code: def fill_n_todo(self):
          
          left = self.left
          right = self.right
          top = self.top
          bottom = self.bottom
          for i in xrange(self.n_chunks):
              self.n_todo.ravel()[i] = np.sum([left.ravel()[i].n_todo,
                                              right.ravel()[i].n_todo,
                                              top.ravel()[i].n_todo,
                                              bottom.ravel()[i].n_todo])
docs: Calculate and record the number of edge pixels left to do on each tile