| 1 | var utils = require('../utils')
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| 2 | , nodes = require('../nodes');
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| 3 |
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| 4 | /**
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| 5 | * Returns the transparent version of the given `top` color,
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| 6 | * as if it was blend over the given `bottom` color.
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| 7 | *
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| 8 | * Examples:
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| 9 | *
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| 10 | * transparentify(#808080)
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| 11 | * => rgba(0,0,0,0.5)
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| 12 | *
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| 13 | * transparentify(#414141, #000)
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| 14 | * => rgba(255,255,255,0.25)
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| 15 | *
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| 16 | * transparentify(#91974C, #F34949, 0.5)
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| 17 | * => rgba(47,229,79,0.5)
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| 18 | *
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| 19 | * @param {RGBA|HSLA} top
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| 20 | * @param {RGBA|HSLA} [bottom=#fff]
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| 21 | * @param {Unit} [alpha]
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| 22 | * @return {RGBA}
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| 23 | * @api public
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| 24 | */
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| 25 |
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| 26 | function transparentify(top, bottom, alpha){
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| 27 | utils.assertColor(top);
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| 28 | top = top.rgba;
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| 29 | // Handle default arguments
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| 30 | bottom = bottom || new nodes.RGBA(255, 255, 255, 1);
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| 31 | if (!alpha && bottom && !bottom.rgba) {
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| 32 | alpha = bottom;
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| 33 | bottom = new nodes.RGBA(255, 255, 255, 1);
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| 34 | }
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| 35 | utils.assertColor(bottom);
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| 36 | bottom = bottom.rgba;
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| 37 | var bestAlpha = ['r', 'g', 'b'].map(function(channel){
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| 38 | return (top[channel] - bottom[channel]) / ((0 < (top[channel] - bottom[channel]) ? 255 : 0) - bottom[channel]);
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| 39 | }).sort(function(a, b){return b - a;})[0];
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| 40 | if (alpha) {
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| 41 | utils.assertType(alpha, 'unit', 'alpha');
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| 42 | if ('%' == alpha.type) {
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| 43 | bestAlpha = alpha.val / 100;
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| 44 | } else if (!alpha.type) {
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| 45 | bestAlpha = alpha = alpha.val;
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| 46 | }
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| 47 | }
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| 48 | bestAlpha = Math.max(Math.min(bestAlpha, 1), 0);
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| 49 | // Calculate the resulting color
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| 50 | function processChannel(channel) {
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| 51 | if (0 == bestAlpha) {
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| 52 | return bottom[channel]
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| 53 | } else {
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| 54 | return bottom[channel] + (top[channel] - bottom[channel]) / bestAlpha
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| 55 | }
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| 56 | }
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| 57 | return new nodes.RGBA(
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| 58 | processChannel('r'),
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| 59 | processChannel('g'),
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| 60 | processChannel('b'),
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| 61 | Math.round(bestAlpha * 100) / 100
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| 62 | );
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| 63 | }
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| 64 | transparentify.params = ['top', 'bottom', 'alpha'];
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| 65 | module.exports = transparentify;
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