1. A method of generating a sketch image of an input image, the method comprising: calculating a representative value corresponding to brightness of a pixel block, the pixel block including a specific pixel of an image and at least one of the pixels of the image which surround the specific pixel; determining whether an input gray scale value of the specific pixel is higher than the representative value corresponding to the brightness of the pixel block; and outputting an output gray scale of the specific pixel, which is adjusted, based on a result of the determining, wherein the outputting comprises outputting the output gray scale of the specific pixel having a maximum gray scale if the input gray scale value of the specific pixel is higher than the representative value, and wherein the outputting comprises outputting the output gray scale of the specific pixel having a value lower than the input gray scale value in response to determining that the input gray scale value of the specific pixel is below the representative value.