A method of designing a double patterning mask set includes dividing a chip into a grid comprising grid cells; and laying out a metal layer of the chip. In substantially each of the grid cells, all left-boundary patterns of the metal layer are assigned with a first one of a first indicator and a second indicator, and all right-boundary patterns of the metal layer are assigned with a second one of the first indicator and the second indicator. Starting from one of the grid cells in a row, indicator changes are propagated throughout the row. All patterns in the grid cells are transferred to the double patterning mask set, with all patterns assigned with the first indicator transferred to a first mask of the double patterning mask set, and all patterns assigned with the second indicator transferred to a second mask of the double patterning mask set.