PROBLEM TO BE SOLVED: To eliminate moire in the case of binarizing a dot image and to speed up processing time. SOLUTION: As to an error caused by binarizing a noted picture element, some picture elements before binarization and in existence in the vicinity of the noted picture element are selected as error sum object picture elements. When the result of binarizing the noted picture element is zero, a maximum value smaller than an upper limit of a picture element value range is selected from an error sum object picture element as a sum object. Then the error is added to the picture element. When the result of binarizing the noted picture element is 1, a minimum value larger than a lower limit of the picture element value range is selected from the error sum object picture element as a sum object. Then the error is added to the picture element. The shape as dots is maintained by adding the error caused in the noted picture element to unprocessed picture elements before binarization in existence in the vicinity of the noted picture element in this way and no moire is produced. When the addition of the error generated from the noted picture element is finished, since the binarization processing with respect to the noted picture element is finished, high speed processing is attained.
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