Make SVG painting independent of GraphicsContext's alpha state

Source/platform/graphics/GraphicsContextState.h

Index: Source/platform/graphics/GraphicsContextState.h
diff --git a/Source/platform/graphics/GraphicsContextState.h b/Source/platform/graphics/GraphicsContextState.h
index 4417d4f218ef4b4521815ff27ee5e9a5c29e1902..b8bcd778f3c5298911a40a417512a1e27798825f 100644
--- a/Source/platform/graphics/GraphicsContextState.h
+++ b/Source/platform/graphics/GraphicsContextState.h
@@ -74,11 +74,11 @@ public:
     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; }
@@ -95,11 +95,11 @@ public:
     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
@@ -145,17 +145,42 @@ public:
     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
+    {
+        return combineWithAlpha(color, m_alpha);
+    }
+
+    // Map alpha values from [0, 1] to [0, 256] for alpha blending.
+    static int clampedAlphaForBlending(float alpha)
     {
-        int a = SkAlphaMul(SkColorGetA(c), m_alpha);
-        return (c & 0x00FFFFFF) | (a << 24);
+        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.