1. An image processing apparatus for outputting an image signal to an image forming device which uses an electrophotographic method, comprising: a generator, configured to generate directional data representing a pattern which indicates a growing direction of a dot on a scanning line and in which the growing direction changes every predetermined number of scanning lines; a setting section, configured to set correction coefficients to be used to distribute image data of a pixel of interest to the pixel of interest and an adjacent pixel adjacent to the pixel of interest in a sub-scanning direction; a detector, configured to detect an edge portion of an image represented by input image data; a distributor, configured to distribute the image data of the pixel of interest of the input image data to the pixel of interest and the adjacent pixel in accordance with the correction coefficients; a corrector, configured to add image data distributed from the adjacent pixel to image data of the pixel of interest on which the distribution is performed, and to output the added image data as corrected image data of the pixel of interest; an selector, configured to select the corrected image data of the pixel of interest and the generated directional data for an edge portion of the image, and to select the image data of the pixel of interest and input directional data supplied together with the input image data for a non-edge portion of the image; and a pulse-width modulator, configured to generate an image signal pulse-width modulated using the image data of the pixel of interest and the directional data selected by said selector, and to output the pulse-width modulated image signal to the image forming device.