[vtkusers] transparency in multi-layer render window

[francesco caruso]

Hello
I need some hints to realize occlusion...
I put in a window two renderers, one to display frame grabbed by a 
webcam, and the other to visualize a dataset.
Both the renderers fill render-window entirely so I used two layers.
I set the dataset's renderer in the layer 0(the transparent one) and the 
image renderer in the layer 1.
Virtual objects are superimposed to the image grabbed by the webcam, 
like I want.
But I'd like occlude (i.e. hide) the virtual objects when they occupy 
certain region of the window...

How can I do?????

Thanks in advance