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If I interpret this correctly (quoting from
<a class="moz-txt-link-freetext" href="http://cairographics.org/operators/">http://cairographics.org/operators/</a>):<br>
<blockquote>Resulting alpha (aR) Resulting color (xR)<br>
aA + aB − 2·aA·aB (xaA·(1−aB) + xaB·(1−aA))/aR<br>
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If the alpha value for both A and B is 1, then nothing will be drawn
anywhere? That is, I have something drawn on surface A with alpha at
1.0, and I have something on the receiving surface drawn with alpha
at 1.0. This results in aR = 0.0 so everything in B becomes
transparent?<br>
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