Characteristic data representing the metallic luster characteristic of each of a plurality of kinds of printing materials to be used by a printing apparatus is acquired. On the basis of a stimulus value represented by the acquired characteristic data corresponding to each of the plurality of kinds of printing materials, an overlay order is determined to form an image on the printing medium by overlaying a printing material having a smaller stimulus value on a printing material having a large stimulus value. The print data is generated in accordance with the determined overlay order.