In an image forming apparatus and a computer program, extraction of information about distortion in a charged particle beam scan area can be implemented. An image forming apparatus integrates image data obtained by a charged particle beam apparatus and calculates, from a plurality of images with different scan directions of the charged particle beam apparatus, first information about the amount of change in a feature quantity in accordance with the time of irradiation of the charged particle beam, second information about the amount of change in the feature quantity before and after a change in beam scan direction, and/or third information about a position error of a pattern on the image before and after the change in beam scan direction.