Make SVG painting independent of GraphicsContext's alpha state

Source/platform/graphics/GraphicsContextState.h

 // Copyright (C) 2013 Google Inc. All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //    * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //    * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
@@ skipping 56 matching lines @@
     uint16_t saveCount() const { return m_saveCount; }
     void incrementSaveCount() { ++m_saveCount; }
     void decrementSaveCount() { --m_saveCount; }
 
     // Stroke data
     Color strokeColor() const { return m_strokeColor; }
     SkColor effectiveStrokeColor() const { return applyAlpha(m_strokeColor.rgb()); }
     void setStrokeColor(const Color&);
 
     Gradient* strokeGradient() const { return m_strokeGradient.get(); }
-    void setStrokeGradient(const PassRefPtr<Gradient>);
+    void setStrokeGradient(const PassRefPtr<Gradient>, float);
     void clearStrokeGradient();
 
     Pattern* strokePattern() const { return m_strokePattern.get(); }
-    void setStrokePattern(const PassRefPtr<Pattern>);
+    void setStrokePattern(const PassRefPtr<Pattern>, float);
     void clearStrokePattern();
 
     const StrokeData& strokeData() const { return m_strokeData; }
     void setStrokeStyle(StrokeStyle);
     void setStrokeThickness(float);
     void setLineCap(LineCap);
     void setLineJoin(LineJoin);
     void setMiterLimit(float);
     void setLineDash(const DashArray&, float);
 
     // Fill data
     Color fillColor() const { return m_fillColor; }
     SkColor effectiveFillColor() const { return applyAlpha(m_fillColor.rgb()); }
     void setFillColor(const Color&);
 
     Gradient* fillGradient() const { return m_fillGradient.get(); }
-    void setFillGradient(const PassRefPtr<Gradient>);
+    void setFillGradient(const PassRefPtr<Gradient>, float);
     void clearFillGradient();
 
     Pattern* fillPattern() const { return m_fillPattern.get(); }
-    void setFillPattern(const PassRefPtr<Pattern>);
+    void setFillPattern(const PassRefPtr<Pattern>, float);
     void clearFillPattern();
 
     // Path fill rule
     WindRule fillRule() const { return m_fillRule; }
     void setFillRule(WindRule rule) { m_fillRule = rule; }
 
     // Shadow. (This will need tweaking if we use draw loopers for other things.)
     SkDrawLooper* drawLooper() const { return m_looper.get(); }
     void setDrawLooper(PassRefPtr<SkDrawLooper>);
     void clearDrawLooper();
@@ skipping 25 matching lines @@
     void setShouldAntialias(bool);
 
     bool shouldClampToSourceRect() const { return m_shouldClampToSourceRect; }
     void setShouldClampToSourceRect(bool shouldClampToSourceRect) { m_shouldClampToSourceRect = shouldClampToSourceRect; }
 
     // Returns true if clip is more than just a rectangle or intersection
     // of rectangles.
     bool hasComplexClip() const;
     void setHasComplexClip();
 
+    // Multiply a color's alpha channel by an additional alpha factor where
+    // alpha is in the range [0, 1].
+    static SkColor combineWithAlpha(SkColor color, float alpha)

[f(malita), 2015/02/03 00:33:28]

Is there any reason to keep these helpers in GC/GCS? They don't have anything to
do with GC and should probably be in SkiaUtils.

[pdr., 2015/02/04 04:04:31]

Moved to the other dumping ground, SkiaUtils :)

+    {
+        return combineWithAlpha(color, clampedAlphaForBlending(alpha));

[f(malita), 2015/02/03 00:33:28]

Naming nit: "combineWithAlpha" sounds pretty weird. How about applyAlpha or
applyOpacity?

[pdr., 2015/02/04 04:04:31]

After much fighting, I renamed this to multiplyAlpha. I originally wanted
applyAlpha but the compiler can't look up the correct function since we're
calling a global applyAlpha from a local function by the same name which
requires some ugly scoping to disambiguate the call.

+    }
+
 private:
     GraphicsContextState();
     explicit GraphicsContextState(const GraphicsContextState&);
     GraphicsContextState& operator=(const GraphicsContextState&);
 
+    // Multiply a color's alpha channel by an additional alpha factor where
+    // alpha is in the range [0, 256].
+    static SkColor combineWithAlpha(SkColor color, int alpha)
+    {
+        int a = (SkColorGetA(color) * alpha) >> 8;
+        return (color & 0x00FFFFFF) | (a << 24);
+    }
+
     // Helper function for applying the state's alpha value to the given input
     // color to produce a new output color.
-    SkColor applyAlpha(SkColor c) const
+    SkColor applyAlpha(SkColor color) const
     {
-        int a = SkAlphaMul(SkColorGetA(c), m_alpha);
-        return (c & 0x00FFFFFF) | (a << 24);
+        return combineWithAlpha(color, m_alpha);
+    }
+
+    // Map alpha values from [0, 1] to [0, 256] for alpha blending.
+    static int clampedAlphaForBlending(float alpha)
+    {
+        if (alpha < 0)
+            return 0;
+        int roundedAlpha = roundf(alpha * 256);
+        if (roundedAlpha > 256)
+            roundedAlpha = 256;
+        return roundedAlpha;
     }
 
     // These are mutbale to enable gradient updates when the paints are fetched for use.
     mutable SkPaint m_strokePaint;
     mutable SkPaint m_fillPaint;
 
     StrokeData m_strokeData;
 
     Color m_strokeColor;
     RefPtr<Gradient> m_strokeGradient;
@@ skipping 19 matching lines @@
     uint16_t m_saveCount;
 
     bool m_shouldAntialias : 1;
     bool m_shouldClampToSourceRect : 1;
     bool m_hasComplexClip : 1;
 };
 
 } // namespace blink
 
 #endif // GraphicsContextState_h