View Full Version : Help. Compositing / Glow / Alpha Channel Question

01-15-2004, 08:33 AM
Hey all, I am stuck again.

Here is the situation:

I have filmed a live action shot of a newsreporter doing a on the spot report (greenscreened)

I have composited this in After Effects with a background of a building, trees and landscape. (its all good so far)

I have created in Lightwave spacecraft flying across the horizontal plane with engines glowing. (still ok)

I attempted to bring the footage of the spacecraft into After Effects to composite it together. I want the spacecraft to fly behind the buildings and the reporter, but in front of the trees. Easy in After Effects because I just mask out the buildings and reporter. However . . .

If I render out my lightwave footage with glow (millions of colours) I can't key out the black background and make them appear cleanly in the comped footage (the glow messes it up). If I render out my lightwave footage with glow (millions+ colours) I get exactly what I want when comped together except NO glow. I think this is because it puts the glow in the Alpha Channel.

I am really stuck, and haven't figured out a way to make this work. I have tried rendering out a separte alpha channel for the glow and putting it in After Effects, but (i dont' know how firstly) it doesn't seem to work.

Any ideas on what to do? Any methods to make this easier?

PS. I tried comping it together in Lightwave, but couldn't figure out how to mask out the reporter and the buildings.

What am I missing? I am a big Noob, but I have read the manual and Inside Lightwave, and still can't figure out a way around this problem.


Thanks in advance.

01-15-2004, 12:42 PM
Lightwave does not add Glow and Lens Flares to the alpha channel. Your best bet would to use the Flare2Alpha LScript or Al Streets Post Alpha Image Filter. They will premultiply the light effects based upon brightness and add that info to the alpha. You could do the same in After Effects using a Filter (Walker Effects Premultiply, XMult, Knoll Unmult). If all that fails, render an extra pass with your glowing elements and black objects (use Matte object on the object property panel or change the surface properties) and then use the screen mode to put your glow on top of the other effects in AE. Yet another way would be to render your glow elements without glow and add the glow in AE with the respective filter. You se, a lot of options, you just have to think a little "around the corner".


01-15-2004, 07:57 PM
uhm, the best solution is render a separate pass with light and luminosity render, than in after effects you can compose and add glow effects that you need.