Add ImageBanner color distance calculation

Support CIE94 color distance calculations for improved ImageBanner
rendering. Based on code originally developed by Michael Simons and
added with his permission.

See gh-4647

spring-boot/src/main/java/org/springframework/boot/ImageBanner.java

@@ -18,6 +18,7 @@ package org.springframework.boot;
 
 import java.awt.Color;
 import java.awt.Image;
+import java.awt.color.ColorSpace;
 
 import java.awt.image.BufferedImage;
 
@@ -214,15 +215,6 @@ public class ImageBanner implements Banner {
 		}
 	}
 
-	private static double getColorDistance(Color color1, Color color2) {
-		double redDelta = (color1.getRed() - color2.getRed()) * RED_WEIGHT;
-		double greenDelta = (color1.getGreen() - color2.getGreen()) * GREEN_WEIGHT;
-		double blueDelta = (color1.getBlue() - color2.getBlue()) * BLUE_WEIGHT;
-
-		return Math.pow(redDelta, 2.0d) + Math.pow(greenDelta, 2.0d)
-				+ Math.pow(blueDelta, 2.0d);
-	}
-
 	private static BufferedImage resizeImage(BufferedImage sourceImage, int maxWidth,
 			double aspectRatio) {
 		int width;
@@ -246,4 +238,79 @@ public class ImageBanner implements Banner {
 		resizedImage.getGraphics().drawImage(image, 0, 0, null);
 		return resizedImage;
 	}
+
+	/**
+	 * Computes the CIE94 distance between two colors.
+	 *
+	 * Contributed by michael-simons
+	 * (original implementation https://github.com/michael-simons/dfx-mosaic/blob/public/src/main/java/de/dailyfratze/mosaic/images/CIE94ColorDistance.java)
+	 *
+	 * @param color1 the first color
+	 * @param color2 the second color
+	 * @return the distance between the colors
+	 */
+	private static double getColorDistance(final Color color1, final Color color2) {
+		// Convert to L*a*b* color space
+		float[] lab1 = toLab(color1);
+		float[] lab2 = toLab(color2);
+
+		// Make it more readable
+		double L1 = lab1[0];
+		double a1 = lab1[1];
+		double b1 = lab1[2];
+		double L2 = lab2[0];
+		double a2 = lab2[1];
+		double b2 = lab2[2];
+
+		// CIE94 coefficients for graphic arts
+		double kL = 1;
+		double K1 = 0.045;
+		double K2 = 0.015;
+		// Weighting factors
+		double sl = 1.0;
+		double kc = 1.0;
+		double kh = 1.0;
+
+		// See http://en.wikipedia.org/wiki/Color_difference#CIE94
+		double c1 = Math.sqrt(a1 * a1 + b1 * b1);
+		double deltaC = c1 - Math.sqrt(a2 * a2 + b2 * b2);
+		double deltaA = a1 - a2;
+		double deltaB = b1 - b2;
+		double deltaH = Math.sqrt(Math.max(0.0, deltaA * deltaA + deltaB * deltaB - deltaC * deltaC));
+
+		return Math.sqrt(Math.max(0.0, Math.pow((L1 - L2) / (kL * sl), 2) + Math.pow(deltaC / (kc * (1 + K1 * c1)), 2) + Math.pow(deltaH / (kh * (1 + K2 * c1)), 2.0)));
+	}
+
+	/**
+	 * Returns the CIE L*a*b* values of this color.
+	 *
+	 * Implements the forward transformation described in
+	 * https://en.wikipedia.org/wiki/Lab_color_space
+	 *
+	 * @param color the color to convert
+	 * @return the xyz color components
+	 */
+	static float[] toLab(Color color) {
+		float[] xyz = color.getColorComponents(
+				ColorSpace.getInstance(ColorSpace.CS_CIEXYZ), null);
+
+		return xyzToLab(xyz);
+	}
+
+	static float[] xyzToLab(float[] colorvalue) {
+		double l = f(colorvalue[1]);
+		double L = 116.0 * l - 16.0;
+		double a = 500.0 * (f(colorvalue[0]) - l);
+		double b = 200.0 * (l - f(colorvalue[2]));
+		return new float[]{(float) L, (float) a, (float) b};
+	}
+
+	private static double f(double t) {
+		if (t > 216.0 / 24389.0) {
+			return Math.cbrt(t);
+		}
+		else {
+			return (1.0 / 3.0) * Math.pow(29.0 / 6.0, 2) * t + (4.0 / 29.0);
+		}
+	}
 }