CGTalk - Effect-only layer

alona

04-17-2005, 06:30 PM

Hi,

Here's the scenario: I'd like to export a movie that ONLY has a "lens
flare" effect, say, with all the rest being fully transparent.
I create a new composition, new layer, solid black, render a lens
flare, animate it a bit. This gives a nice flare but I do have the
solid black object there which I don't want; I'd like to turn it into a
transparent color. On the other hand, if I don't create that solid
black - for instance, if I use a null object only - then I don't get
the lens flare at all. I suppose the effect needs something to "work
on"; fine, but I would like to be able to separate the effect from the
object it's applied to, and cannot figure out how.

One possible technique would be to color-pick all the black pixels and
make them transparent. This is far from ideal though, since the effect
itself may contain some "truly" black pixels which I DON'T want to
change. Anyway I'm also curios to know how to do that, I can do it in
Photoshop but I'd like to do it inside AE.

I tried playing with the blending modes but this didn't help.

Any tips would be greatly appreciated.

AA

AAAron

04-17-2005, 07:48 PM

This is the best technique I can come up with..

1. Take the layer you have with the lensflare on a black solid, duplicate it.

2. Put an hue saturation filter on the original layer, slide the saturation to 0. you should have a lensflare in greyscale against a black bg.

3.Precomp the original layer with a atributes moved along.

4.Precomp the duplicate layer with a atributes moved along.

5.Put a set matte filter on the precomp of the duplicate layer. Use the precomp of the original layer a source, then set the chanel to luminance.

6. Hide the precomp of the original layer.

dwkim

04-17-2005, 09:03 PM

Try using a screen blending mode. You won't need an alpha that way.
Screen is sort of like lighten, but instead of comparing for lightness (value) it's an additive transfer mode. black = 0,0,0 so when you screen (or add) black pixels on top of anything, the black pixels don't change any values. but the lighter pixels (the rings in the flare) will add their value to whatever pixel it's on top of. Fire and other "glowing" elements can be treated the same way.

alona

04-18-2005, 12:14 AM

AAAron: Thanks for the detailed explanation, I'll try that asap! If I run into trouble I'll let you know...

dwkim: The movie needs to leave AE with an alpha channel for use in other applications that do not have a screen blending mode. That's why I do need an actual alpha channel in the final comp. I agree that it's a bit awkward but there's not much I can do about it; within AE it would have been very easy, indeed, to keep the black solid and blend away.

Thanks again!

AA

alona

04-18-2005, 08:15 AM

AAAron: thanks again, that was a great idea and it works perfectly.

AA

deano

04-18-2005, 02:46 PM

Maybe you could try Knoll Unmult to get the alpha channel for the lens flare

http://www.redgiantsoftware.com/unmult.html

OR

XMult

http://www.fandev.com/xmult.html

Lemme know if its helps u out and if it is a faster method

Cheers,

Deano.:thumbsup:

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