Alpha Channel Background

[Johan]

I want to compose with alpha channel as background, say 50% of the screen. Is there any simple way to do that in AE?

[Jive]

Not sure what you're asking here. You want to composite something over a background? Please clarify.

[PurposeDriven]

Do you mean you want the black in Background to have an alpha? In that case... it is already there. Just specify in your Render Que to export rgb+alpha and your NLE will pick it up. May need to render in your NLE as well...

But if that's not what you are looking for, please clarify. ;D

[Johan]

Hi all,
Thanks for replying :)

I want to compose a ring for example, with alpha channel inside the inner circle, black background outside the outer circle

[kai]

if you're wanting to have (say some footage playing inside a circular shape), you could use track mattes.

[Johan]

Thanks Kai,

I understand it, but what I need is different.
I want to make a composition, with some part consist of alpha channel, then I export as avi.
The reason is that someone else will do the rest on NLE, and I want that they just simply drop the avi to the time line on upper layer to create masking effect.