Add functions to alpha-blend two colors and pick the more readable of two colors. These will be used to make the UI more readable on non-default themes.

Also removes a function that no one was calling.

BUG=92, 18367
TEST=none
Review URL: http://codereview.chromium.org/190001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@25340 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 50 insertions, 4 deletions

diff --git a/app/gfx/color_utils.cc b/app/gfx/color_utils.cc
index 76c17cb..2cacd9b 100644
--- a/app/gfx/color_utils.cc
+++ b/app/gfx/color_utils.cc
@@ -250,8 +250,47 @@ void BuildLumaHistogram(SkBitmap* bitmap, int histogram[256]) {
   }
 }
 
-SkColor SetColorAlpha(SkColor c, SkAlpha alpha) {
-  return SkColorSetARGB(alpha, SkColorGetR(c), SkColorGetG(c), SkColorGetB(c));
+SkColor AlphaBlend(SkColor foreground, SkColor background, SkAlpha alpha) {
+  if (alpha == 0)
+    return background;
+  else if (alpha == 0xFF)
+    return foreground;
+
+  return SkColorSetRGB(
+    ((SkColorGetR(foreground) * alpha) +
+     (SkColorGetR(background) * (0xFF - alpha))) / 0xFF,
+    ((SkColorGetG(foreground) * alpha) +
+     (SkColorGetG(background) * (0xFF - alpha))) / 0xFF,
+    ((SkColorGetB(foreground) * alpha) +
+     (SkColorGetB(background) * (0xFF - alpha))) / 0xFF);
+}
+
+// Next three functions' formulas from:
+// http://www.w3.org/TR/WCAG20/#relativeluminancedef
+// http://www.w3.org/TR/WCAG20/#contrast-ratiodef
+static double ConvertSRGB(double eight_bit_component) {
+  const double component = eight_bit_component / 255.0;
+  return (component <= 0.03928) ?
+      (component / 12.92) : pow((component + 0.055) / 1.055, 2.4);
+}
+
+static double RelativeLuminance(SkColor color) {
+  return (0.2126 * ConvertSRGB(SkColorGetR(color))) +
+      (0.7152 * ConvertSRGB(SkColorGetG(color))) +
+      (0.0722 * ConvertSRGB(SkColorGetB(color)));
+}
+
+static double ContrastRatio(SkColor color1, SkColor color2) {
+  const double l1 = RelativeLuminance(color1) + 0.05;
+  const double l2 = RelativeLuminance(color2) + 0.05;
+  return (l1 > l2) ? (l1 / l2) : (l2 / l1);
+}
+
+SkColor PickMoreReadableColor(SkColor foreground1,
+                              SkColor foreground2,
+                              SkColor background) {
+  return (ContrastRatio(foreground1, background) >=
+      ContrastRatio(foreground2, background)) ? foreground1 : foreground2;
 }
 
 SkColor GetSysSkColor(int which) {
diff --git a/app/gfx/color_utils.h b/app/gfx/color_utils.h
index f28f003..878561b 100644
--- a/app/gfx/color_utils.h
+++ b/app/gfx/color_utils.h
@@ -52,8 +52,15 @@ SkColor GetAverageColorOfFavicon(SkBitmap* bitmap, SkAlpha alpha);
 // this image.
 void BuildLumaHistogram(SkBitmap* bitmap, int histogram[256]);
 
-// Create a color from a base color and a specific alpha value.
-SkColor SetColorAlpha(SkColor c, SkAlpha alpha);
+// Returns a blend of the supplied colors, ranging from |background| (for
+// |alpha| == 0) to |foreground| (for |alpha| == 255).
+SkColor AlphaBlend(SkColor foreground, SkColor background, SkAlpha alpha);
+
+// Given two possible foreground colors, return the one that is more readable
+// over |background|.
+SkColor PickMoreReadableColor(SkColor foreground1,
+                              SkColor foreground2,
+                              SkColor background);
 
 // Gets a Windows system color as a SkColor
 SkColor GetSysSkColor(int which);