[vtkusers] Render radial trasparency

[vespasiano at libero.it]

Hi all,
I'm writing an augmented reality application where I have two renders 
connected to a render window. 

In the first render, that is on layer 0, I have a plane source where i have 
connected a camera input as a texture to see images from a webcam. In the other 
render, that is on layer 1, I render over the layer 0 my 3d scene. 

Now i need to adjust the opacity of the rendered layer1. I would that the 
opacity of my rendered 3d scene should decrease radially around a center point 
that i define.

I thought that i can use a graylevel map to define the opacity map of my 
rendered scene but i can't find in vtk documentation any ideas to make such 
filter. I need like a filter that i connect between the output of the render 
and the input of the render window and set the pixel opacity. 

Note that i wouldn't modify the opacity of the video but only the opacity of 
my rendered 3d scene.

Any ideas to do so? Thanks a lot. 
Francesco Ricciardi.