android-34/com/android/systemui/monet/contrast/Contrast.java

/*
 * Copyright (C) 2023 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.android.systemui.monet.contrast;


import static java.lang.Math.max;

import com.android.systemui.monet.utils.ColorUtils;

/**
 * Color science for contrast utilities.
 *
 * <p>Utility methods for calculating contrast given two colors, or calculating a color given one
 * color and a contrast ratio.
 *
 * <p>Contrast ratio is calculated using XYZ's Y. When linearized to match human perception, Y
 * becomes HCT's tone and L*a*b*'s' L*.
 */
public final class Contrast {
    // The minimum contrast ratio of two colors.
    // Contrast ratio equation = lighter + 5 / darker + 5, if lighter == darker, ratio == 1.
    public static final double RATIO_MIN = 1.0;

    // The maximum contrast ratio of two colors.
    // Contrast ratio equation = lighter + 5 / darker + 5. Lighter and darker scale from 0 to 100.
    // If lighter == 100, darker = 0, ratio == 21.
    public static final double RATIO_MAX = 21.0;
    public static final double RATIO_30 = 3.0;
    public static final double RATIO_45 = 4.5;
    public static final double RATIO_70 = 7.0;

    // Given a color and a contrast ratio to reach, the luminance of a color that reaches that ratio
    // with the color can be calculated. However, that luminance may not contrast as desired, i.e.
    // the contrast ratio of the input color and the returned luminance may not reach the contrast
    // ratio asked for.
    //
    // When the desired contrast ratio and the result contrast ratio differ by more than this
    // amount, an error value should be returned, or the method should be documented as 'unsafe',
    // meaning, it will return a valid luminance but that luminance may not meet the requested
    // contrast ratio.
    //
    // 0.04 selected because it ensures the resulting ratio rounds to the same tenth.
    private static final double CONTRAST_RATIO_EPSILON = 0.04;

    // Color spaces that measure luminance, such as Y in XYZ, L* in L*a*b*, or T in HCT, are
    // known as perceptually accurate color spaces.
    //
    // To be displayed, they must gamut map to a "display space", one that has a defined limit on
    // the number of colors. Display spaces include sRGB, more commonly understood as RGB/HSL/HSV.
    //
    // Gamut mapping is undefined and not defined by the color space. Any gamut mapping algorithm
    // must choose how to sacrifice accuracy in hue, saturation, and/or lightness.
    //
    // A principled solution is to maintain lightness, thus maintaining contrast/a11y, maintain hue,
    // thus maintaining aesthetic intent, and reduce chroma until the color is in gamut.
    //
    // HCT chooses this solution, but, that doesn't mean it will _exactly_ matched desired
    // lightness, if only because RGB is quantized: RGB is expressed as a set of integers: there may
    // be an RGB color with, for example, 47.892 lightness, but not 47.891.
    //
    // To allow for this inherent incompatibility between perceptually accurate color spaces and
    // display color spaces, methods that take a contrast ratio and luminance, and return a
    // luminance that reaches that contrast ratio for the input luminance, purposefully
    // darken/lighten their result such that the desired contrast ratio will be reached even if
    // inaccuracy is introduced.
    //
    // 0.4 is generous, ex. HCT requires much less delta. It was chosen because it provides a rough
    // guarantee that as long as a percetual color space gamut maps lightness such that the
    // resulting lightness rounds to the same as the requested, the desired contrast ratio will be
    // reached.
    private static final double LUMINANCE_GAMUT_MAP_TOLERANCE = 0.4;

    private Contrast() {
    }

    /**
     * Contrast ratio is a measure of legibility, its used to compare the lightness of two colors.
     * This method is used commonly in industry due to its use by WCAG.
     *
     * <p>To compare lightness, the colors are expressed in the XYZ color space, where Y is
     * lightness,
     * also known as relative luminance.
     *
     * <p>The equation is ratio = lighter Y + 5 / darker Y + 5.
     */
    public static double ratioOfYs(double y1, double y2) {
        final double lighter = max(y1, y2);
        final double darker = (lighter == y2) ? y1 : y2;
        return (lighter + 5.0) / (darker + 5.0);
    }

    /**
     * Contrast ratio of two tones. T in HCT, L* in L*a*b*. Also known as luminance or perpectual
     * luminance.
     *
     * <p>Contrast ratio is defined using Y in XYZ, relative luminance. However, relative luminance
     * is linear to number of photons, not to perception of lightness. Perceptual luminance, L* in
     * L*a*b*, T in HCT, is. Designers prefer color spaces with perceptual luminance since they're
     * accurate to the eye.
     *
     * <p>Y and L* are pure functions of each other, so it possible to use perceptually accurate
     * color spaces, and measure contrast, and measure contrast in a much more understandable way:
     * instead of a ratio, a linear difference. This allows a designer to determine what they need
     * to adjust a color's lightness to in order to reach their desired contrast, instead of
     * guessing & checking with hex codes.
     */
    public static double ratioOfTones(double t1, double t2) {
        return ratioOfYs(ColorUtils.yFromLstar(t1), ColorUtils.yFromLstar(t2));
    }

    /**
     * Returns T in HCT, L* in L*a*b* >= tone parameter that ensures ratio with input T/L*. Returns
     * -1 if ratio cannot be achieved.
     *
     * @param tone  Tone return value must contrast with.
     * @param ratio Desired contrast ratio of return value and tone parameter.
     */
    public static double lighter(double tone, double ratio) {
        if (tone < 0.0 || tone > 100.0) {
            return -1.0;
        }
        // Invert the contrast ratio equation to determine lighter Y given a ratio and darker Y.
        final double darkY = ColorUtils.yFromLstar(tone);
        final double lightY = ratio * (darkY + 5.0) - 5.0;
        if (lightY < 0.0 || lightY > 100.0) {
            return -1.0;
        }
        final double realContrast = ratioOfYs(lightY, darkY);
        final double delta = Math.abs(realContrast - ratio);
        if (realContrast < ratio && delta > CONTRAST_RATIO_EPSILON) {
            return -1.0;
        }

        final double returnValue = ColorUtils.lstarFromY(lightY) + LUMINANCE_GAMUT_MAP_TOLERANCE;
        // NOMUTANTS--important validation step; functions it is calling may change implementation.
        if (returnValue < 0 || returnValue > 100) {
            return -1.0;
        }
        return returnValue;
    }

    /**
     * Tone >= tone parameter that ensures ratio. 100 if ratio cannot be achieved.
     *
     * <p>This method is unsafe because the returned value is guaranteed to be in bounds, but, the
     * in bounds return value may not reach the desired ratio.
     *
     * @param tone  Tone return value must contrast with.
     * @param ratio Desired contrast ratio of return value and tone parameter.
     */
    public static double lighterUnsafe(double tone, double ratio) {
        double lighterSafe = lighter(tone, ratio);
        return lighterSafe < 0.0 ? 100.0 : lighterSafe;
    }

    /**
     * Returns T in HCT, L* in L*a*b* <= tone parameter that ensures ratio with input T/L*. Returns
     * -1 if ratio cannot be achieved.
     *
     * @param tone  Tone return value must contrast with.
     * @param ratio Desired contrast ratio of return value and tone parameter.
     */
    public static double darker(double tone, double ratio) {
        if (tone < 0.0 || tone > 100.0) {
            return -1.0;
        }
        // Invert the contrast ratio equation to determine darker Y given a ratio and lighter Y.
        final double lightY = ColorUtils.yFromLstar(tone);
        final double darkY = ((lightY + 5.0) / ratio) - 5.0;
        if (darkY < 0.0 || darkY > 100.0) {
            return -1.0;
        }
        final double realContrast = ratioOfYs(lightY, darkY);
        final double delta = Math.abs(realContrast - ratio);
        if (realContrast < ratio && delta > CONTRAST_RATIO_EPSILON) {
            return -1.0;
        }

        // For information on 0.4 constant, see comment in lighter(tone, ratio).
        final double returnValue = ColorUtils.lstarFromY(darkY) - LUMINANCE_GAMUT_MAP_TOLERANCE;
        // NOMUTANTS--important validation step; functions it is calling may change implementation.
        if (returnValue < 0 || returnValue > 100) {
            return -1.0;
        }
        return returnValue;
    }

    /**
     * Tone <= tone parameter that ensures ratio. 0 if ratio cannot be achieved.
     *
     * <p>This method is unsafe because the returned value is guaranteed to be in bounds, but, the
     * in bounds return value may not reach the desired ratio.
     *
     * @param tone  Tone return value must contrast with.
     * @param ratio Desired contrast ratio of return value and tone parameter.
     */
    public static double darkerUnsafe(double tone, double ratio) {
        double darkerSafe = darker(tone, ratio);
        return max(0.0, darkerSafe);
    }
}