Using XCMYK and eciCMYK color spaces to prepare images for printing

An analysis of XCMYK and eciCMYK spaces with an extended color gamut is performed. These color spaces are useful tools for creating and sharing print files. The general indicators of the gamut volume of the studied spaces are determined. The larger gamut corresponds to the eciCMYK space, but for this space there is a large difference between the colorimetric measurements and the optimized values obtained from the ICC-profile. Projections of color gamut on the chromatic plane of CIE Lab are constructed. eciCMYK is more than XCMYK in green, blue and purple shades. XCMYK color gamut, in turn, is larger than FOGRA51 (PSO Сoated v3) and can reproduce a wider range of colors in green, yellow, and red.

The increase of the colorimetric tone value of CMYK inks for the studied spaces is determined. The tone value for XCMYK is increased by 1–3% due to the use of non-classical type of screening. According to the characteristic data of XCMYK 2017, the average value of the nonlinearity of the gradation scales of color inks is 1.67, black ink – 1.69. Compared to other characteristic data, the nonlinearity index for XCMYK space is higher.

When preparing the test image to print, the maximum value of the total area coverage of ink for XCMYK is 301–303%. The maximum total area coverage of ink for eciCMYK is 365–390%, which exceeds the tolerances for offset printing. Out of printing gamut pixels on the test image are counted. The largest number of such pixels is obtained for the space eciCMYK. Use eciCMYK space only for file sharing, as this conversion will not accurately reproduce colors in dark areas when printing. The use of the XCMYK 2017 profile in the preparation of images for printing will provide a stable and predictable process of reproducing an expanded color range.

Keywords: color space, color gamut, ICC-profile, tone value increase, nonlinearity of gradation, rendering intent, color separation.

doi: 10.32403/0554-4866-2021-2-82-52-61