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Abstract

In a thermal printer, the print mode that has been set by the user is judged at the start of printing. When the high-speed print mode is set, the standard deviation .sigma. of the original image data is calculated. According to the standard deviation .sigma., it is judged whether the densities are evenly distributed. When the densities are evenly distributed, a gradation conversion characteristic is selected for converting the gradation in the entire density range only by decreasing the number of gradation levels of the output image. When the densities are intensively concentrated in a specific density range, it is further judged whether the specific density range is the low, middle, or high density range. According to the judgement, the gradation values of input data are converted using a gradation conversion characteristic for decreasing the number of gradation levels of the output image and accurately reproducing the gradation in the specific density range.

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First Claim