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ContrastOverlay.js
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ContrastOverlay.js
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// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import * as Common from '../common/common.js';
import {ContrastInfo, Events} from './ContrastInfo.js'; // eslint-disable-line no-unused-vars
export class ContrastOverlay {
/**
* @param {!ContrastInfo} contrastInfo
* @param {!Element} colorElement
*/
constructor(contrastInfo, colorElement) {
/** @type {!ContrastInfo} */
this._contrastInfo = contrastInfo;
this._visible = false;
this._contrastRatioSVG = colorElement.createSVGChild('svg', 'spectrum-contrast-container fill');
this._contrastRatioLines = {
aa: this._contrastRatioSVG.createSVGChild('path', 'spectrum-contrast-line'),
aaa: this._contrastRatioSVG.createSVGChild('path', 'spectrum-contrast-line')
};
this._width = 0;
this._height = 0;
this._contrastRatioLineBuilder = new ContrastRatioLineBuilder(this._contrastInfo);
this._contrastRatioLinesThrottler = new Common.Throttler.Throttler(0);
this._drawContrastRatioLinesBound = this._drawContrastRatioLines.bind(this);
this._contrastInfo.addEventListener(Events.ContrastInfoUpdated, this._update.bind(this));
}
_update() {
if (!this._visible || this._contrastInfo.isNull() || !this._contrastInfo.contrastRatio()) {
return;
}
this._contrastRatioLinesThrottler.schedule(this._drawContrastRatioLinesBound);
}
/**
* @param {number} width
* @param {number} height
*/
setDimensions(width, height) {
this._width = width;
this._height = height;
this._update();
}
/**
* @param {boolean} visible
*/
setVisible(visible) {
this._visible = visible;
this._contrastRatioSVG.classList.toggle('hidden', !visible);
this._update();
}
async _drawContrastRatioLines() {
for (const level in this._contrastRatioLines) {
const path = this._contrastRatioLineBuilder.drawContrastRatioLine(this._width, this._height, level);
if (path) {
this._contrastRatioLines[level].setAttribute('d', path);
} else {
this._contrastRatioLines[level].removeAttribute('d');
}
}
}
}
export class ContrastRatioLineBuilder {
/**
* @param {!ContrastInfo} contrastInfo
*/
constructor(contrastInfo) {
/** @type {!ContrastInfo} */
this._contrastInfo = contrastInfo;
}
/**
* @param {number} width
* @param {number} height
* @param {string} level
* @return {?string}
*/
drawContrastRatioLine(width, height, level) {
const requiredContrast = this._contrastInfo.contrastRatioThreshold(level);
if (!width || !height || !requiredContrast) {
return null;
}
const dS = 0.02;
const epsilon = 0.0002;
const H = 0;
const S = 1;
const V = 2;
const A = 3;
const color = this._contrastInfo.color();
const bgColor = this._contrastInfo.bgColor();
if (!color || !bgColor) {
return null;
}
const fgRGBA = color.rgba();
const fgHSVA = color.hsva();
const bgRGBA = bgColor.rgba();
const bgLuminance = Common.Color.Color.luminance(bgRGBA);
const blendedRGBA = [];
Common.Color.Color.blendColors(fgRGBA, bgRGBA, blendedRGBA);
const fgLuminance = Common.Color.Color.luminance(blendedRGBA);
const fgIsLighter = fgLuminance > bgLuminance;
const desiredLuminance = Common.Color.Color.desiredLuminance(bgLuminance, requiredContrast, fgIsLighter);
let lastV = fgHSVA[V];
let currentSlope = 0;
const candidateHSVA = [fgHSVA[H], 0, 0, fgHSVA[A]];
let pathBuilder = [];
const candidateRGBA = [];
Common.Color.Color.hsva2rgba(candidateHSVA, candidateRGBA);
Common.Color.Color.blendColors(candidateRGBA, bgRGBA, blendedRGBA);
/**
* @param {number} index
* @param {number} x
*/
function updateCandidateAndComputeDelta(index, x) {
candidateHSVA[index] = x;
Common.Color.Color.hsva2rgba(candidateHSVA, candidateRGBA);
Common.Color.Color.blendColors(candidateRGBA, bgRGBA, blendedRGBA);
return Common.Color.Color.luminance(blendedRGBA) - desiredLuminance;
}
/**
* Approach a value of the given component of `candidateHSVA` such that the
* calculated luminance of `candidateHSVA` approximates `desiredLuminance`.
* @param {number} index The component of `candidateHSVA` to modify.
* @return {?number} The new value for the modified component, or `null` if
* no suitable value exists.
*/
function approach(index) {
let x = candidateHSVA[index];
let multiplier = 1;
let dLuminance = updateCandidateAndComputeDelta(index, x);
let previousSign = Math.sign(dLuminance);
for (let guard = 100; guard; guard--) {
if (Math.abs(dLuminance) < epsilon) {
return x;
}
const sign = Math.sign(dLuminance);
if (sign !== previousSign) {
// If `x` overshoots the correct value, halve the step size.
multiplier /= 2;
previousSign = sign;
} else if (x < 0 || x > 1) {
// If there is no overshoot and `x` is out of bounds, there is no
// acceptable value for `x`.
return null;
}
// Adjust `x` by a multiple of `dLuminance` to decrease step size as
// the computed luminance converges on `desiredLuminance`.
x += multiplier * (index === V ? -dLuminance : dLuminance);
dLuminance = updateCandidateAndComputeDelta(index, x);
}
// The loop should always converge or go out of bounds on its own.
console.error('Loop exited unexpectedly');
return null;
}
// Plot V for values of S such that the computed luminance approximates
// `desiredLuminance`, until no suitable value for V can be found, or the
// current value of S goes of out bounds.
let s;
for (s = 0; s < 1 + dS; s += dS) {
s = Math.min(1, s);
candidateHSVA[S] = s;
// Extrapolate the approximate next value for `v` using the approximate
// gradient of the curve.
candidateHSVA[V] = lastV + currentSlope * dS;
const v = approach(V);
if (v === null) {
break;
}
// Approximate the current gradient of the curve.
currentSlope = s === 0 ? 0 : (v - lastV) / dS;
lastV = v;
pathBuilder.push(pathBuilder.length ? 'L' : 'M');
pathBuilder.push((s * width).toFixed(2));
pathBuilder.push(((1 - v) * height).toFixed(2));
}
// If no suitable V value for an in-bounds S value was found, find the value
// of S such that V === 1 and add that to the path.
if (s < 1 + dS) {
s -= dS;
candidateHSVA[V] = 1;
s = approach(S);
if (s !== null) {
pathBuilder = pathBuilder.concat(['L', (s * width).toFixed(2), '-0.1']);
}
}
if (pathBuilder.length === 0) {
return null;
}
return pathBuilder.join(' ');
}
}