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alpha channel question

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Doug Wolfalpha channel question
by on Jun 17, 2011 at 10:08:11 pm

Hello everyone,
Have a question that is making my head hurt.
I have two separate image sequences, each of a character dancing on a stage in my comp. Now what I want to do is have control over the intersection of the alpha whenever the characters overlap each other. So make the intersection 100% and the parts of the alpha that don't overlap be 50% transparent, etc..

Any help would be greatly appreciated!

Doug


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Tudor "Ted" JelescuRe: alpha channel question
by on Jun 17, 2011 at 10:16:25 pm

I'm not sure I completely understand what you're trying to achieve, but here's an idea:
Use the alpha from each clip as matte (channel/set matte) on a white solid layer(one for each clip). Set both white solid layers to 50% opacity and render over black. The result is your alpha, where the two intersected shapes are at 100% and the rest at 50%.

Tudor "Ted" Jelescu
Senior VFX Artist


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Doug WolfRe: alpha channel question
by on Jun 17, 2011 at 10:27:59 pm

I did try that, and I must be doing something wrong, cause when the white solids are set to 50%, the intersection isn't 100 white. It is closer to to white than the non intersecting parts.. but not 100% white.


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Tudor "Ted" JelescuRe: alpha channel question
by on Jun 17, 2011 at 10:36:15 pm

Did you comp the white shapes on a solid black background and then render, and then use the final?

Anyway, here's another method I thought of: render the two at 100% separate on black. Use one as the alpha for the other to comp again on black and render. You'll get the intersection alpha this way.

Tudor "Ted" Jelescu
Senior VFX Artist


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Doug WolfRe: alpha channel question
by on Jun 17, 2011 at 10:43:35 pm

Hey Ted,
Thanks for all the help! I found out what was missing. It only works if the topmost white solid is set to Add. Then I get pure white in the intersection.

Thanks!

Doug


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Todd KoprivaRe: alpha channel question
by on Jun 18, 2011 at 7:58:57 pm

See my post here:
http://forums.creativecow.net/readpost/2/987050

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Todd Kopriva, Adobe Systems Incorporated
Technical Support for professional video software
After Effects Help & Support
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