[vtkusers] get points in a volume

[Thomas Faust]

Hi VTK Users (and math PhD's)

I'm looking for a computer graphic algorithm, that I need in VTK.

I'm rendering a volume that is created by vtkImageData. (To make it easy: 
spacing is 1.0 in each direction and the origin is 0,0,0)

The algorithm that I need has to return all points (from vtkImageData) that 
are inside a hexahedron.

Background: The user draws a rubberband rectangel on the screen. Depending on 
the projection mode, I end up with a 4 faced pyramid on perspective 
projection or a cuboid in parallel projection. This volume gets cut by the 
clipping planes.
I want to know, which points of the volume (vtkImageData) are laying in the 
space the user selected.

I'm appreciate every help.
Thanks