UIDefaultsLoader: added contrast() color function (inspired by Less CSS), which is useful to choose a foreground color that is readable, based on the luma (perceptual brightness) of a background color

flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java

@@ -611,6 +611,7 @@ class UIDefaultsLoader
 				case "mix":				return parseColorMix( null, params, resolver, reportError );
 				case "tint":			return parseColorMix( "#fff", params, resolver, reportError );
 				case "shade":			return parseColorMix( "#000", params, resolver, reportError );
+				case "contrast":		return parseColorContrast( params, resolver, reportError );
 			}
 		} finally {
 			parseColorDepth--;
@@ -804,14 +805,10 @@ class UIDefaultsLoader
 		if( color1Str == null )
 			color1Str = params.get( i++ );
 		String color2Str = params.get( i++ );
-		int weight = 50;
-
-		if( params.size() > i )
-			weight = parsePercentage( params.get( i++ ) );
+		int weight = (params.size() > i) ? parsePercentage( params.get( i ) ) : 50;
 
 		// parse second color
-		String resolvedColor2Str = resolver.apply( color2Str );
-		ColorUIResource color2 = (ColorUIResource) parseColorOrFunction( resolvedColor2Str, resolver, reportError );
+		ColorUIResource color2 = (ColorUIResource) parseColorOrFunction( resolver.apply( color2Str ), resolver, reportError );
 		if( color2 == null )
 			return null;
 
@@ -822,6 +819,34 @@ class UIDefaultsLoader
 		return parseFunctionBaseColor( color1Str, function, false, resolver, reportError );
 	}
 
+	/**
+	 * Syntax: contrast(color,dark,light[,threshold])
+	 *   - color: a color to compare against
+	 *   - dark: a designated dark color (e.g. #000) or a color function
+	 *   - light: a designated light color (e.g. #fff) or a color function
+	 *   - threshold: the threshold (in range 0-100%) to specify where the transition
+	 *                from "dark" to "light" is (default is 43%)
+	 */
+	private static Object parseColorContrast( List<String> params, Function<String, String> resolver, boolean reportError ) {
+		String colorStr = params.get( 0 );
+		String darkStr = params.get( 1 );
+		String lightStr = params.get( 2 );
+		int threshold = (params.size() > 3) ? parsePercentage( params.get( 3 ) ) : 43;
+
+		// parse color to compare against
+		ColorUIResource color = (ColorUIResource) parseColorOrFunction( resolver.apply( colorStr ), resolver, reportError );
+		if( color == null )
+			return null;
+
+		// check luma and determine whether to use dark or light color
+		String darkOrLightColor = (ColorFunctions.luma( color ) * 100 < threshold)
+			? lightStr
+			: darkStr;
+
+		// parse dark or light color
+		return parseColorOrFunction( resolver.apply( darkOrLightColor ), resolver, reportError );
+	}
+
 	private static Object parseFunctionBaseColor( String colorStr, ColorFunction function,
 		boolean derived, Function<String, String> resolver, boolean reportError )
 	{

flatlaf-core/src/main/java/com/formdev/flatlaf/util/ColorFunctions.java

@@ -79,6 +79,34 @@ public class ColorFunctions
 			Math.round( a2 + ((a1 - a2) * weight) ) );
 	}
 
+	/**
+	 * Calculates the luma (perceptual brightness) of the given color.
+	 * <p>
+	 * Uses SMPTE C / Rec. 709 coefficients, as recommended in
+	 * <a href="https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef">WCAG 2.0</a>.
+	 *
+	 * @param color a color
+	 * @return the luma (in range 0-1)
+	 *
+	 * @see <a href="https://en.wikipedia.org/wiki/Luma_(video)">https://en.wikipedia.org/wiki/Luma_(video)</a>
+	 * @since 1.6
+	 */
+	public static float luma( Color color ) {
+		// see https://en.wikipedia.org/wiki/Luma_(video)
+		// see https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
+		// see https://github.com/less/less.js/blob/master/packages/less/src/less/tree/color.js
+		float r = gammaCorrection( color.getRed() / 255f );
+		float g = gammaCorrection( color.getGreen() / 255f );
+		float b = gammaCorrection( color.getBlue() / 255f );
+		return (0.2126f * r) + (0.7152f * g) + (0.0722f * b);
+	}
+
+	private static float gammaCorrection( float value ) {
+		return (value <= 0.03928f)
+			? value / 12.92f
+			: (float) Math.pow( (value + 0.055) / 1.055, 2.4 );
+	}
+
 	//---- interface ColorFunction --------------------------------------------
 
 	public interface ColorFunction {

flatlaf-core/src/test/java/com/formdev/flatlaf/util/TestColorFunctions.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2021 FormDev Software GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.formdev.flatlaf.util;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import java.awt.Color;
+import org.junit.jupiter.api.Test;
+
+/**
+ * @author Karl Tauber
+ */
+public class TestColorFunctions
+{
+	@Test
+	void luma() {
+		assertEquals( 0, ColorFunctions.luma( Color.black ) );
+		assertEquals( 1, ColorFunctions.luma( Color.white ) );
+
+		assertEquals( 0.2126f, ColorFunctions.luma( Color.red ) );
+		assertEquals( 0.7152f, ColorFunctions.luma( Color.green ) );
+		assertEquals( 0.0722f, ColorFunctions.luma( Color.blue ) );
+
+		assertEquals( 0.9278f, ColorFunctions.luma( Color.yellow ) );
+		assertEquals( 0.7874f, ColorFunctions.luma( Color.cyan ) );
+
+		assertEquals( 0.051269464f, ColorFunctions.luma( Color.darkGray ) );
+		assertEquals( 0.21586052f, ColorFunctions.luma( Color.gray ) );
+		assertEquals( 0.52711517f, ColorFunctions.luma( Color.lightGray ) );
+	}
+}