[vtkusers] Actor Transparency

[Richard Haney]

Does anyone know of a quick way to render just part of a vtkActor 
transparent?  I have a triangular geometry rendered from a 
vtkXMLUnstructuredGridReader and I would like only certain sections of the 
total actor to be solid and the rest transparent.

Is there a quick and easy way to do this with vtk4.2 ?

Thanks in advance for any help/hints.