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.

def extract_geometry(dataset):
        """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.
        """
        alg = vtk.vtkGeometryFilter()
        alg.SetInputDataObject(dataset)
        alg.Update()
        return _get_output(alg)