[vtkusers] vtkCoordinate, what i am doing wrong ?

[c.kleinhuis]

Hi,

i want to realise some kind of ruler, it should be aligned to screen 
coordinates, e.g. the lower left to the lower right
corner of the display ,  but i am encountering a  severe problem  with 
that i  am using 2  vtkCoordinate objects, which

i place in
SetCoordinateSystemToNormalizedDisplay

with  (  0.1,0)    and (  0.9 ,0),  now i want to calculate the distance 
between these two coordinates in WORLD space,

when using

GetComputedWorldValue(renderer)


i am getting strange results, an the distance between these two points 
differ very strong between several calculations ...


what i am doing wrong? do i need to update the vtkCoordinate object 
somehow ?


thanks
c.Kleinhuis