Use color classifier's brightness algorithm for dark theme detection.

This is a follow-up to a comment in https://crrev.com/c/1637036 about
moving IsLight() to within DarkModeColorClassifier().

This change removes IsLight() and makes CalculateColorBrightness() from
the .cc file publicly available via a static method in
DarkModeColorClassifier.

Change-Id: I71497cbd4d6776134debade82960df7af01b1157
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1755054Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Aran Gilman <gilmanmh@google.com>
Cr-Commit-Position: refs/heads/master@{#687368}

third_party/blink/renderer/core/accessibility/apply_dark_mode.cc

@@ -16,20 +16,27 @@
 namespace blink {
 namespace {
 
+const int kAlphaThreshold = 100;
+const int kBrightnessThreshold = 128;
+
 // TODO(https://crbug.com/925949): Add detection and classification of
 // background image color. Most sites with dark background images also have a
 // dark background color set, so this is less of a priority than it would be
 // otherwise.
 bool HasLightBackground(const LayoutView& root) {
   const ComputedStyle& style = root.StyleRef();
-  if (style.HasBackground()) {
-    Color color = style.VisitedDependentColor(GetCSSPropertyBackgroundColor());
-    return IsLight(color);
-  }
 
   // If we can't easily determine the background color, default to inverting the
   // page.
-  return true;
+  if (!style.HasBackground())
+    return true;
+
+  Color color = style.VisitedDependentColor(GetCSSPropertyBackgroundColor());
+  if (color.Alpha() < kAlphaThreshold)
+    return true;
+
+  return DarkModeColorClassifier::CalculateColorBrightness(color) >
+         kBrightnessThreshold;
 }
 
 DarkMode GetMode(const Settings& frame_settings) {

third_party/blink/renderer/platform/graphics/dark_mode_color_classifier.cc

@@ -9,20 +9,6 @@
 namespace blink {
 namespace {
 
-// Determine perceived brightness of a color.
-//
-// Based on this algorithm suggested by the W3:
-// https://www.w3.org/TR/AERT/#color-contrast
-//
-// We don't use HSL or HSV here because perceived brightness is a function of
-// hue as well as lightness/value.
-int CalculateColorBrightness(const Color& color) {
-  int weighted_red = color.Red() * 299;
-  int weighted_green = color.Green() * 587;
-  int weighted_blue = color.Blue() * 114;
-  return (weighted_red + weighted_green + weighted_blue) / 1000;
-}
-
 class SimpleColorClassifier : public DarkModeColorClassifier {
  public:
   static std::unique_ptr<SimpleColorClassifier> NeverInvert() {
@@ -83,25 +69,16 @@ class InvertHighBrightnessColorsClassifier : public DarkModeColorClassifier {
 
 }  // namespace
 
-// Values below which a color is considered sufficiently transparent that a
-// lighter color behind it would make the final color as seen by the user light.
-// TODO(https://crbug.com/925949): This is a placeholder value. It should be
-// replaced with a better researched value before launching dark mode.
-const float kOpacityThreshold = 0.4;
-
-// TODO(https://crbug.com/925949): Find a better algorithm for this.
-bool IsLight(const Color& color) {
-  // Use the alpha value as the opacity.
-  float opacity = (color.Alpha() / 255);
-  // Assume the color is light if the background is sufficiently transparent.
-  if (opacity < kOpacityThreshold) {
-    return true;
-  }
-  double hue;
-  double saturation;
-  double lightness;
-  color.GetHSL(hue, saturation, lightness);
-  return lightness > 0.5;
+// Based on this algorithm suggested by the W3:
+// https://www.w3.org/TR/AERT/#color-contrast
+//
+// We don't use HSL or HSV here because perceived brightness is a function of
+// hue as well as lightness/value.
+int DarkModeColorClassifier::CalculateColorBrightness(const Color& color) {
+  int weighted_red = color.Red() * 299;
+  int weighted_green = color.Green() * 587;
+  int weighted_blue = color.Blue() * 114;
+  return (weighted_red + weighted_green + weighted_blue) / 1000;
 }
 
 std::unique_ptr<DarkModeColorClassifier>

third_party/blink/renderer/platform/graphics/dark_mode_color_classifier.h

@@ -14,10 +14,11 @@
 
 namespace blink {
 
-bool PLATFORM_EXPORT IsLight(const Color& color);
-
 class PLATFORM_EXPORT DarkModeColorClassifier {
  public:
+  // Determine perceived brightness of a color.
+  static int CalculateColorBrightness(const Color& color);
+
   static std::unique_ptr<DarkModeColorClassifier> MakeTextColorClassifier(
       const DarkModeSettings& settings);
   static std::unique_ptr<DarkModeColorClassifier> MakeBackgroundColorClassifier(