code: def facets_normal(self):
          
          if len(self.facets) == 0:
              return np.array([])
  
          area_faces = self.area_faces
          # sum the area of each group of faces represented by facets
  
          # the face index of the first face in each facet
          index = np.array([i[area_faces[i].argmax()]
                            for i in self.facets])
          # (n,3) float, unit normal vectors of facet plane
          normals = self.face_normals[index]
          # (n,3) float, points on facet plane
          origins = self.vertices[self.faces[:, 0][index]]
          # save origins in cache
          self._cache['facets_origin'] = origins
  
          return normals
docs: Return the normal of each facet
  
          Returns
          ---------
          normals: (len(self.facets), 3) float
            A unit normal vector for each facet