[vtkusers] Picking points in a volume

[Robb Brown]

I have a volume rendering of a dataset that I can crop (hardware 
cropping).  I would like to determine the scalar value at a point the 
user clicks on.

I've tried vtkPointPicker but it always returns a point on the surface 
of the bounding box of the uncropped volume.  I can get the pick 
position using vtkPropPicker.GetPickPosition() but what space are these 
coordinates in?  The documentation says they're global coordinates 
(world?) but if I rotate the volume and click on the same point I get 
different z values.

Anybody know how to do this?

Thanks,

Robb
_____________________________
Robb Brown
Seaman Family MR Center
Calgary, AB