本論文提出近似K值演算法(approximate-K algorithm)以節省彩色墨水或碳粉的使用量。一般印表機使用C、M和Y三色混合出彩色與灰階影像，因此利用CMY混合的灰階像素印刷成本比直接使用黑色(K)墨水貴上4-4.5倍。為求降低成本要避免使用CMY三色去混合灰階像素，所以在處理灰階與近似灰階部分皆改用K色印刷。我們發現人眼對低飽和度之色彩差異敏感度較低，因此可以將近似灰階的像素用K色墨水取代原有的彩色墨水，如此就可以減少彩色墨水使用量，也不會造成視覺品質太大的影響。我們使用HSV色彩空間中的色彩飽和度，篩選出接近灰階範圍的彩色像素並將其色彩像素值改為灰階。本論文採用客觀的方法(PSNR)和主觀的方法(DSCQS)來評估影像品質。從我們研究的結果得知，在印刷彩色影像時使用本演算法，平均成本只要原來的84~86%，相對可節省約15%的成本。

This thesis proposes a novel scheme to save the usage of ink or toner by an approximate-K algorithm for color printers. Existing printers use the mixtures of three color toners (C, M and Y) to print all the pixels for color images, and it makes color printing cost 4-4.5 times more than monochromic printing. Since human eyes are not sensitive to distinguish neighboring colors in the color space, we can use the K toner to replace the colors close to gray-scale. We can then reduce the ink usage without affecting the image visual qualities. We use the saturation in the HSV color model to discover the near gray-scale pixels and transform those pixels to gray level. We then evaluate the objective image quality using the PSNR and use the Double Stimulus Continuous Quality Scale (DSCQS) as the subjective evaluation method. From our experimental results, printing a color image using our algorithm needs only 84~86% of the original price in average.

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