def __undo_filter_average(self, scanline):
        """Undo average filter."""
        ai = -self.fu
        previous = self.prev
        for i in range(len(scanline)):
            x = scanline[i]
            if ai < 0:
                a = 0
            else:
                a = scanline[ai]  # result
            b = previous[i]
            scanline[i] = (x + ((a + b) >> 1)) & 0xff  # result
            ai += 1