1. An electrophotographic image forming apparatus, using a halftone spot consisting of multiple dot images formed in pixel areas so as to represent a gray scale and to reproduce an image, comprising: a halftone processor for generating image reproduction data for each pixel by referring to a transformation table wherein gray level data for said image and correlated image reproduction data are stored, wherein said transformation table includes: look-up tables, in each of which gray level data and correlated image reproduction data are stored; and a pattern matrix, elements of which are so correlated with multiple pixels in a predetermined area of the image that each indicates a look-up table to be referred to; and wherein said image reproduction data stored by each of said look-up tables includes size data and position data for virtual dots, each of which corresponds to one of said multiple dot images.