Image processing method, apparatus and program

A computer readable storage medium having embodied thereon a computer executable image processing program for obtaining a quantized image from a multiple tone image by a computer, the image processing program comprising:

• a first code segment for an image dividing step of dividing an input image into a plurality of image blocks of a unit block size;
• a second code segment for a threshold value matrix specification step of specifying threshold value matrices for the input image with respect to each of the image blocks from a plurality of threshold value matrices including a first threshold value matrix having the unit block size and a second threshold value matrix dividable into a plurality of sub-matrices each having the unit block size;
• a third code segment for a threshold value matrix setting step of setting threshold value matrices and the second threshold value matrix in image blocks for which the second threshold value matrix has been specified as the threshold value matrix, in such a manner that adjacent positional relationships between sub-matrices formed when the plurality of sub-matrices serve as base units and the second threshold value matrix is divided into sub-matrix units are preserved.