1. An image forming apparatus, comprising: a noise adding unit configured to change pixel values of image data corresponding to a print image with a noise pattern; and a controller configured to control a printing device to perform printing of the print image in which the noise pattern has been added; wherein the noise pattern is a pattern obtained by two-dimensionally arranging plural noises of which each has a local planar distribution; and each of the plural noises has a local planar distribution in which pixel values monotonically vary with a distance from a pixel having a peak pixel value; wherein the plural noises include a positive value noise and a negative value noise; the positive value noise has a local planar distribution in which pixel values monotonically increase with a distance from a pixel having a positive peak pixel value; and the negative value noise has a local planar distribution in which pixel values monotonically decrease with a distance from a pixel having a negative peak pixel value.