| 摘要 |
PROBLEM TO BE SOLVED: To generate a half tone image with high quality. SOLUTION: A gradation dependent type error spread half tone processing system 150 uses a gradation dependent threshold 152 and an error weighting 156 that are optimized by minimizing a cost function of a model base. When a corrected pixel value is higher than an upper threshold, a dot is printed and when smaller than a lower threshold, not dot is printed, and when the corrected pixel value is between the upper and the lower threshold, a binary bit map is used to discriminate whether or not a dot is printed out. A half tone image is generated by a direct binary tree retrieval method and an error spread method as to respective medium tone threshold levels and magnitude of fast Fourier transform of the half tone image is compared by using a visual cost function. As to a highlight and shadow gradation levels, the image is filtered via a human visual system model, and the filtered half tone image from the error spread system is compared with a continuous gradation image. The threshold value and the error weighting are optimized by minimizing the cost function in both the cases.
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