Bug 1316869 - Choose black or white title text color for custom tabs toolbar based on background color. r=sebastian

mobile/android/base/java/org/mozilla/gecko/customtabs/CustomTabsActivity.java

@@ -164,7 +164,10 @@ public class CustomTabsActivity extends GeckoApp implements Tabs.OnTabsChangedLi
             toolbarColor = color;
         }
 
+        final int titleTextColor = ColorUtil.getReadableTextColor(toolbarColor);
+
         toolbar.setBackgroundColor(toolbarColor);
+        toolbar.setTitleTextColor(titleTextColor);
         final Window window = getWindow();
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
             window.addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS);

mobile/android/base/java/org/mozilla/gecko/util/ColorUtil.java

@@ -19,7 +19,26 @@ public class ColorUtil {
         return Color.argb(alpha, red, green, blue);
     }
 
+    public static int getReadableTextColor(final int backgroundColor) {
+        final int greyValue = grayscaleFromRGB(backgroundColor);
+        // 186 chosen rather than the seemingly obvious 128 because of gamma.
+        if (greyValue < 186) {
+            return Color.WHITE;
+        } else {
+            return Color.BLACK;
+        }
+    }
+
     private static int darkenColor(final int color, final double fraction) {
         return (int) Math.max(color - (color * fraction), 0);
     }
+
+    private static int grayscaleFromRGB(final int color) {
+        final int red = Color.red(color);
+        final int green = Color.green(color);
+        final int blue = Color.blue(color);
+        // Magic weighting taken from a stackoverflow post, supposedly related to how
+        // humans perceive color.
+        return (int) (0.299*red + 0.587*green + 0.114*blue);
+    }
 }