z-buffer use

[Stef Desmet]

Hi,

I'm using following procedure to decide on the visibility of a given point.
Get world coordinates of point -> Transform to display coordinates -> Use a vtkWorldPointPicker
to pick the point with the given display coordinqtes -> Get the world coordinates of picked point
-> Compare both world coordinates.

This procedure works fine on different platfroms (SGI, DEC alpha, Linux) using VTK3.1. (both with
OpenGL and Mesa)
However NOT on our DEC alpha XP1000 with a Powerstorm 300 graphic card. Comparing the returned
values for z in the vtkWorldPointPicker using the GetZ method reveals that this computer returns
totally different values for z.

So my questions. 
   - Can I assume that the value of the z buffer is really related to the distance of
   that point of is it only a relative measure defining what is closer to the viewpoint.
   If it is a real distance (thus equal on all platforms) what could be the reason of the different
   answer from the Powerstorm300. 
   - Is this the most appropriate way of deciding on visibility ?

Thans a lot,
-- 
Stef Desmet                                         
Katholieke Universiteit Leuven                      Tel. +32-16-321041
Departement Elektrotechniek - ESAT                  Fax. +32-16-321986
Centrum voor Beeld- en Spraakverwerking - PSI       Telex 25914 ELEKUL
Kardinaal Mercierlaan 94         eMail stef.desmet at esat.kuleuven.ac.be
B-3001 Heverlee Leuven    URL <http://www.esat.kuleuven.ac.be/~desmet>
B E L G I U M 
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