1. An apparatus for generating image data to produce a print sheet in which characters are hidden, wherein the characters appear when the print sheet is reproduced, the apparatus comprising: a generation unit that generates first image data of a size equal to a least common multiple of a size of data representing a bigger dot, a size of data representing smaller dots, a size of data representing a pattern where no dots are to be printed, and a size of data representing a boundary between a layout region of the bigger dot and a layout region of the smaller dots, wherein the boundary is a form of the characters; and a unit that generates second image data for producing the print sheet by repeatedly laying out the generated first image data of a size equal to the least common multiple.