Method for generating threshold patterns and gradation image binarizing device

A gradation image binarizing device comprising:

• a threshold pattern storing section used to store a threshold pattern for binarization;
• a gradation image storing section used to store a gradation image to be binarized by said threshold pattern;
• a parameter storing section used to store a rotation parameter obtained from a screen angle after correction which is carried out in a manner such that an origin is set for said pattern and real-number coordinates of an origin of an adjacent threshold pattern are obtained by rotating said threshold pattern relative to said set origin by a predetermined screen angle and such that coordinates of said origin of said adjacent threshold pattern are corrected so that said real-number coordinates are changed to integer coordinates by rounding fractions of said real-number coordinates and an angle which a segment connecting said set origin and the origin of the adjacent dither pattern obtained after said correction forms with a horizontal line is selected as a screen angle;
• and
• a means, responsive to said threshold pattern and a rotation parameter stored in said parameter storing section, for binarizing said gradation image.