code: def extractSurface(image, radius=0.5):
      
      fe = vtk.vtkExtractSurface()
      fe.SetInputData(image)
      fe.SetRadius(radius)
      fe.Update()
      return Actor(fe.GetOutput())
docs: ``vtkExtractSurface`` filter. Input is a ``vtkImageData``.
      Generate zero-crossing isosurface from truncated signed distance volume.