1. An image data generation method embedding data to be embedded in original image data to generate synthesized image data, said synthesized image data being data for forming an image by an image formation apparatus having a difference in performance between a main scanning direction and a vertical scanning direction in reproducibility for a line, the method comprising the steps of: generating watermarking image data from said data to be embedded, said watermarking image data being data that said image formation apparatus is caused to generate such that a first dot having pixels arranged two dimensionally and a second dot having pixels arranged one dimensionally in one of said main scanning direction and said vertical scanning direction that allows higher performance in reproducibility for said line having their respective center positions substantially equidistantly; and synthesizing said original image data and said watermarking image data together.