To solve a problem that a halftone to be printed in any color is shielded by a mask pattern and is not printed at all, resulting in an unexpected print.

In a processing system for dither processing of a halftone, command data are analyzed, a foreground pattern and a background color are acquired, an estimated HT-processed image that is obtained when the background color is applied, is generated, the foreground pattern is compared with the created background HT image, and a level of background color omission caused by interference is determined. If the level of the background color omission exceeds a threshold, processing is applied so as to provide a background HT image wherein the level of the omission is equal to or lower than the threshold.