1. A method for generating optically readable dot image data when binary data corresponds to presence or absence of an optically readable dot and said dot is printed in a virtual cell virtually formed on a printout medium, comprising: a first step of determining the number of pixels constituting said virtual cell as a minimum print unit in a typesetting device used for printing said dot on a printout medium; a second step of determining the number of pixels in said virtual cell associated with printout of said dot in consideration of enlargement of said dot when printed on a printout medium based on said dot image data; and a third step of selecting a composition pattern which minimizes dispersion of pixels constituting said dot image data when a plurality of composition patterns is available as composition patterns for dot image data comprising the number of pixels determined by said second step.