Alpha Channels Dissappears after Applying a Mask

#1

I don’t see this question answered, so pardon me if it’s covered in a previous thread I can’t locate.

I have a scene with 3 aircraft flying toward the camera. There are 3 vehicle passes: color(32bit TGA w/Alpha), spec(24bit TGA), and a propeller blur(32bit w/ alpha). Each pass contains all 3 planes. The problem is that the planes weren’t animated with enough bounce to simulate mild wind turbulence, so I want to add some movement to each aircraft separately.

I made three nested comps(one for each craft). I applied a mask at the sub-comp level for each aircraft that masks out the other 2 planes. I figured I could then add a unique bit of animation to each plane independently.

The problem is that when I apply the mask downstream from the 3 passes, Combustion seems to ignore the existing alpha channel information in the source footage. What am I missing here? This seems a relatively basic task.

Gooch

#2

I believe that when adding masks you would be “annulling” the original alpha channel. You can use the Compound Alpha or set matte op to reestablish the Alpha channel.

#3

I’m not in front of combustion to check how the buttons are labelled but a gmask has two “apply modes”. One local (that is to say how several shapes of one gmask operator relate to each other) and a global apply mode which says how gmask and existing alpha should be combined. You would have to go for “add”.

Edit:
Sorry, did not read carefully enough. If you want to mask out the other planes (drawing a mask around those you want to get rid of, or one around the one you want to keep with “invert” on…you get the idea :argh: ), set local mode to add and global mode to subtract

-k

#4

Hey thanks for the tips. I did manage to get the compound alpha operator to work last night, but I’m going to try switching the local and global settings on one of the other plane subcomps to try that option as well. I figure it can’t hurt to have more than one way doing things.

#5

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