Estimate the brightness of the primary color (#10071)

If the caller doesn't explicitly give the brightness of the primary
color, we now estimate it using an algorithm from the Web Content
Accessibility Guidelines.

Also, this patch contains a function that converts RGB colors to
HSVColors. I was originally going to use that, but the WCAG algorithm
ended up seeming like a better choice. The patch still includes this
function because it's generally useful.

Fixes #5718

packages/flutter/lib/src/material/theme_data.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
 import 'dart:ui' show Color, hashValues;
 
 import 'package:flutter/foundation.dart';
@@ -42,6 +43,32 @@ const Color _kLightThemeSplashColor = const Color(0x66C8C8C8);
 const Color _kDarkThemeHighlightColor = const Color(0x40CCCCCC);
 const Color _kDarkThemeSplashColor = const Color(0x40CCCCCC);
 
+// See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
+double _linearizeColorComponent(double component) {
+  if (component <= 0.03928)
+    return component / 12.92;
+  return math.pow((component + 0.055) / 1.055, 2.4);
+}
+
+Brightness _estimateBrightnessForColor(Color color) {
+  // See <https://www.w3.org/TR/WCAG20/#relativeluminancedef>
+  final double R = _linearizeColorComponent(color.red / 0xFF);
+  final double G = _linearizeColorComponent(color.green / 0xFF);
+  final double B = _linearizeColorComponent(color.blue / 0xFF);
+  final double L = 0.2126 * R + 0.7152 * G + 0.0722 * B;
+
+  // See <https://www.w3.org/TR/WCAG20/#contrast-ratiodef>
+  // The spec says to use kThreshold=0.0525, but Material Design appears to bias
+  // more towards using light text than WCAG20 recommends. Material Design spec
+  // doesn't say what value to use, but 0.15 seemed close to what the Material
+  // Design spec shows for its color palette on
+  // <https://material.io/guidelines/style/color.html#color-color-palette>.
+  const double kThreshold = 0.15;
+  if ((L + 0.05) * (L + 0.05) > kThreshold )
+    return Brightness.light;
+  return Brightness.dark;
+}
+
 /// Holds the color and typography values for a material design theme.
 ///
 /// Use this class to configure a [Theme] widget.
@@ -107,10 +134,10 @@ class ThemeData {
     final bool isDark = brightness == Brightness.dark;
     primarySwatch ??= Colors.blue;
     primaryColor ??= isDark ? Colors.grey[900] : primarySwatch[500];
-    primaryColorBrightness ??= Brightness.dark;
+    primaryColorBrightness ??= _estimateBrightnessForColor(primaryColor);
     final bool primaryIsDark = primaryColorBrightness == Brightness.dark;
     accentColor ??= isDark ? Colors.tealAccent[200] : primarySwatch[500];
-    accentColorBrightness ??= isDark ? Brightness.light : Brightness.dark;
+    accentColorBrightness ??= _estimateBrightnessForColor(accentColor);
     final bool accentIsDark = accentColorBrightness == Brightness.dark;
     canvasColor ??= isDark ? Colors.grey[850] : Colors.grey[50];
     scaffoldBackgroundColor ??= canvasColor;

packages/flutter/lib/src/painting/colors.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
 import 'dart:ui' show Color, lerpDouble, hashValues;
 
 import 'package:flutter/foundation.dart';
@@ -23,6 +24,37 @@ class HSVColor {
         assert(saturation != null),
         assert(value != null);
 
+  /// Creates an [HSVColor] from an RGB [Color].
+  ///
+  /// This constructor does not necessarily round-trip with [toColor] because
+  /// of floating point imprecision.
+  factory HSVColor.fromColor(Color color) {
+    final double alpha = color.alpha / 0xFF;
+    final double red = color.red / 0xFF;
+    final double green = color.green / 0xFF;
+    final double blue = color.blue / 0xFF;
+
+    final double max = math.max(red, math.max(green, blue));
+    final double min = math.min(red, math.min(green, blue));
+    final double delta = max - min;
+
+    double hue = 0.0;
+
+    if (max == 0.0) {
+      hue = 0.0;
+    } else if (max == red) {
+      hue = 60.0 * (((green - blue) / delta) % 6);
+    } else if (max == green) {
+      hue = 60.0 * (((blue - red) / delta) + 2);
+    } else if (max == blue) {
+      hue = 60.0 * (((red - green) / delta) + 4);
+    }
+
+    final double saturation = max == 0.0 ? 0.0 : delta / max;
+
+    return new HSVColor.fromAHSV(alpha, hue, saturation, max);
+  }
+
   /// Alpha, from 0.0 to 1.0.
   final double alpha;
 

packages/flutter/test/material/theme_data_test.dart

@@ -89,4 +89,17 @@ void main() {
     expect(themeData.primaryTextTheme.title.fontFamily, equals('Ahem'));
     expect(themeData.accentTextTheme.display4.fontFamily, equals('Ahem'));
   });
+
+  test('Can estimate brightness', () {
+    expect(new ThemeData(primaryColor: Colors.white).primaryColorBrightness, equals(Brightness.light));
+    expect(new ThemeData(primaryColor: Colors.black).primaryColorBrightness, equals(Brightness.dark));
+    expect(new ThemeData(primaryColor: Colors.blue).primaryColorBrightness, equals(Brightness.dark));
+    expect(new ThemeData(primaryColor: Colors.yellow).primaryColorBrightness, equals(Brightness.light));
+    expect(new ThemeData(primaryColor: Colors.deepOrange).primaryColorBrightness, equals(Brightness.dark));
+    expect(new ThemeData(primaryColor: Colors.orange).primaryColorBrightness, equals(Brightness.light));
+    expect(new ThemeData(primaryColor: Colors.lime).primaryColorBrightness, equals(Brightness.light));
+    expect(new ThemeData(primaryColor: Colors.grey).primaryColorBrightness, equals(Brightness.light));
+    expect(new ThemeData(primaryColor: Colors.teal).primaryColorBrightness, equals(Brightness.dark));
+    expect(new ThemeData(primaryColor: Colors.indigo).primaryColorBrightness, equals(Brightness.dark));
+  });
 }

packages/flutter/test/painting/colors_test.dart

@@ -26,4 +26,16 @@ void main() {
     expect(result.saturation, lessThan(0.4001));
     expect(result.value, 0.5);
   });
+
+  test('HSVColor.fromColor test', () {
+    final HSVColor black = new HSVColor.fromColor(const Color.fromARGB(0xFF, 0x00, 0x00, 0x00));
+    final HSVColor red = new HSVColor.fromColor(const Color.fromARGB(0xFF, 0xFF, 0x00, 0x00));
+    final HSVColor green = new HSVColor.fromColor(const Color.fromARGB(0xFF, 0x00, 0xFF, 0x00));
+    final HSVColor blue = new HSVColor.fromColor(const Color.fromARGB(0xFF, 0x00, 0x00, 0xFF));
+
+    expect(black.toColor(), equals(const Color.fromARGB(0xFF, 0x00, 0x00, 0x00)));
+    expect(red.toColor(), equals(const Color.fromARGB(0xFF, 0xFF, 0x00, 0x00)));
+    expect(green.toColor(), equals(const Color.fromARGB(0xFF, 0x00, 0xFF, 0x00)));
+    expect(blue.toColor(), equals(const Color.fromARGB(0xFF, 0x00, 0x00, 0xFF)));
+  });
 }