[vtkusers] conversion between coordinate systems

[Nurwijaya]

First of all, thank you so much Miss Amy for the help on my previous
question about vtkArcPlotter.

Now I have another unsolved problem due to my lack of experience and
knowledge, I tried to do this :

...
vtkCoordinate coord
  coord SetCoordinateSystemToWorld
  coord SetValue 2 2 2

set pos [coord GetComputedViewportValue myRenderer]
...

But it keeps crashes all the time, it's a classic crash : "This program
has performed an illegal ..bla..bla.." (I use Windows 98) 
As always, I would greatly appreciate any help I can get

Thank you,

Jaya