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<!--
Any copyright is dedicated to the Public Domain.
http://creativecommons.org/publicdomain/zero/1.0/
-->
<html>
<style>
.reftest {
background-color: rgba(0, 255, 0, 0.5);
}
.child {
width: 100px;
height: 100px;
opacity: 0.5;
background-color: rgb(0, 127, 0);
}
.topleft {
float:left;
width: 100px;
height: 100px;
/*First blend black rect with green in place*/
/* Cs = (1 - αb) x Cs + αb x B(Cb, Cs)*/
/* B(Cb, Cs) = | Cb - Cs | = (0, 0, 0) */
/* 0.5 * (0, 255, 0) + 0.5 * (0, 0, 0) = (0, 127.5, 0)*/
/* Now, composite the resulting color with src-over; the alpha is the original alpha for the top layer
while the color is the blending result*/
/* co = αs x Fa x Cs + αb x Fb x Cb - this is premultiplied */
/* αo = αs + αb x (1 – αs) */
/* Source over: Fa = 1; Fb = 1 – αs */
/*co = 0.5 * 1 * (0, 127.5, 0) + 0.5 * 0.5 * (0, 255, 0) = (0, 63.5, 0) + 0.25 * (0, 255, 0) = (0, 127.5, 0)*/
/*ao = 0.5 + 0.5 * 0.5 = 0.75*/
/* Co = co/ao = (0, 127.5, 0) / 0.75*/
/* Now alpha composite on white background */
/*co = 0.75 * 1 * (0, 127.5, 0) / 0.75 + 1 * 0.25 * (255, 255, 255) = (0, 127.5, 0) + (63.75, 63.75, 63.75) = (63.75, 159, 63.75) = (64, 191, 64) */
background-color: rgb(64, 191, 64);
}
.topright {
float:left;
width: 100px;
height: 100px;
}
.bottom {
width:200px;
height: 100px;
clear:both;
}
</style>
<div class="topleft"></div>
<div class="reftest topright"></div>
<div class="reftest bottom"></div>
</html>
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