[vtkusers] Getting Depth of points of being displayed

[Syed Hassan Amin]

Hi,
 I display a 3D face surface in a window in a particular pose. Now , 
depending on pose , I want to get z-component of points that are 
currently being displayed. I am not exactly sure how to do that. Anyway 
, when I try to use GetZ function of vtkRenderer or GetZBufferData 
function of vtkRenderWindow , my window simply hangs without displaying 
anything. Can any body guide about this.

Thanks,

Hassan