1. An image processing method of converting a binary image into a grayscale image, comprising: generating a plurality of lookup tables having gray level values assigned to a predetermined pattern of the binary image such that the plurality of lookup tables corresponds to frequency components of the binary image, wherein the plurality of lookup tables includes an image lookup table corresponding to an image having a low frequency component and a text lookup table corresponding to an image having a high frequency component; and converting the binary image into the grayscale image by retrieving a gray level value from one of the plurality of lookup tables that corresponds to one of the frequency components of the binary image, wherein the converting the binary image into the grayscale image includes retrieving the gray level value from one of the image lookup table or the text lookup table according to a text ratio of the binary image, the text ratio being a ratio of a frequency at which a pattern to be processed occurs in an overall training image to a frequency at which the pattern to be processed occurs in a training image having the high frequency component.