In a matrix area (720), a plurality of highlight-side dot centers (731) and a plurality of shadow-side dot centers (741) are regularly arranged and a plurality of reference points (751) are set while being distributed almost uniformly. Each of the dot centers (731, 741) is rotated about a nearest reference point (751). Threshold values of the matrix area (720) are determined so that halftone dot areas should be changed around a plurality of highlight-side dot centers (731) after being rotated in accordance with the variation in gray level on the highlight side and a halftone dot area (i.e., joined dot areas) should be changed around a plurality of shadow-side dot centers after being rotated in accordance with the variation in gray level on the shadow side. In a halftone dot image generated by using the matrix area which is thus generated, it is possible to suppress moire and graininess.
