[vtkusers] How to get silhouette/mask of rendered polydata?

[Michael Rice]

I am rendering a vtkImageData as the background image and a  
vtkPolyData on top of it. What I need to do is get the elements from  
the vtkImageData that are beneath the rendered vtkPolyData. Basically  
I'm trying to use the vtkPolyData to generate a mask for the image.

I thought of using a clip plane on the poly data, but I don't think  
that will take into account the perspective rendering and the pixel  
aspect ratio. Is that correct?

Does anyone have any hints or examples on how I can do this?

Thanks.
Michael