Patent ID: 7471421

Claim:
A method of generating a threshold matrix which is used for generation of a halftone dot image representing a grayscale original image, comprising the steps of: a) arranging a plurality of highlight-side dot centers and a plurality of shadow-side dot centers alternately at regular intervals in predetermined two arrangement directions in a matrix area for storing a threshold matrix; b) setting reference points as many as or less than a quarter of the number of said plurality of highlight-side dot centers and said plurality of shadow-side dot centers, to be distributed substantially uniformly in said matrix area; c) rotating each dot center included in said plurality of highlight-side dot centers and said plurality of shadow-side dot centers about a nearest reference point; and d) determining threshold values in said matrix area so that halftone dot areas are changed around said plurality of highlight-side dot centers after being rotated in accordance with variation in gray level on a highlight side and a halftone dot area is changed around said plurality of shadow-side dot centers after being rotated in accordance with variation in gray level on a shadow side.