Importance

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Abstract

This invention can generate encoded data of a high compression ratio of not only a halftone image but also a vertical stripe portion or text portion, which is weak in the conventional method, by a simple process or circuit scale, and can decode the encoded data on the decoding side by a simple process or circuit scale. To this end, image data is input from an input terminal (109). A reference pixel value storage unit (101) stores image data for one period of a dither matrix. A first prediction unit (102) calculates a predicted value of a pixel of interest with reference to threshold values less than one period of pixel values, which are stored in the reference pixel value storage unit (101). A second prediction unit (104) determines a pixel value one period before the pixel of interest as a predicted value. A continuity checking unit (105) compares first to third pixel values before the pixel of interest with three pixel values one period before the dither matrix, checks whether or not the number of matches is larger than the number of mismatches, and outputs the result to a predicted value selection unit (106). The predicted value selection unit selects one predicted value according to the checking result, and an encoding unit (107) encodes a difference between the selected predicted value and the pixel of interest.

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First Claim