1. An image forming apparatus, comprising: a stamp shape data generation unit that configured to generate stamp shape data indicating a shape of a stamp image, wherein the stamp image includes stamp color data indicating a color of the stamp image; a stamp combining unit configured to (i) generate planes of the stamp shape data corresponding to color components of the stamp color data, wherein a color density of each plane of the stamp shape data is equal to a color density of a corresponding color component of the stamp color data, and (ii) combine the planes of the stamp shape data with respective planes of image data to form a combined image; and an output execution unit configured to execute an output operation on the combined image.