xref: /aosp_15_r20/external/libmonet/contrast/Contrast.java (revision 970e10460f970939fd510dd6ad3e0d65908272e3)

/*
 * Copyright 2022 Google LLC
 *
 * 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.google.ux.material.libmonet.contrast;

import static java.lang.Math.max;

import com.google.ux.material.libmonet.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/HSB.
  // 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 perceptual 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);
  }
}