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<!doctype>
<html>
<head>
<style>
div {
background:rgba(0,255,255,0.3);
}
#div1 {
width:300px;
height:300px;
}
#div2 {
width:200px;
height:200px;
}
</style>
</head>
<body>
<div id="div1"><div id="div2"></div></div><br/>
</body>
what is the actually computed transparency of div2? I am confused. It is a unknown number that is not 0.3+0.3 or 0.3+0.3*0.3
843
The sum of alpha channels can be calculated as:
alpha_top + alpha_bottom * (1 - alpha_top) =
0.3 + 0.3 * (1 - 0.3) =
0.3 + 0.3 * 0.7 =
0.3 + 0.21 =
0.51
For more explanation, see Alpha Compositing @ wikipedia, which lists the formula as:
or
This is essentially the same formula used by Bogdan Kuštan and it can also be found in this answer by bwoebi.
180
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