Patent ID: 8149462

Claim:
A line printer comprising: a plurality of print heads disposed in a line across a printing range; a half toning unit that performs a half toning process, using a dithering mask, on image data; and a printing controlling unit that drives said print heads using results of the half toning process to form an image, wherein: said dithering mask has a pattern generated by: postulating a plurality of types of positional shift patterns each of which represents a shifted or a non-shifted state in specific directions and distances between said plurality of print heads; generating a plurality of base dithering masks, each of which corresponds to each of the plurality of types of positional shift patterns in a one-to-one relation, by considering dot dispersion characteristics that occur in each of the plurality of types of positional shift patterns; and combining a plurality of threshold value sets each of which structures not all but part of one of the plurality of base dithering masks and is included in a different base dithering mask.