Calculates the minimum color alpha value which can be applied to foreground

Description

Demo Code

/*/*from   w  ww  . j ava  2  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 {
    private static final int MIN_ALPHA_SEARCH_MAX_ITERATIONS = 10;
    private static final int MIN_ALPHA_SEARCH_PRECISION = 10;

    /**
     * Calculates the minimum alpha value which can be applied to {@code foreground} so that would
     * have a contrast value of at least {@code minContrastRatio} when compared to
     * {@code background}.
     *
     * @param foreground       the foreground color.
     * @param background       the background color. Should be opaque.
     * @param minContrastRatio the minimum contrast ratio.
     * @return the alpha value in the range 0-255, or -1 if no value could be calculated.
     */
    public static int calculateMinimumAlpha(int foreground, int background,
            float minContrastRatio) {
        if (Color.alpha(background) != 255) {
            throw new IllegalArgumentException(
                    "background can not be translucent");
        }

        // First lets check that a fully opaque foreground has sufficient contrast
        int testForeground = setAlphaComponent(foreground, 255);
        double testRatio = calculateContrast(testForeground, background);
        if (testRatio < minContrastRatio) {
            // Fully opaque foreground does not have sufficient contrast, return error
            return -1;
        }

        // Binary search to find a value with the minimum value which provides sufficient contrast
        int numIterations = 0;
        int minAlpha = 0;
        int maxAlpha = 255;

        while (numIterations <= MIN_ALPHA_SEARCH_MAX_ITERATIONS
                && (maxAlpha - minAlpha) > MIN_ALPHA_SEARCH_PRECISION) {
            final int testAlpha = (minAlpha + maxAlpha) / 2;

            testForeground = setAlphaComponent(foreground, testAlpha);
            testRatio = calculateContrast(testForeground, background);

            if (testRatio < minContrastRatio) {
                minAlpha = testAlpha;
            } else {
                maxAlpha = testAlpha;
            }

            numIterations++;
        }

        // Conservatively return the max of the range of possible alphas, which is known to pass.
        return maxAlpha;
    }

    /**
     * Set the alpha component of {@code color} to be {@code alpha}.
     */
    public static int setAlphaComponent(int color, int alpha) {
        if (alpha < 0 || alpha > 255) {
            throw new IllegalArgumentException(
                    "alpha must be between 0 and 255.");
        }
        return (color & 0x00ffffff) | (alpha << 24);
    }

    /**
     * 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);
    }
}
Calculates the minimum color alpha value which can be applied to foreground - Android Graphics

Description

Demo Code

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