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 21bf8858b54d7fb0b3f4edf8fe4d8504be3c2963..24dd8ab57298a4622b266e879110a2b4149486f0 100644
--- a/src/effects/gradients/SkGradientShader.cpp
+++ b/src/effects/gradients/SkGradientShader.cpp
@@ -251,6 +251,28 @@ bool SkGradientShaderBase::isOpaque() const {
     return fColorsAreOpaque;
 }
 
+static unsigned rndivbyte(unsigned numer, unsigned denom) {

[bungeman-skia, 2014/08/21 14:15:43]

rounded_divide?

It isn't totally clear that the 'd' is part of 'rnd' (which also suggests
'random') and 'div'. Not sure of a better way of getting 'assert is byte, but
returned unsigned' into the name and also short enough to keep your one liner
below.

[reed1, 2014/08/21 20:07:39]

Done.

+    unsigned result = (numer + (denom >> 1)) / denom;
+    SkASSERT(SkToU8(result) == result);
+    return result;
+}
+
+bool SkGradientShaderBase::onAsLuminanceColor(SkColor* lum) const {
+    // we just compute an average color
+    int r = 0;
+    int g = 0;
+    int b = 0;
+    const int n = fColorCount;
+    for (int i = 0; i < n; ++i) {
+        SkColor c = fOrigColors[i];
+        r += SkColorGetR(c);
+        g += SkColorGetG(c);
+        b += SkColorGetB(c);
+    }
+    *lum = SkColorSetRGB(rndivbyte(r, n),rndivbyte(g, n), rndivbyte(b, n));
+    return true;
+}
+
 SkGradientShaderBase::GradientShaderBaseContext::GradientShaderBaseContext(
         const SkGradientShaderBase& shader, const ContextRec& rec)
     : INHERITED(shader, rec)