In order to form a high-quality image, upon execution of multi-pass printing using a printing head with a plurality of nozzles, an image processing apparatus calculates, using a scan duty setting unit (105), scan duty data for respective nozzles for each scan of the printing head in accordance with input image data. A halftone processor (108) generates a dot pattern to be formed by applying N-ary processing. The constraining condition information is set so that the phase of spatial frequencies of a next dot pattern to be generated has an opposite phase relationship in a low frequency region with respect to the already printed dot pattern.