An image that is robust against position shift and that has improved graininess is formed. In the embodiments, a halftone processor creates a first dot arrangement and a second dot arrangement from related weighting functions and weighting maps (S1508, S1509), evaluates the image quality when the first dot arrangement and the second dot arrangement are overlapped and combined (S1511, S1512), updates the weightings for only pixels where a dot is arranged in case where the evaluation results in the evaluating step are not within a specified range (S1513), obtains a first dot arrangement and a second dot arrangement that were recreated based on the updated weightings (S1508, S1509), and repeats steps S1508 to S1514 until it is determined that the change in image quality due to position shift when the recreated first and second dot arrangements are overlapped and combined is within a specified range.