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64 lines
1.7 KiB
64 lines
1.7 KiB
9 years ago
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var utils = require('../utils')
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, nodes = require('../nodes');
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/**
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* Returns the transparent version of the given `top` color,
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* as if it was blend over the given `bottom` color.
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*
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* Examples:
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*
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* transparentify(#808080)
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* => rgba(0,0,0,0.5)
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*
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* transparentify(#414141, #000)
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* => rgba(255,255,255,0.25)
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*
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* transparentify(#91974C, #F34949, 0.5)
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* => rgba(47,229,79,0.5)
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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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* @param {Unit} [alpha]
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* @return {RGBA}
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* @api public
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*/
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module.exports = function transparentify(top, bottom, alpha){
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utils.assertColor(top);
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top = top.rgba;
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// Handle default arguments
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bottom = bottom || new nodes.RGBA(255, 255, 255, 1);
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if (!alpha && bottom && !bottom.rgba) {
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alpha = bottom;
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bottom = new nodes.RGBA(255, 255, 255, 1);
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}
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utils.assertColor(bottom);
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bottom = bottom.rgba;
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var bestAlpha = ['r', 'g', 'b'].map(function(channel){
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return (top[channel] - bottom[channel]) / ((0 < (top[channel] - bottom[channel]) ? 255 : 0) - bottom[channel]);
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}).sort(function(a, b){return a < b;})[0];
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if (alpha) {
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utils.assertType(alpha, 'unit', 'alpha');
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if ('%' == alpha.type) {
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bestAlpha = alpha.val / 100;
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} else if (!alpha.type) {
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bestAlpha = alpha = alpha.val;
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}
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}
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bestAlpha = Math.max(Math.min(bestAlpha, 1), 0);
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// Calculate the resulting color
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function processChannel(channel) {
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if (0 == bestAlpha) {
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return bottom[channel]
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} else {
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return bottom[channel] + (top[channel] - bottom[channel]) / bestAlpha
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}
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}
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return new nodes.RGBA(
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processChannel('r'),
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processChannel('g'),
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processChannel('b'),
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Math.round(bestAlpha * 100) / 100
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);
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}
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