Add canvas blending modes under a runtime flag

Source/core/html/canvas/CanvasRenderingContext2D.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 17 matching lines @@
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "config.h"
 #include "core/html/canvas/CanvasRenderingContext2D.h"
 
 #include "CSSPropertyNames.h"
 #include "HTMLNames.h"
+#include "RuntimeEnabledFeatures.h"
 #include "core/css/CSSFontSelector.h"
 #include "core/css/CSSParser.h"
 #include "core/css/StylePropertySet.h"
 #include "core/css/StyleResolver.h"
 #include "core/dom/ExceptionCode.h"
 #include "core/dom/ExceptionCodePlaceholder.h"
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/HTMLImageElement.h"
 #include "core/html/HTMLMediaElement.h"
 #include "core/html/HTMLVideoElement.h"
@@ skipping 556 matching lines @@
 {
     return compositeOperatorName(state().m_globalComposite, state().m_globalBlend);
 }
 
 void CanvasRenderingContext2D::setGlobalCompositeOperation(const String& operation)
 {
     CompositeOperator op = CompositeSourceOver;
     BlendMode blendMode = BlendModeNormal;
     if (!parseCompositeAndBlendOperator(operation, op, blendMode))
         return;
+    if (!RuntimeEnabledFeatures::cssCompositingEnabled() && blendMode != BlendModeNormal)

[eseidel, 2013/05/08 19:34:44]

I'm confused.  Did we change the parsing?  Was it already possible to pass a
non-normal blend mode?  Did it do anything?

[rosca, 2013/05/08 21:37:17]

As described in
https://dvcs.w3.org/hg/FXTF/rawfile/tip/compositing/index.html#canvascomposit...,
globalCompositeOperation has been extended with blend modes. When
globalCompositeOperation is a blend mode, composition operator is suposed to be
'source over'. Parsing is done in parseCompositeAndBlendOperator.

On 2013/05/08 19:34:44, Eric Seidel (Google) wrote:

+        blendMode = BlendModeNormal;
     if ((state().m_globalComposite == op) && (state().m_globalBlend == blendMode))
         return;
     realizeSaves();
     modifiableState().m_globalComposite = op;
     modifiableState().m_globalBlend = blendMode;
     GraphicsContext* c = drawingContext();
     if (!c)
         return;
     c->setCompositeOperation(op, blendMode);
 }
@@ skipping 1629 matching lines @@
 }
 
 PassRefPtr<Canvas2DContextAttributes> CanvasRenderingContext2D::getContextAttributes() const
 {
     RefPtr<Canvas2DContextAttributes> attributes = Canvas2DContextAttributes::create();
     attributes->setAlpha(m_hasAlpha);
     return attributes.release();
 }
 
 } // namespace WebCore