Setting of growing directions of dots in a dither matrix is facilitated. During defining of the growing directions of the dots corresponding to cells in the dither matrix, a target cell in the dither matrix is defined. When the growing directions of the dots in cells located within a range of one cell from the target cell are not set, the target cell is set as a reference point. When the growing direction of the dot in the cell located at the immediate left or the immediate right of the target cell is set, the growing direction of the dot in the target cell is set based on setting states of the growing directions of the dots in the cells located at the immediate left and the immediate right of the target cell and the reference point.