[vtkusers] how to get volume or surface area of a vtkImplicitFunction ?

[michal you]

I already tried one way:
   
vtkImplicitFunction-->vtkSampleFunction-->vtkContourFilter-->vtkPolyData-->vtkMassProperties
but the result correctness largely depends on the sample dimension and 
position,sometimes it is even wrong result.
   I am wondering if there is better way to get volume of a 
vtkImplicitFunction or vtkImplicitBoolean.thanks for any suggestions..:)

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