[vtkusers] Seems have bug in vtkPolyPlane...

[hellow]

Hello all:
    I want to cut a surface model to extract intersection lines, I uses a line extrude along z-axis as cut plane.
currently, I have done a test for this task:
   The surface model is consisit of a planar 2x2 rectangle grid that splitted into 8 triangles. and the cut-line
consist of two points. I have using two methods to cut Model and got different result. 
    Sorry for my poor english, and the source code I have attatched. The result from vtkPolyPlane is not correct
and the result from 3-planes is right.
    I using VTK-5.10.1.


  thanks all.


Liangaibin



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