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;
+		int weight = (params.size() > i) ? parsePercentage( params.get( i ) ) : 50;
-
-		if( params.size() > i )
-			weight = parsePercentage( params.get( i++ ) );
 
 		// parse second color
-		String resolvedColor2Str = resolver.apply( color2Str );
+		ColorUIResource color2 = (ColorUIResource) parseColorOrFunction( resolver.apply( color2Str ), resolver, reportError );
-		ColorUIResource color2 = (ColorUIResource) parseColorOrFunction( resolvedColor2Str, 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 ) );
+	}
+}

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatCompletionProvider.java

@@ -471,6 +471,12 @@ class FlatCompletionProvider
 			addFunction( "shade",
 				"color", colorParamDesc,
 				"weight", weightParamDesc );
+
+			addFunction( "contrast",
+				"color", colorParamDesc,
+				"dark", colorParamDesc,
+				"light", colorParamDesc,
+				"threshold", "(optional) 0-100%, default is 43%" );
 		}
 
 		private void addFunction( String name, String... paramNamesAndDescs ) {

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorOverlay.java

@@ -31,6 +31,7 @@ import org.fife.ui.rsyntaxtextarea.Token;
 import com.formdev.flatlaf.FlatLaf;
 import com.formdev.flatlaf.UIDefaultsLoaderAccessor;
 import com.formdev.flatlaf.ui.FlatUIUtils;
+import com.formdev.flatlaf.util.ColorFunctions;
 import com.formdev.flatlaf.util.HSLColor;
 import com.formdev.flatlaf.util.UIScale;
 
@@ -46,6 +47,7 @@ class FlatThemeEditorOverlay
 
 	static boolean showHSL = true;
 	static boolean showRGB;
+	static boolean showLuma;
 
 	private Font font;
 	private Font baseFont;
@@ -80,19 +82,21 @@ class FlatThemeEditorOverlay
 		}
 
 		FontMetrics fm = c.getFontMetrics( font );
+		int space = fm.stringWidth( "  " );
 		int maxTextWidth = 0;
 		if( showHSL )
-			maxTextWidth += fm.stringWidth( "HSL 360 100 100" );
+			maxTextWidth += fm.stringWidth( "HSL 360 100 100" ) + space;
 		if( showRGB )
-			maxTextWidth += fm.stringWidth( "#ffffff" );
+			maxTextWidth += fm.stringWidth( "#ffffff" ) + space;
-		if( showHSL && showRGB )
+		if( showLuma )
-			maxTextWidth += fm.stringWidth( "  " );
+			maxTextWidth += fm.stringWidth( "100" ) + space;
+		maxTextWidth = Math.max( maxTextWidth - space, 0 );
 		int textHeight = fm.getAscent() - fm.getLeading();
 
 		int width = c.getWidth();
 		int previewWidth = UIScale.scale( COLOR_PREVIEW_WIDTH );
 		int gap = UIScale.scale( 4 );
-		int textGap = (showHSL || showRGB) ? UIScale.scale( 6 ) : 0;
+		int textGap = (showHSL || showRGB || showLuma) ? UIScale.scale( 6 ) : 0;
 
 		// check whether preview is outside of clip bounds
 		if( clipBounds.x + clipBounds.width < width - previewWidth - maxTextWidth - gap - textGap )
@@ -123,7 +127,7 @@ class FlatThemeEditorOverlay
 				}
 
 				// paint text
-				if( showHSL || showRGB ) {
+				if( showHSL || showRGB || showLuma ) {
 					int textX = px - textGap - maxTextWidth;
 					if( textX > r.x + gap) {
 						String colorStr = null;
@@ -141,6 +145,14 @@ class FlatThemeEditorOverlay
 							else
 								colorStr = rgbStr;
 						}
+						if( showLuma ) {
+							String lumaStr = String.format( "%3d",
+								Math.round( ColorFunctions.luma( color ) * 100 ) );
+							if( colorStr != null )
+								colorStr += "  " + lumaStr;
+							else
+								colorStr = lumaStr;
+						}
 
 						int textWidth = fm.stringWidth( colorStr );
 						if( textWidth > maxTextWidth )

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.java

@@ -83,8 +83,9 @@ class FlatThemeFileEditor
 	private static final String KEY_PREVIEW = "preview";
 	private static final String KEY_LAF = "laf";
 	private static final String KEY_FONT_SIZE_INCR = "fontSizeIncr";
-	private static final String KEY_HSL_COLORS = "hslColors";
+	private static final String KEY_SHOW_HSL_COLORS = "showHslColors";
-	private static final String KEY_RGB_COLORS = "rgbColors";
+	private static final String KEY_SHOW_RGB_COLORS = "showRgbColors";
+	private static final String KEY_SHOW_COLOR_LUMA = "showColorLuma";
 
 	private File dir;
 	private Preferences state;
@@ -701,9 +702,11 @@ class FlatThemeFileEditor
 	private void colorModelChanged() {
 		FlatThemeEditorOverlay.showHSL = showHSLColorsMenuItem.isSelected();
 		FlatThemeEditorOverlay.showRGB = showRGBColorsMenuItem.isSelected();
+		FlatThemeEditorOverlay.showLuma = showColorLumaMenuItem.isSelected();
 
-		putPrefsBoolean( state, KEY_HSL_COLORS, FlatThemeEditorOverlay.showHSL, true );
+		putPrefsBoolean( state, KEY_SHOW_HSL_COLORS, FlatThemeEditorOverlay.showHSL, true );
-		putPrefsBoolean( state, KEY_RGB_COLORS, FlatThemeEditorOverlay.showRGB, false );
+		putPrefsBoolean( state, KEY_SHOW_RGB_COLORS, FlatThemeEditorOverlay.showRGB, false );
+		putPrefsBoolean( state, KEY_SHOW_COLOR_LUMA, FlatThemeEditorOverlay.showLuma, false );
 
 		repaint();
 	}
@@ -755,13 +758,15 @@ class FlatThemeFileEditor
 		directoryField.setModel( model );
 
 		// restore overlay color models
-		FlatThemeEditorOverlay.showHSL = state.getBoolean( KEY_HSL_COLORS, true );
+		FlatThemeEditorOverlay.showHSL = state.getBoolean( KEY_SHOW_HSL_COLORS, true );
-		FlatThemeEditorOverlay.showRGB = state.getBoolean( KEY_RGB_COLORS, false );
+		FlatThemeEditorOverlay.showRGB = state.getBoolean( KEY_SHOW_RGB_COLORS, false );
+		FlatThemeEditorOverlay.showLuma = state.getBoolean( KEY_SHOW_COLOR_LUMA, false );
 
 		// restore menu item selection
 		previewMenuItem.setSelected( state.getBoolean( KEY_PREVIEW, true ) );
 		showHSLColorsMenuItem.setSelected( FlatThemeEditorOverlay.showHSL );
 		showRGBColorsMenuItem.setSelected( FlatThemeEditorOverlay.showRGB );
+		showColorLumaMenuItem.setSelected( FlatThemeEditorOverlay.showLuma );
 	}
 
 	private void saveState() {
@@ -877,6 +882,7 @@ class FlatThemeFileEditor
 		resetFontSizeMenuItem = new JMenuItem();
 		showHSLColorsMenuItem = new JCheckBoxMenuItem();
 		showRGBColorsMenuItem = new JCheckBoxMenuItem();
+		showColorLumaMenuItem = new JCheckBoxMenuItem();
 		windowMenu = new JMenu();
 		activateEditorMenuItem = new JMenuItem();
 		nextEditorMenuItem = new JMenuItem();
@@ -1024,6 +1030,11 @@ class FlatThemeFileEditor
 				showRGBColorsMenuItem.setText("Show RGB colors (hex)");
 				showRGBColorsMenuItem.addActionListener(e -> colorModelChanged());
 				viewMenu.add(showRGBColorsMenuItem);
+
+				//---- showColorLumaMenuItem ----
+				showColorLumaMenuItem.setText("Show color luma");
+				showColorLumaMenuItem.addActionListener(e -> colorModelChanged());
+				viewMenu.add(showColorLumaMenuItem);
 			}
 			menuBar.add(viewMenu);
 
@@ -1133,6 +1144,7 @@ class FlatThemeFileEditor
 	private JMenuItem resetFontSizeMenuItem;
 	private JCheckBoxMenuItem showHSLColorsMenuItem;
 	private JCheckBoxMenuItem showRGBColorsMenuItem;
+	private JCheckBoxMenuItem showColorLumaMenuItem;
 	private JMenu windowMenu;
 	private JMenuItem activateEditorMenuItem;
 	private JMenuItem nextEditorMenuItem;

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.jfd

@@ -177,6 +177,11 @@ new FormModel {
 						"text": "Show RGB colors (hex)"
 						addEvent( new FormEvent( "java.awt.event.ActionListener", "actionPerformed", "colorModelChanged", false ) )
 					} )
+					add( new FormComponent( "javax.swing.JCheckBoxMenuItem" ) {
+						name: "showColorLumaMenuItem"
+						"text": "Show color luma"
+						addEvent( new FormEvent( "java.awt.event.ActionListener", "actionPerformed", "colorModelChanged", false ) )
+					} )
 				} )
 				add( new FormContainer( "javax.swing.JMenu", new FormLayoutManager( class javax.swing.JMenu ) ) {
 					name: "windowMenu"

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeTokenMaker.java

@@ -71,6 +71,7 @@ public class FlatThemeTokenMaker
 		tokenMap.put( "mix", TOKEN_FUNCTION );
 		tokenMap.put( "tint", TOKEN_FUNCTION );
 		tokenMap.put( "shade", TOKEN_FUNCTION );
+		tokenMap.put( "contrast", TOKEN_FUNCTION );
 		tokenMap.put( "lazy", TOKEN_FUNCTION );
 
 		// function options

flatlaf-theme-editor/theme-editor-test.properties

@@ -70,3 +70,32 @@ Prop.colorFunc42 = tint(#f0f,75%)
 Prop.colorFunc45 = shade(#f0f,25%)
 Prop.colorFunc46 = shade(#f0f)
 Prop.colorFunc47 = shade(#f0f,75%)
+
+Prop.colorFunc50 = contrast(#111,#000,#fff)
+Prop.colorFunc51 = contrast(#eee,#000,#fff)
+Prop.colorFunc52 = contrast(#111,$Prop.colorFunc1,$Prop.colorFunc3)
+Prop.colorFunc53 = contrast(#eee,$Prop.colorFunc1,$Prop.colorFunc3)
+Prop.colorFunc54 = contrast(lighten(#111,5%),$Prop.colorFunc1,$Prop.colorFunc3)
+Prop.colorFunc55 = contrast(lighten(#eee,5%),$Prop.colorFunc1,$Prop.colorFunc3)
+Prop.colorFunc56 = contrast(contrast(#222,#111,#eee),contrast(#eee,#000,#fff),contrast(#111,#000,#fff))
+
+Prop.1.selectionBackground = #2675BF
+Prop.1.selectionForeground = contrast($Prop.1.selectionBackground,#000,#fff)
+
+Prop.2.selectionBackground = #95c0e9
+Prop.2.selectionForeground = contrast($Prop.2.selectionBackground,#000,#fff)
+
+Prop.3.selectionBackground = #f00
+Prop.3.selectionForeground = contrast($Prop.3.selectionBackground,#000,#fff)
+
+Prop.4.selectionBackground = #0f0
+Prop.4.selectionForeground = contrast($Prop.4.selectionBackground,#000,#fff)
+
+Prop.5.selectionBackground = #00f
+Prop.5.selectionForeground = contrast($Prop.5.selectionBackground,#000,#fff)
+
+Prop.6.selectionBackground = #FFCC00
+Prop.6.selectionForeground = contrast($Prop.6.selectionBackground,#000,#fff)
+
+Prop.7.selectionBackground = #FF9500
+Prop.7.selectionForeground = contrast($Prop.7.selectionBackground,#000,#fff)