Add luminance getter to Color (flutter#4252)

* add luminance getter to color

* tests

* nit

lib/ui/painting.dart

@@ -200,6 +200,27 @@ class Color {
     return new Color.fromARGB(alpha, red, green, b);
   }
 
+  // See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
+  static double _linearizeColorComponent(double component) {
+    if (component <= 0.03928)
+      return component / 12.92;
+    return math.pow((component + 0.055) / 1.055, 2.4);
+  }
+
+  /// Returns a brightness value between 0 for darkest and 1 for lightest.
+  ///
+  /// Represents the relative luminance of the color. This value is computationally
+  /// expensive to calculate.
+  ///
+  /// See <https://en.wikipedia.org/wiki/Relative_luminance>.
+  double computeLuminance() {
+    // See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
+    final double R = _linearizeColorComponent(red / 0xFF);
+    final double G = _linearizeColorComponent(green / 0xFF);
+    final double B = _linearizeColorComponent(blue / 0xFF);
+    return 0.2126 * R + 0.7152 * G + 0.0722 * B;
+  }
+
   /// Linearly interpolate between two colors.
   ///
   /// If either color is null, this function linearly interpolates from a

testing/dart/color_test.dart

@@ -77,4 +77,20 @@ void main() {
       const Color(0xFFFFFFFF),
     );
   });
+
+  test('compute gray luminance', () {
+    // Each color component is at 20%.
+    final Color lightGray = const Color(0xFF333333);
+    // Relative luminance's formula is just the linearized color value for gray.
+    // ((0.2 + 0.055) / 1.055) ^ 2.4.
+    expect(lightGray.computeLuminance(), equals(0.033104766570885055));
+  });
+
+  test('compute color luminance', () {
+    final Color brightRed = const Color(0xFFFF3B30);
+    // 0.2126 * ((1.0 + 0.055) / 1.055) ^ 2.4 +
+    // 0.7152 * ((0.23137254902 +0.055) / 1.055) ^ 2.4 +
+    // 0.0722 * ((0.18823529411 + 0.055) / 1.055) ^ 2.4
+    expect(brightRed.computeLuminance(), equals(0.24601329637099723));
+  });
 }