1. A method of controlling an image processing apparatus, the method comprising: obtaining image data expressed by hue, luminance, and saturation components corresponding to the respective pixels using R, G, and B components of an image of a document scanned by a scanner; distinguishing a black character from a scanned image; determining whether a subject pixel of the distinguished black character is an edge pixel or a background pixel of an original image, by comparing the subject pixel with a calculated threshold value based on the maximum and minimum values among pixels in a window on which the subject pixel of the distinguished black character is centered; controlling a print thickness of the distinguished black character based on whether the subject pixel is the edge pixel; and printing out the black character having the controlled print thickness by using only a K color.