1. An image processing apparatus comprising: a first generation portion that generates a first low-gradation image by performing a first process on an input image to reduce the number of gradations of the input image; a second generation portion that generates a second low-gradation image by performing a second process on the input image to reduce the number of gradations of the input image; a density detector that detects a density of the input image; a determination portion that determines, based on the density, a ratio at which the first low-gradation image and the second low-gradation image are blended together; a mixing portion that blends the first low-gradation image and the second low-gradation image in accordance with the ratio; and a line-appearance-frequency detector that detects a frequency indicating how often a line constituted by a plurality of dots continuously disposed appears, or, indicating how often a line constituted by a plurality of dots regularly and intermittently disposed appears, wherein the determination portion determines the ratio in such a manner that the higher the frequency is, the higher the ratio becomes.