1. A smoothing method comprising the steps of: (a) selectively outputting one of an extracted window pattern which is obtained by extracting a region having a predetermined size with a target pixel formed by each pixel of an input image data, and an inverted window pattern which is obtained by inverting a polarity of each pixel of the extracted window pattern; (b) collating an output window pattern which is output by said step (a) with a plurality of collating templates, and outputting corrected dot data corresponding to one of the collating templates matching the output window pattern; and (c) selectively outputting one of the corrected dot data output by said step (b), and an inverted corrected dot data which is obtained by inverting a polarity of each dot of the corrected dot data.