[vtkusers] Need a fast way to identify pixels

[Erik Sobel]

Does anyone know of a FAST way to identify the actor under a particular 
RenderWindow pixel.
I am currently using renderer->PickProp(x,y) but it is too slow.

I have speedy access to the window's z-buffer with...
renderer->GetRenderWindow()->GetZbufferData().
Is there anyway to get a pointer to a buffer that identifies the actor 
under each pixel in a similar way?

Thanks,
Erik