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76 lines
2.3 KiB
76 lines
2.3 KiB
9 years ago
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var utils = require('../utils')
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, nodes = require('../nodes')
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, blend = require('./blend')
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, luminosity = require('./luminosity');
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/**
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* Returns the contrast ratio object between `top` and `bottom` colors,
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* based on http://leaverou.github.io/contrast-ratio/
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* and https://github.com/LeaVerou/contrast-ratio/blob/gh-pages/color.js#L108
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*
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* Examples:
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*
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* contrast(#000, #fff).ratio
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* => 21
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*
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* contrast(#000, rgba(#FFF, 0.5))
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* => { "ratio": "13.15;", "error": "7.85", "min": "5.3", "max": "21" }
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*
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* @param {RGBA|HSLA} top
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* @param {RGBA|HSLA} [bottom=#fff]
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* @return {Object}
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* @api public
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*/
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module.exports = function contrast(top, bottom){
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if ('rgba' != top.nodeName && 'hsla' != top.nodeName) {
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return new nodes.Literal('contrast(' + (top.isNull ? '' : top.toString()) + ')');
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}
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var result = new nodes.Object();
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top = top.rgba;
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bottom = bottom || new nodes.RGBA(255, 255, 255, 1);
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utils.assertColor(bottom);
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bottom = bottom.rgba;
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function contrast(top, bottom) {
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if (1 > top.a) {
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top = blend(top, bottom);
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}
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var l1 = luminosity(bottom).val + 0.05
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, l2 = luminosity(top).val + 0.05
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, ratio = l1 / l2;
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if (l2 > l1) {
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ratio = 1 / ratio;
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}
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return Math.round(ratio * 10) / 10;
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}
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if (1 <= bottom.a) {
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var resultRatio = new nodes.Unit(contrast(top, bottom));
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result.set('ratio', resultRatio);
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result.set('error', new nodes.Unit(0));
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result.set('min', resultRatio);
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result.set('max', resultRatio);
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} else {
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var onBlack = contrast(top, blend(bottom, new nodes.RGBA(0, 0, 0, 1)))
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, onWhite = contrast(top, blend(bottom, new nodes.RGBA(255, 255, 255, 1)))
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, max = Math.max(onBlack, onWhite);
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function processChannel(topChannel, bottomChannel) {
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return Math.min(Math.max(0, (topChannel - bottomChannel * bottom.a) / (1 - bottom.a)), 255);
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}
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var closest = new nodes.RGBA(
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processChannel(top.r, bottom.r),
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processChannel(top.g, bottom.g),
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processChannel(top.b, bottom.b),
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1
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);
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var min = contrast(top, blend(bottom, closest));
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result.set('ratio', new nodes.Unit(Math.round((min + max) * 50) / 100));
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result.set('error', new nodes.Unit(Math.round((max - min) * 50) / 100));
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result.set('min', new nodes.Unit(min));
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result.set('max', new nodes.Unit(max));
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}
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return result;
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}
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