PURPOSE:To increase the efficiency of coding without reducing picture quality by comparing a binary pattern obtained by binarizing processing using a systematic dither method of a halftone image with a previously processed binary pattern and coding only their difference. CONSTITUTION:For instance, a Bayer type dither matrix is prepared in a dither processing circuit 2, the n-th half tone image is inputted and the binarizing processing of the input is executed by the systematic dither method. The n-th binary pattern is compared with the (n-1)th binary pattern processed in the precedent time. When the n-th binary pattern is the same as the (n-1)th binary pattern, a coding circuit 5 adds a coincidence code E and counts only 'the number of times of coincidence +1'. The counted contents are stored in the succeeding code memory 6. When both the patterns are different from each other, a discrepancy code B is added and the discrepancy position is coded correspondingly to a discrepancy position address and stored in the memory 6.
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