1. An image forming apparatus comprising: a halftone processor that applies a halftone process to an input image signal with a predetermined halftone processing period; an add-on processor that attaches, to the image signal subjected to the halftone process, an add-on signal with an add-on period which is determined according to the halftone process period; and an image forming unit which forms an image on a recording medium, based on an output of the add-on processor.