[vtkusers] vtkImplicitDataSet to contouring 2D shape

[man]

Hi all vtkUsers,

I would like to use vtkImplicitDataSet and vtkContourFilter to contouring a 2D shape at various offset.
I hope vtkImplicitDataSet to generate scalar values from a vtkPolyData. This poly data contains a shape in 2D, only lines and points are filled. After, I would like to input into a vtkContourFilter to extract contours but I trap the error message "cannot evaluate vtkImplicitDateSet". 
How can I create an implicit function (to connect into vtkContourFilter) from the geometry of my vtkPolyData. I think vtkImplictDataSet should be a possible way but I miss something....

Thanks a lot. 
Manfred

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