code: def extract_geometry(dataset):
          
          alg = vtk.vtkGeometryFilter()
          alg.SetInputDataObject(dataset)
          alg.Update()
          return _get_output(alg)
docs: Extract the outer surface of a volume or structured grid dataset as
          PolyData. This will extract all 0D, 1D, and 2D cells producing the
          boundary faces of the dataset.