1. An image forming apparatus comprising: an image-data acquiring unit that acquires image data composed of a plurality of pixels; a resolution converting unit that converts a resolution of the acquired image data into a higher resolution than the resolution of the image data; a position determining unit that performs a position determining process of determining a position of an additional pixel to be added on the basis of main-scanning directional positions of pixels composing the converted image data and respective shift amounts of the pixels shifted to a sub-scanning direction from the main-scanning directional positions; a density determining unit that determines a density of the additional pixel on the basis of a density of a pixel located at a position corresponding to the determined position in the acquired image data; a correcting unit that performs a correction process of adding the additional pixel having the determined density to the determined position; and a scaling unit that controls the position determining unit and the correcting unit so as to scale the image data up or down by causing the position determining unit and the correcting unit to repeatedly perform the position determining process and the correction process with respect to each of sub-scanning lines of pixels and then repeatedly perform the position determining process and the correction process with respect to each of main-scanning lines of pixels.