Returns the contrast ratio between foreground and background color. - Android Graphics

Android examples for Graphics:Color

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Description

Returns the contrast ratio between foreground and background color.

Demo Code

/*//from  ww  w. j a  v  a2  s.  c  o  m
 * 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.
 */
//package com.java2s;
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);
    }
}

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