update color brightness calculation

application/src/main/java/org/opentripplanner/gtfs/graphbuilder/GtfsModule.java

@@ -1,5 +1,7 @@
 package org.opentripplanner.gtfs.graphbuilder;
 
+import static org.opentripplanner.utils.color.ColorUtils.computeBrightness;
+
 import java.awt.Color;
 import java.io.IOException;
 import java.io.Serializable;
@@ -52,6 +54,7 @@
 import org.opentripplanner.standalone.config.BuildConfig;
 import org.opentripplanner.transit.model.framework.FeedScopedId;
 import org.opentripplanner.transit.service.TimetableRepository;
+import org.opentripplanner.utils.color.Brightness;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -415,8 +418,7 @@ private boolean skipEntityClass(Class<?> entityClass) {
    * If a route doesn't have color or already has routeColor and routeTextColor nothing is done.
    * <p>
    * textColor can be black or white. White for dark colors and black for light colors of
-   * routeColor. If color is light or dark is calculated based on luminance formula: sqrt(
-   * 0.299*Red^2 + 0.587*Green^2 + 0.114*Blue^2 )
+   * routeColor.
    */
   private void generateRouteColor(Route route) {
     String routeColor = route.getColor();
@@ -431,16 +433,7 @@ private void generateRouteColor(Route route) {
     }
 
     Color routeColorColor = Color.decode("#" + routeColor);
-    //gets float of RED, GREEN, BLUE in range 0...1
-    float[] colorComponents = routeColorColor.getRGBColorComponents(null);
-    //Calculates luminance based on https://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
-    double newRed = 0.299 * Math.pow(colorComponents[0], 2.0);
-    double newGreen = 0.587 * Math.pow(colorComponents[1], 2.0);
-    double newBlue = 0.114 * Math.pow(colorComponents[2], 2.0);
-    double luminance = Math.sqrt(newRed + newGreen + newBlue);
-
-    //For brighter colors use black text color and reverse for darker
-    if (luminance > 0.5) {
+    if (computeBrightness(routeColorColor) == Brightness.LIGHT) {
       textColor = "000000";
     } else {
       textColor = "FFFFFF";

utils/src/main/java/org/opentripplanner/utils/color/Brightness.java

@@ -0,0 +1,6 @@
+package org.opentripplanner.utils.color;
+
+public enum Brightness {
+  DARK,
+  LIGHT,
+}

utils/src/main/java/org/opentripplanner/utils/color/ColorUtils.java

@@ -0,0 +1,44 @@
+package org.opentripplanner.utils.color;
+
+import java.awt.Color;
+
+public final class ColorUtils {
+
+  private ColorUtils() {}
+
+  /**
+   * Calculates luminance according to
+   * <a href="https://www.w3.org/TR/WCAG21/#dfn-relative-luminance">W3C Recommendation</a>
+   */
+  public static double computeLuminance(Color color) {
+    //gets float of RED, GREEN, BLUE in range 0...1
+    float[] colorComponents = color.getRGBColorComponents(null);
+    //Calculates luminance based on https://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
+    double r = linearizeColorComponent(colorComponents[0]);
+    double g = linearizeColorComponent(colorComponents[1]);
+    double b = linearizeColorComponent(colorComponents[2]);
+    return 0.2126 * r + 0.7152 * g + 0.0722 * b;
+  }
+
+  private static double linearizeColorComponent(double srgb) {
+    return srgb <= 0.04045 ? srgb / 12.92 : Math.pow((srgb + 0.055) / 1.055, 2.4);
+  }
+
+  /**
+   * Determine if a color is light or dark
+   * <p>
+   * A light color is a color where the contrast ratio with black is larger than with white.
+   * <p>
+   * The contrast ratio is defined per Web Content Accessibility Guidelines (WCAG) 2.1.
+   */
+  public static Brightness computeBrightness(Color color) {
+    // The contrast ratio between two colors is defined as (L1 + 0.05) / (L2 + 0.05)
+    // where L1 is the lighter of the two colors.
+    //
+    // Therefore, the contrast ratio with black is (L + 0.05) / 0.05 and the contrast ratio with
+    // white is 1.05 / (L + 0.05)
+    //
+    // Solving (L + 0.05) / 0.05 > 1.05 / (L + 0.05) gets L > 0.179
+    return computeLuminance(color) > 0.179 ? Brightness.LIGHT : Brightness.DARK;
+  }
+}

utils/src/test/java/org/opentripplanner/utils/color/ColorUtilsTest.java

@@ -0,0 +1,31 @@
+package org.opentripplanner.utils.color;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.params.provider.Arguments.arguments;
+import static org.opentripplanner.utils.color.ColorUtils.computeBrightness;
+
+import java.awt.Color;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.FieldSource;
+
+public class ColorUtilsTest {
+
+  static final Arguments[] brightnessExpectations = {
+    arguments(Color.black, Brightness.DARK),
+    arguments(Color.green, Brightness.LIGHT),
+    arguments(Color.blue, Brightness.DARK),
+    arguments(Color.red, Brightness.LIGHT),
+    arguments(Color.yellow, Brightness.LIGHT),
+    arguments(Color.white, Brightness.LIGHT),
+    arguments(Color.pink, Brightness.LIGHT),
+    arguments(Color.orange, Brightness.LIGHT),
+    arguments(Color.cyan, Brightness.LIGHT),
+  };
+
+  @ParameterizedTest
+  @FieldSource("brightnessExpectations")
+  void testBrightness(Color color, Brightness brightness) {
+    assertEquals(computeBrightness(color), brightness);
+  }
+}