[vtkusers] checking visibility of a vtkActor

[Juan Cardelino]

I've been looking for some time a way to determine if an actor is
visible or not in a scene. So far I've been unsuccesful and I'm about
to implement it myself, but I want to be sure before. What makes me
doubt is the behaviour of the vtkRenderWindowInteractor. When you
Interact with a vtkRenderWindow and you press the 'r' key, all actors
are made visible. So it has to compute their visibility in some way.
Can anyone give me a hint on where to look.
Thanks in advance,
                            Juan