A plurality of types of positional shift postulated patterns are postulated wherein the positional shift of print head tips in a printer are postulated to have positional shift directions and distances. For each of the postulated positional shift postulated patterns, a base dithering mask is generated considering the ability to produce excellent dot dispersion characteristics when this postulated positional shift has occurred. Additionally, optimal dithering masks are generated through multiply combining the sets of threshold values that comprise the base dithering masks. Doing so enables the suppression of degradation of printed image quality, even when there are positional shifts between the print heads, in a half toning process for printing using a printer that is provided with a plurality of print heads that are arranged in a line across a printing range.