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