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Opacity frustrations

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Alan SmithOpacity frustrations
by on Oct 26, 2009 at 11:09:28 pm

How come in Aftereffects, if you put two black solids on top of a white background and set both their opacity to 50%, you still get a 50% opacity black?

Surely you would expect that two identical 50% opacity layers on top of each other should give you a 100% opacity result, no?

Does this bug anyone else or is it just me?

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Danny WinnRe: Opacity frustrations
by on Oct 26, 2009 at 11:50:02 pm

Actually it makes perfect sense. You're asking for 50% of each Black Solid and that's what it's giving you. If you want 100% then just keep the one Black at 100%. I don't get it:/

What exactly are you trying to do with all this?

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Xinlai NiRe: Opacity frustrations
by on Oct 26, 2009 at 11:51:44 pm

opacities are not additive, if you will, transparencies are multiplicative:
Your set up has two black layers with opacity 50%, that's the same as saying they both have transaparency of 50%, which means if you overlay them they have a transparency of 50%x50%=25%, which means 75% opacity.
The fact that transparencies are multiplicative can be justified by thinking of rays going through these layers, after the first layer, 50% of the original light passed through, and after the 2nd, 50% of the 50% passed through.
Hope this helps.

Xinlai Ni
Software Engineer, Google Inc.

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Harry FrankRe: Opacity frustrations
by on Oct 27, 2009 at 4:12:58 pm

If you want the Alphas to add together, set the blending modes to "Alpha Add"

Harry J Frank
Freelance Motion Designer

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Todd KoprivaRe: Opacity frustrations
by on Oct 27, 2009 at 5:06:02 pm

Xinlai and Harry have it.

Here's an excerpt from the Alpha Add section of the blending mode reference in After Effects Help:

"When two areas of 50% transparency overlap, the result is not 100% opacity but 75% opacity, because the default operation is multiplication. (50% of the light gets through one layer, and then 50% of the remainder gets through the next layer, so 25% gets through the system.) This is like partial transparency in the real world. But, in some cases, you don't want this default blending. You want the two 50% opacity areas to combine to make a seamless, opaque join. You want the alpha values to be added. In these cases, use the Alpha Add blending mode."

Todd Kopriva, Adobe Systems Incorporated
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Alan SmithRe: Opacity frustrations
by on Oct 30, 2009 at 11:30:28 am

Great thanks for the replies
Yep 75% makes complete sense.

I just naturally assumed that transparencies were additive and got frustrated when doing cross fades and the mid point was still transparent.

Thanks for the help.

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