1. An image processing apparatus comprising: a first generating unit that generates a plurality of histograms of images with respect to each color based on an input image; a second generating unit that generates output plane information with respect to each color based on the input image; a calculating unit that calculates a plurality of first image densities of the images from at least one of the histograms and the output plane information; a receiving unit that receives a content of image quality adjustment performed on the input image; a determination unit that select one of the histograms and the output plane information to be used based on the content of the image quality adjustment; and an executing unit that execute calculation process to calculates a second image density corresponding to one of the images on which the image quality adjustment is performed, based on the content and the selected one of the histograms and the output plane information.