1. An image processing method for generating a halftone image by forming a halftone dot represented by a set of one or plural output dots corresponding to an intensity of an input image signal while making a part of the dots constituting the halftone dot to be an actual non-output dot so as to reduce an amount of a coloring material of a halftone-dot portion, the image processing method comprising: holding (1) halftone-dot profile data representing a threshold-value set for forming the halftone dot corresponding to the intensity of the image signal and (2) gap-size profile data representing a gap size, which is a size of a set of one or plural non-output dots representing the actual non-output dot; correcting threshold values, which are other than a threshold value corresponding to the intensity of the image signal to be processed and are in the halftone-dot profile data, by referring to the gap size of the gap-size profile data, the gap size corresponding non-linearly to the intensity of the image signal to be processed; comparing the corrected threshold values and the intensity of the image signal to be processed; and supplementing the halftone dot with the output dots to an outside of the halftone dot on the basis of the number of the non-output dots so as to compensate the gap represented by the non-output dots.