1. An image processing method comprising the steps of: performing resolution conversion processing on input image data to increase the number of grayscale levels; performing edge enhancement processing to increase the number of grayscale levels of the input image data; performing dithering processing to reduce the number of grayscale levels of each pixel of the input image data; and determining the number of pseudo grayscale levels based on a combination of a parameter concerning the resolution conversion processing and a parameter indicating the level of the edge enhancement processing.