Get the normal vector of a polygon using vtkPolyDataNormals

I have a simple polygon where all points belong to the same plane, therefore the normal vectors at each point should be the same. The problem I have is that I can’t get those vectors.

For example, I have the following code.

normals = vtk.vtkPolyDataNormals()

normals.SetInputData(poly_data) # existing polydata

From here I do not know how to obtain these vectors. I have tried different ways but apparently I am doing something wrong.

Point normal is computed by averaging neighbor polygon cell normals, so it is expected to be different from the polygon cell normal. See details in Chapter 9 of VTK textbook.

Thanks Andras. My polygon only has one cell, does this also apply in this case?

Can you provide a full example (including setting of poly_data and retrieving of normal vectors)?
What result do you expect and what do you get?

Yes of course.