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Confused about additive layers

May 31, 2014, 17:29 (This post was last modified: May 31, 2014 17:30 by braddabug.)
Post: #1
Confused about additive layers
I'm not new to Gimp, but I still don't understand additive layers. They should be simple, but they're not working the way I expect.

Here's what I mean. If I create a new image, and create 3 white layers, set each to an opacity of 33%, make each of them additive, then add a normal black layer underneath it all, shouldn't the final image be white? Since that becomes 0.33 + 0.33 + 0.33 + 0 = 0.99? But instead the image is definitely gray.

Why isn't it white?
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May 31, 2014, 19:00 (This post was last modified: May 31, 2014 19:01 by ofnuts.)
Post: #2
RE: Confused about additive layers
Your computation is wrong because what happens is not an addition, but a "composition" and the math are a bit different. This is fairly well explained on Wikipedia.

A consequence of this, for instance, is that if you have N stacked layers, and want each of them to have equal importance in the result, then they should have decreasing opacity, corresponding to their order in the stack: 1/1 (bottom), 1/2, 1/3, 1/4... until 1/N (at top) (and not 1/N for all as most people would believe).

Now answering your questions on
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