I'm missing something really obvious here, but what is it?
I want to have a subject casting shadows on a moving clouds background. Here's the clip so far using a method that doesn't works quite well enough :
The shadow doesn't work on the darker parts of the cloud. That's not really so much of a prob in 2D, but this is a stereo 3D project and having the shadow intermittently disappear looks odd.
So, to create a shadow that I can then insert between the two Images, I need to combine the alpha channels from the two videos (one created using green as a key, the other the blue sky).
How do I do that? I bet it's really obvious!
Currently using CS5
Use the Channel Combiner effect or things like Shift Channel to expose multiple channels and merge them differently. Or you could use a pre-comp that has duplicates of your stuff and then use matte and stencil blending modes. It's really just a matter of using one on top of the ther and combining them sensibly...
Stencil blending modes fixed it.