1. A computer image processing method, the method comprising: providing a computer having an image input apparatus, a storage section, a segmentation process section and an image output apparatus; providing to the storage section an image comprising a plurality of pixels; segmenting the pixels into one or more segmentation classes to which each pixel to be processed belongs; performing a dither process using a dither matrix of a same configuration and a same size with a plurality of pairs of threshold values based on the segmentation class to which each pixel belongs, wherein according to the result of determination of the segmentation class a first dither processing is performed on one part of the dither matrix and a second dither processing is performed on another part of the dither matrix, the first dither processing being different from the second dither processing, and to perform the dither processing, any of a plurality of pairs of threshold values is selected according to the result of determination of the segmentation class, and the selected pair of threshold values are compared with a value of the pixel to be processed, which are set so that in correspondence with each position of the dither matrix of the same configuration and the same size, a different dither processing is performed according to the result of determination of the segmentation class on part of the dither matrix and a same dither processing is performed irrespective of the result of determination of the segmentation class on a remainder of the dither matrix.