Patent ID: 7733531

Claim:
An image processing apparatus comprising: an image data acquiring unit that acquires image data formed of plural pixel data having an M-ary (M≧3) gradation value; a grouping unit that divides the image data acquired into J (J≧2) image data areas and sections the J image data areas into two groups; a pixel data selecting unit that selects, for each of the image data areas forming the respective groups, predetermined pixel data out of pixel data included in the image data area; an N-arization processing unit that applies N-arization (M>N≧2) processing to the pixel data selected based on a predetermined threshold value; and an error diffusing unit that diffuses a difference between a value of the selected pixel data and a value after the N-arization processing for the selected pixel data to pixel data not subjected to the N-arization processing, wherein the predetermined pixel data selected for a first group is selected from among pixels near a first margin of the image and the predetermined pixel data selected for the second group is selected from among pixels located near the opposite margin of the image and diffusion errors associated with pixels in one group are not propagated to pixels in the other group and the error diffusing directions are opposite to one another in the two groups.