is3d()) {
-            m_context->paintRenderingResultsToCanvas(CanvasRenderingContext::Front);
-        }
+        if (m_context && m_context->is3d())
+            m_context->paintRenderingResultsToCanvas(convertSourceBuffer(sourceBuffer));
         m_copiedImage = buffer()->copyImage(CopyBackingStore, Unscaled);
         updateExternallyAllocatedMemory();
     }