forked from mirrors/gecko-dev
b503616295
# ignore-this-changeset Differential Revision: https://phabricator.services.mozilla.com/D36056 --HG-- extra : source : 2616392f26053ee376b9126fbca696de5d4bb15b
113 lines
3.7 KiB
JavaScript
113 lines
3.7 KiB
JavaScript
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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"use strict";
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var EXPORTED_SYMBOLS = ["Color"];
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/**
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* A list of minimum contrast ratio's per WCAG 2.0 conformance level.
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* Please refer to section 1.4.3 of the WCAG 2.0 spec at http://www.w3.org/TR/WCAG20/.
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* Simply put:
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* A = Large text only.
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* AA = Regular sized text or large text in enhanced contrast mode.
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* AAA = Regular sized text in enhanced contrast mode.
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*
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* @type {Object}
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*/
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const CONTRAST_RATIO_LEVELS = {
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A: 3,
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AA: 4.5,
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AAA: 7,
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};
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/**
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* For text legibility on any background color, you need to determine which text
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* color - black or white - will yield the highest contrast ratio.
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* Since you're always comparing `contrastRatio(bgcolor, black) >
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* contrastRatio(bgcolor, white) ? <use black> : <use white>`, we can greatly
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* simplify the calculation to the following constant.
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*
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* @type {Number}
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*/
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const CONTRAST_BRIGHTTEXT_THRESHOLD = Math.sqrt(1.05 * 0.05) - 0.05;
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/**
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* Color class, which describes a color.
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* In the future, this object may be extended to allow for conversions between
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* different color formats and notations, support transparency.
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*
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* @param {Number} r Red color component
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* @param {Number} g Green color component
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* @param {Number} b Blue color component
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*/
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class Color {
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constructor(r, g, b) {
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this.r = r;
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this.g = g;
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this.b = b;
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}
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/**
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* Formula from W3C's WCAG 2.0 spec's relative luminance, section 1.4.1,
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* http://www.w3.org/TR/WCAG20/.
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*
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* @return {Number} Relative luminance, represented as number between 0 and 1.
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*/
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get relativeLuminance() {
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let colorArr = [this.r, this.g, this.b].map(color => {
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color = parseInt(color, 10);
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if (color <= 10) {
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return color / 255 / 12.92;
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}
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return Math.pow((color / 255 + 0.055) / 1.055, 2.4);
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});
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return colorArr[0] * 0.2126 + colorArr[1] * 0.7152 + colorArr[2] * 0.0722;
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}
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/**
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* @return {Boolean} TRUE if you need to use a bright color (e.g. 'white'), when
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* this color is set as the background.
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*/
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get useBrightText() {
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return this.relativeLuminance <= CONTRAST_BRIGHTTEXT_THRESHOLD;
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}
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/**
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* Get the contrast ratio between the current color and a second other color.
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* A common use case is to express the difference between a foreground and a
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* background color in numbers.
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* Formula from W3C's WCAG 2.0 spec's contrast ratio, section 1.4.1,
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* http://www.w3.org/TR/WCAG20/.
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*
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* @param {Color} otherColor Color instance to calculate the contrast with
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* @return {Number} Contrast ratios can range from 1 to 21, commonly written
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* as 1:1 to 21:1.
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*/
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contrastRatio(otherColor) {
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if (!(otherColor instanceof Color)) {
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throw new TypeError("The first argument should be an instance of Color");
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}
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let luminance = this.relativeLuminance;
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let otherLuminance = otherColor.relativeLuminance;
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return (
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(Math.max(luminance, otherLuminance) + 0.05) /
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(Math.min(luminance, otherLuminance) + 0.05)
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);
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}
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/**
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* Method to check if the contrast ratio between two colors is high enough to
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* be discernable.
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*
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* @param {Color} otherColor Color instance to calculate the contrast with
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* @param {String} [level] WCAG conformance level that maps to the minimum
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* required contrast ratio. Defaults to 'AA'
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* @return {Boolean}
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*/
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isContrastRatioAcceptable(otherColor, level = "AA") {
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return this.contrastRatio(otherColor) > CONTRAST_RATIO_LEVELS[level];
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}
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}
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