masking geo with a full screen texture

I’m trying to make an effect where I have a full screen circle gradient that would mask out a layer to be able to see through it.

ex: I have trees on one layer rendered by it’s own camera. That layer also has a plane with a screen-space shader on it which I would like to use to mask out those trees to be able to see through them at the center. For whatever reason I can’t get this to work. To give you a visual - imagine using a vignette as a mask for a layer’s transparency.

is this at all possible? could it have anything to do with trying to mask out transparency with a deferred render?

hmmm. this seem pretty close actually. That sample shader didn’t seem to work though. here’s a thread that I put up so that you can see an example:

that seems pretty close. I added a url to a thread I started with some images to better explain. I tried the sample shader included but it didn’t seem to work.