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: supplementing the halftone dot with the output dots so as to compensate a gap represented by the non-output dot.