painting on alpha with blending modes?

[Aharon Rabinowitz]

I’m really not clear on what you are attempting to do here, but I don’t believe you can effect the alpha channel that way, though I’m not sure. I’ve never seen that done in AE before. Maybe in photoshop?

If you are trying to fill in some of the transparent areas in your alpha channels, maybe try applying the effect called Effect > Adjust > Levels, and set it to adjust the alpha channel (By defualt it is set to RGB). then try moving the arrow sliders (Especially the Gamma Correction – the gray one) closer to the white area.

Yoiu can use this effects to tell the grayer areas of your alpha channel to become darker or lighter (more transparet or opaque).

—————————————-
Aharon Rabinowitz
aharon(AT)yahoo(DOT)com
http://www.allbetsareoff.com
—————————————-
Creative Cow Master Series DVD
particleIllusion Fusion Volume 1
available @ http://www.pIllusionFusion.com

[Peter O’connell]

Ok I’ll explain in a little more detail. If I key out a background from a lousy green screen or if I am pulling a luma key, I often get an alpha channel that isn’t as black and white as I would like it. I would rather not use levels etc… in many situations because often if I am happy with the matte in a certain portion, I don’t want the levels adjustment to affect that area, only the problem area. What I am able to do in photoshop and combustion is paint on the gray areas of the alpha channel dodging the light grays to white and burning the dark grays to black. In photoshop for example, if you create a so-so alpha using calculations you can clean it up in this way by selecting the alpha channel only and painting on it with the brush set to the burn and dodge for darks and lights respectively. After effects offers this ability in theory because when I have the brush tool selected, I can choose from many blending modes for my brush, only they all seem to be applying as normal not as the blending mode to which they are assigned. Maybe it’s a bug or am I just missing something?
Any advice would be greatly appreciated
Pete

[Aharon Rabinowitz]

Since even on star wars they have to clean up their keying, you’re going to have to do a little work. There’s no free greenscreen ride. Serioulsy.

If you try yo key out the entire shot, instead of an area of focus (you actors for example), you will only make yourself miserable.

First things first – Create an animated junk matte around the area of focus. In other words – if you have a person on a green screen, before you key them, create a loose animated mask around them that moves with them. With the mask set to “add,” everything outside the masked area will be invisible. Then key that. This way, you only have to worry about the stuff closest to the actor.

Since your stuff is bad, there are probably changes in color and light of the BG – in that case, you may need to keyframe the keying effect’s properties to change over time.

You may still need to roto it a bit and use levels. Bad footage is bad footage, no matter how you spin it.

OK, now that I’ve said that – let me make a suggestion that works sometimes, and sometimes not:

Select your green screen footage, and add the Keylight effect to it. Get the best key you can – it doesn’t need to be perfect.

Next, with the layer selected, choose Layer > Auto Trace

Tell it to use the work area and set the channel to alpha.

This creates several animated masks on the layer that when combined make a very tight junk matte.

Set all of the masks to “add” – you may have to make only the first one add and the rest subtract or vice versa. Play around until you get what you need.

Set all of the masks to be feathered about 10 pixels

Set all of the mask expansions on the masks to at least 5 or 10 pixels.

Play with all of these values and see what you get. You may want more feathering and expansion.

This often works, but sometimes doesn’t. It’s really iffy, but iof you get lucky, You should have a tight key+junk matte at this point.

You may need to actually duplicate the layer (so you have 2 exact copies, one on top of the other) so that it fills in some of the more transparent areas. Or you could try to precompose it and use a levels effect as I described previosly.

—————————————-
Aharon Rabinowitz
aharon(AT)yahoo(DOT)com
http://www.allbetsareoff.com
—————————————-
Creative Cow Master Series DVD
particleIllusion Fusion Volume 1
available @ http://www.pIllusionFusion.com