code: def flip_normals(self):
          
          if self.faces.size % 4:
              raise Exception('Can only flip normals on an all triangular mesh')
  
          f = self.faces.reshape((-1, 4))
          f[:, 1:] = f[:, 1:][:, ::-1]
docs: Flip normals of a triangular mesh by reversing the point ordering.