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commonMain.contrast.Contrast.kt Maven / Gradle / Ivy

/*
 * Copyright (c) 2024, Google LLC, OpenSavvy and contributors.
 *
 * 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 opensavvy.material3.colors.contrast

import opensavvy.material3.colors.utils.Color.Companion.lstarFromY
import opensavvy.material3.colors.utils.Color.Companion.yFromLstar
import kotlin.math.abs
import kotlin.math.max

/**
 * Color science for contrast utilities.
 *
 *
 * Utility methods for calculating contrast given two colors, or calculating a color given one
 * color and a contrast ratio.
 *
 *
 * 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*.
 */
object Contrast {
	// The minimum contrast ratio of two colors.
	// Contrast ratio equation = lighter + 5 / darker + 5, if lighter == darker, ratio == 1.
	const val RATIO_MIN: Double = 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.
	const val RATIO_MAX: Double = 21.0
	const val RATIO_30: Double = 3.0
	const val RATIO_45: Double = 4.5
	const val RATIO_70: Double = 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 const val 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 const val LUMINANCE_GAMUT_MAP_TOLERANCE = 0.4

	/**
	 * 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.
	 *
	 *
	 * To compare lightness, the colors are expressed in the XYZ color space, where Y is lightness,
	 * also known as relative luminance.
	 *
	 *
	 * The equation is ratio = lighter Y + 5 / darker Y + 5.
	 */
	fun ratioOfYs(y1: Double, y2: Double): Double {
		val lighter: Double = max(y1, y2)
		val darker = if ((lighter == y2)) y1 else 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.
	 *
	 *
	 * 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.
	 *
	 *
	 * 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.
	 */
	fun ratioOfTones(t1: Double, t2: Double): Double {
		return ratioOfYs(yFromLstar(t1), 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.
	 */
	fun lighter(tone: Double, ratio: Double): Double {
		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.
		val darkY = yFromLstar(tone)
		val lightY = ratio * (darkY + 5.0) - 5.0
		if (lightY < 0.0 || lightY > 100.0) {
			return -1.0
		}
		val realContrast = ratioOfYs(lightY, darkY)
		val delta = abs(realContrast - ratio)
		if (realContrast < ratio && delta > CONTRAST_RATIO_EPSILON) {
			return -1.0
		}

		val returnValue = 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.
	 *
	 *
	 * 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.
	 */
	fun lighterUnsafe(tone: Double, ratio: Double): Double {
		val lighterSafe = lighter(tone, ratio)
		return if (lighterSafe < 0.0) 100.0 else 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.
	 */
	fun darker(tone: Double, ratio: Double): Double {
		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.
		val lightY = yFromLstar(tone)
		val darkY = ((lightY + 5.0) / ratio) - 5.0
		if (darkY < 0.0 || darkY > 100.0) {
			return -1.0
		}
		val realContrast = ratioOfYs(lightY, darkY)
		val delta = 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).
		val returnValue = 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.
	 *
	 *
	 * 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.
	 */
	fun darkerUnsafe(tone: Double, ratio: Double): Double {
		val darkerSafe = darker(tone, ratio)
		return max(0.0, darkerSafe)
	}
}




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