code: def fill_percent_done(self):
          
          left = self.left
          right = self.right
          top = self.top
          bottom = self.bottom
          for i in xrange(self.n_chunks):
              self.percent_done.ravel()[i] = \
                  np.sum([left.ravel()[i].percent_done,
                          right.ravel()[i].percent_done,
                          top.ravel()[i].percent_done,
                          bottom.ravel()[i].percent_done])
              self.percent_done.ravel()[i] /= \
                  np.sum([left.ravel()[i].percent_done > 0,
                          right.ravel()[i].percent_done > 0,
                          top.ravel()[i].percent_done > 0,
                          bottom.ravel()[i].percent_done > 0, 1e-16])
docs: Calculate the percentage of edge pixels that would be done if the tile
          was reprocessed. This is done for each tile.