def __undo_filter_sub(self, scanline):
        """Undo sub filter."""
        ai = 0
        # Loops starts at index fu.
        for i in range(self.fu, len(scanline)):
            x = scanline[i]
            a = scanline[ai]  # result
            scanline[i] = (x + a) & 0xff  # result
            ai += 1