1. An image forming apparatus comprising: an image-data acquiring unit that acquires image data composed of a plurality of pixels; a storage unit that stores a position in the image data and a density of each of the plurality of pixels; a resolution converting unit that converts a resolution of the image data into converted image data with 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 pixels of the converted image data shifted in a sub-scanning direction from the main-scanning directional positions; a density determining unit that determines a density of the additional pixel based on a density of a pixel of the plurality of pixels located at a respective position in the image data which is stored in the storage unit and corresponds to the position of the additional pixel; a correcting unit that performs a correction process of adding the additional pixel to the converted image data; and a scaling unit that controls the position determining unit and the correcting unit so as to scale the converted 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; wherein the pixel of the plurality of pixels located at the respective position in the image data which is stored in the storage unit and corresponds to the position of the additional pixel is a pixel of a reference image.