1. An image forming apparatus, comprising: a stamp shape data generation unit 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 image data to form a combined image, wherein the image data has planes each corresponding to a color component, and wherein the stamp combining unit is configured to combine the planes of the stamp shape data with corresponding planes of the image data to form the combined image; and an output execution unit configured to execute an output operation on the combined image.