A dither mask is used in a halftone process that is performed by the printer that includes the preceding and following heads. The dither mask includes preceding and following head storage regions. The preceding head storage region is used for a printing portion of the preceding head. The following head storage region is used for a printing portion of the following head. The dither mask is generated by determining, using comprehensive evaluation values, storage elements in which thresholds are to be stored under the condition that dots are permitted to be formed by the preceding and following heads at the same position in the region corresponding the overlapping regions.