[vtkusers] perspective transform in vtkCamera

[francesco caruso]

hello vtkusers

I've noticed that in every instance of vtkCamera there are two protected
variables of type vtkPerspectiveTransform:
- vtkPerspectiveTransform* PerspectiveTransform
- vtkPerspectiveTransform* Transform

I'd like to understand what are the meaning of this two variables
and how they are concatenated to obtain the effective projection matrix.

Thanks in Advance