[vtkusers] picking vtkImageSlice using vtkCellPicker?

[Miro Drahos]

Hi all,
I am rendering an image slice using vtkImageSlice and a surface (vtkActor).
When I perform picking using vtkCellPicker, I always pick the surface 
actor, even if it is in the background, behind the vtkImageSlice.
If I try to pick the vtkImageSlice (and the ray does not intersect the 
surface actor), the Pick() method returns 0, as well as 
picker->GetPath() returns NULL. The vtkImageSlice is set to be pickable.
I am quite baffled by this, since I'd expect to be able to pick 
vtkImageSlice with vtkCellPicker, since it is a vtkProp3D.
If I use vtkPropPicker, I can pick the image slice fine, but I would 
prefer to use vtkCellPicker so that I can specify picking tolerance.
Can someone shed some light on this?
Thank you tons!
Miro