Patent ID: 7684081

Claim:
An image processing apparatus comprising: a N-valued conversion unit that converts color component values of plural colors comprised in pixels constituting input image data into N-valued values in a unit of color based on threshold values of plural colors; a dither matrix storage unit that stores one of an n dither matrix in which the number of threshold values constituting a row is n (where n is an integer) and an m dither matrix in which the number of threshold values constituting a row is m (where m is an integer) in a unit of dither pattern, each dither matrix comprising such a configuration that one dither pattern or one kind of dither pattern is repeated, wherein m is greater than n; a counter that holds a value specifying a readout position of a row of the n dither matrix and the m dither matrix stored in the dither matrix storage unit; a reference value storage unit that stores reference values specifying the reference positions of the n dither matrix and the m dither matrix stored in the dither matrix storage unit; a readout unit that reads out the threshold values of the colors used for the N-valued conversion unit from the n dither matrix and the m dither matrix stored in the dither matrix storage unit based on values obtained by adding the value of the counter to the reference values of the dither matrixes stored in the reference value storage unit, and the readout unit that reads out the threshold values from the head of a row of the n dither matrix when the value of the counter is an initial value; a counter updating unit that adds a predetermined value to the value of the counter in response to reading out the threshold values from the n dither matrix and the m dither matrix by the readout unit, and counter updating unit that returns the value of the counter to the initial value in response to reading out the threshold values of a row from the n dither matrix; and an adding unit that adds n to the reference value of the m dither matrix stored in the reference value storage unit in response to returning the value of the counter to the initial value by the counter updating unit.