I'm a bit confused about how to use the animated selection created with a rotobrush as a reference for the application of an effect over a different layer.

I'll explain what I'm trying to do, in case the problem is in my process:
I want a person to become translucent. For that, I've got a clip for the background with and without that person over it. I create two layers, one with the person over the one with the background. I rotobrush the person.

Then, I want to create, with that rotobrush selection, a layer that's just white inside the selection and black outside, to use that layer as a reference for a displacement effect over the background layer. And here I find my problems:

- How to change the layer to white over black/transparent? I've done that by touching brightness contrast. Is there a faster/better way?
- How to turn that "white over alpha" layer into a grey degradation from the outside to a white center?
- How to tell AE that I want that layer to be the reference for a displacement (or any other effect, for that matter) on the background layer?