the image data by a dither matrix of fig5 due to this for instance when the concentration levels of the input image data are continuous at 17 the processing result is as shown in fig7 the dither matrix of fig5 is a dither matrix for a low resolution the dither matrix of fig5 comprises four small dither matrices of a 0 0 b 1 0 c 0 1 and d 1 1 each of the small dither matrices is a spiral type dither matrix in which threshold values are sequentially spirally arranged from the center coordinate values i j correspond to numerical values which were written every four pixels for instance in the matrix of a 0 0 the threshold values sequentially increase from the center in a manner such that 1 u2192 5 u2192 9 u2192 13 u2192 17 u2192 u2192 53 u2192 57 u2192 61 on the other hand the average values of the small dither matrices sequentially increase such that a 0 0 u2192 d 1 1 u2192 b 0 1 u2192 c 1 0 that is since the dither matrix for a low resolution is constructed by using a plurality of small spiral type dither matrices if the input image data concentration level 17 of 8 u00d7 8 is binarized by using such a large dither matrix of 8 u00d7 8 four mesh points.