extend SkShader to report a luminance-color to be used for gamma correction

src/effects/gradients/SkGradientShader.cpp

Index: src/effects/gradients/SkGradientShader.cpp
diff --git a/src/effects/gradients/SkGradientShader.cpp b/src/effects/gradients/SkGradientShader.cpp
index b14a3f7e4fb9ae6972b0f1ccc6c3be24b86d8264..791d34dcb79931d2cdf53b73628125754fac55e7 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -326,6 +326,28 @@ bool SkGradientShaderBase::isOpaque() const {
     return fColorsAreOpaque;
 }
 
+static unsigned rounded_divide(unsigned numer, unsigned denom) {
+    return (numer + (denom >> 1)) / denom;
+}
+
+bool SkGradientShaderBase::onAsLuminanceColor(SkColor* lum) const {
+    // we just compute an average color.
+    // possibly we could weight this based on the proportional width for each color
+    //   assuming they are not evenly distributed in the fPos array.
+    int r = 0;
+    int g = 0;
+    int b = 0;
+    const int n = fColorCount;
+    for (int i = 0; i < n; ++i) {

[tomhudson, 2014/08/29 22:52:43]

Seems like this works well for the trivial case (2 colors, or equidistant color
samples), but would need weighting to look representative for several
non-uniformly-spaced color stops?

[reed1, 2014/09/02 13:52:55]

Agreed. I have a comment to that effect just above.

+        SkColor c = fOrigColors[i];
+        r += SkColorGetR(c);
+        g += SkColorGetG(c);
+        b += SkColorGetB(c);
+    }
+    *lum = SkColorSetRGB(rounded_divide(r, n), rounded_divide(g, n), rounded_divide(b, n));
+    return true;
+}
+
 SkGradientShaderBase::GradientShaderBaseContext::GradientShaderBaseContext(
         const SkGradientShaderBase& shader, const ContextRec& rec)
     : INHERITED(shader, rec)