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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
- */
- 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 b - a;})[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
- );
- }
- transparentify.params = ['top', 'bottom', 'alpha'];
- module.exports = transparentify;
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