[vtkusers] Problem with vtkQuadric (hole)

[vlibertiaux]

Hi everybody,

I am currently experiencing a weird problem with vtkQuadric.  I compute the
non zeros coefficients of a paraboloid non centred at the origin (ax2 + by2
+ cx +dy +ez -1 = 0).

The fit is very good for a large range of input data but I had a strange
result for one of them.  It appeared that the quadric encloses a hole (see
picture)
http://vtk.1045678.n5.nabble.com/file/n4268464/Screen_shot_2011-03-28_at_15.26.41.png 

What could be the reason for this ? 
I want to point out that this is not a rendering problem.  I am computing
the normals wherever the surface is defined and nothing is computed in the
black ellipse.

Thank you very much for your help,

V.

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