Java tutorial
//package com.java2s; /* * Copyright 2015 The Android Open Source Project * * 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 * * http://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. */ import android.graphics.Color; public class Main { /** * Returns the contrast ratio between {@code foreground} and {@code background}. * {@code background} must be opaque. * <p> * Formula defined * <a href="http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef">here</a>. */ public static double calculateContrast(int foreground, int background) { if (Color.alpha(background) != 255) { throw new IllegalArgumentException("background can not be translucent"); } if (Color.alpha(foreground) < 255) { // If the foreground is translucent, composite the foreground over the background foreground = compositeColors(foreground, background); } final double luminance1 = calculateLuminance(foreground) + 0.05; final double luminance2 = calculateLuminance(background) + 0.05; // Now return the lighter luminance divided by the darker luminance return Math.max(luminance1, luminance2) / Math.min(luminance1, luminance2); } /** * Composite two potentially translucent colors over each other and returns the result. */ public static int compositeColors(int foreground, int background) { final float alpha1 = Color.alpha(foreground) / 255f; final float alpha2 = Color.alpha(background) / 255f; float a = (alpha1 + alpha2) * (1f - alpha1); float r = (Color.red(foreground) * alpha1) + (Color.red(background) * alpha2 * (1f - alpha1)); float g = (Color.green(foreground) * alpha1) + (Color.green(background) * alpha2 * (1f - alpha1)); float b = (Color.blue(foreground) * alpha1) + (Color.blue(background) * alpha2 * (1f - alpha1)); return Color.argb((int) a, (int) r, (int) g, (int) b); } /** * Returns the luminance of a color. * * Formula defined here: http://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef */ public static double calculateLuminance(int color) { double red = Color.red(color) / 255d; red = red < 0.03928 ? red / 12.92 : Math.pow((red + 0.055) / 1.055, 2.4); double green = Color.green(color) / 255d; green = green < 0.03928 ? green / 12.92 : Math.pow((green + 0.055) / 1.055, 2.4); double blue = Color.blue(color) / 255d; blue = blue < 0.03928 ? blue / 12.92 : Math.pow((blue + 0.055) / 1.055, 2.4); return (0.2126 * red) + (0.7152 * green) + (0.0722 * blue); } }