[vtkusers] cutting or extracting surfaces (please help)

[Theodore Papatheodorou]

Hi all,
I have been trying to write a program that will extract part of a 
cylinder. I define a box as an explicit fuction but I can't get it to 
work. The problems are that vtkCutter does not extract the whole 
geometry but instead just where the implicit fuction intersects the 
other surface.
So I tried to use vtkExtractGeometry which takes all of the surface 
inside the area defined by the explicit function. That filter however, 
does not output the data as polydata.
Any suggestions?
The problem in simple terms is that I have a surface and I want to cut 
and keep half of it. Say I have a sphere, I want to keep half of it, as 
a polydata. Could someone give me some pointers before I waist my youth 
on this?
theodoros