Opacity frustrations

[Danny Winn]

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?

[Xinlai Ni]

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.

[Harry Frank]

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

Harry J Frank
Freelance Motion Designer
graymachine.com

[Todd Kopriva]

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.”

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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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[Alan Smith]

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.