1. An image forming apparatus comprising: a shape information acquirer configured to acquire shape information indicating a shape of a three-dimensional object on which an image forming process is to be performed; a surface determiner configured to determine a plurality of surfaces from which the three-dimensional object is constructed, based on the shape information; and an output image information generator configured to generate surface image information and join region image information, the surface image information being information on images to be formed on respective surfaces, each image being of a region obtained by cutting an end of a corresponding surface by a predetermined amount, the join region image information being information on an image to be formed on a join region between adjacent surfaces, wherein the surface determiner is configured to determine that the adjacent surfaces of the three-dimensional object are different surfaces when an angle formed between the adjacent surfaces of the three-dimensional object is equal to or smaller than a predetermined angle.