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