Reflections on the Application of Hi-Fi Color Printing Technology (5)

People's recognition of natural scenery can not be separated from the shape and color. Nature scenes cannot be directly printed and reproduced, and their shape and color must be recorded on a flat medium such as a photosensitive material by means of conversion conditions to form an analog quantity of color or recorded in a photoelectric conversion device such as a CCD to directly generate a digital quantity of color. The color analogs generated on the planar media ultimately still need to be converted into digital quantities by means of scanners and other devices, which can then be accepted by computer software for the print reproduction process. The natural scene printing and copying process is illustrated in FIG. 12 .

For color reproduction, the copied content can be divided into two parts, namely a color component and a neutral gray component. This decomposition of color is to simulate the human eye's visual perception of color, rod cells feel light and dark, cones feel color. The color perception generated by the external color to stimulate the human visual system is the combined effect of color shading and color perception. In color, the three-dimensional sense is mainly described by the neutral gray component. The human color perception system diagram is shown in Figure 13.

The colors displayed by nature scenes are perceived in the visible color gamut. When it is copied, the authenticity of the color may be lost due to the limitation of the color gamut of the photosensitive material or the influence of the performance of the photoelectric conversion device. When the digital color quantity is printed and reproduced, the ink printing range will cause the color space to be compressed again. The color separation algorithm will change the color tone characteristics. The colors that are printed and reproduced in this way, especially the colors that have more color components, greatly change from the original appearance of colors in the visible color gamut. The conventional four-color printing color gamut area is less than half of the visible light color gamut, so the printed color is often darker than the color in the visible color gamut, and the color gradation compression loss is more.

The print color gamut range can be determined in CIE xyY color coordinates, and can also be determined in the CIE Lab system (see Figure 14).

Due to the limitation of the ink colorant and the production process, there is a large gap between the spectral curve of the color printing of the C, M, Y, and K color inks used in the four-color printing and the ideal color spectrum (see FIG. 15).

The secondary colors of ink superimposition CM, CY, MY color effect, especially the difference between blue, green and ideal color is greater (see Figure 16).

With the existing ink production process, the performance of the ink is greatly improved, leaving little space. Therefore, the four-color printing color gamut is hardly greatly expanded.

The high-fidelity six-color color separation technology represented by Pantone Hexachrome in the world now increases the orange-red ink in the hue region composed of magenta and yellow ink, and increases the green ink in the hue region formed by yellow and cyan inks. Hexachrome C, M, Y, O, G color inks, CIE Lab chromaticity coordinates of secondary color CM, MO, OY, YG, GC, tertiary color MOY, YGC, constitute a printing gamut range, compared with conventional four color The domain is expanded by about 20%.

We know that the color separation technology used in four-color printing is based on the use of Neugebauer's equations. The three-stimulus values ​​of the original color space R, G, and B and the ink color C, M, and Y tristimulus values ​​are listed. In the ternary equation system, ink values ​​are used as variables, and the equations are solved by the approximation method. The ink value when the two sides of the equation are closest is the equation set solution, which is the color separation value. In the four-color separation software, the ink brand number should be specified first, which means that the color product coordinates of the primary color ink and the secondary color are determined, that is, the ink printing range. The color separation software has selected hundreds of sample colors according to a certain rule in the printing color gamut, and uses the above method to preliminarily obtain the color score values. When color separation is performed, the internal predetermined color interpolation method between the sample colors can be used to quickly perform color separation. Converts all pixel colors of the original color space to ink values ​​in the ink color space.

High-fidelity color printing increases the color gamut by adding new base color inks. There are more than three kinds of base color inks that participate in color separation. Therefore, the algorithm for solving color separation by solving triple primary equations is no longer applicable, and a new high quality insurance must be created. True color separation algorithm.

The principle of high-fidelity color separation, first, refers to the human visual perception mode, that is, the spectral color perception effect as the target color. In the process of color separation from the original color space to the printing color space, the color is divided into two parts: color and neutral gray. Control, and finally in the primary color ink channel, each of the two primary ink components in the ink composition are combined to form the final color separation ink parameters. In this way, the proportion of color and neutral gray components in the printed color can be adjusted to optimize the simulated visual brightness perception. Secondly, color components in color are color-separated. Based on the color three-dimensional mode, the color components of the manuscript that are mapped in the color gamut and the color gamut of the printing ink are distinguished and separated by different color separation models. In this way, in the printing color gamut range determined by the primary color ink, the hue, color saturation, and color brightness of the printing color can be maximized, and the simulated visual color perception can be optimized. Third, to adapt the color separation algorithm, all the neutral gray components are duplicated, and the neutral gray component and the color gray component are two parts, and the related changes are respectively matched. There should also be a variety of other related regulatory technologies.
The goal of high fidelity color separation and printing is to strive to make the visual perception of color in the printed color space as close as possible to the visual perception of the color of the visible color space.

The color gamut range of the separation printing color needs to comprehensively consider factors such as the color gamut range, the color gamut distribution, the hue continuous, the proportion of the color component and the neutral gray component, and the color separation printing color gamut range may be slightly smaller than the ink in some hue regions. Printing gamut range.

High-fidelity printing ink gamut range is affected by printing costs and equipment conditions, etc. Newly added basic color inks should generally be selected according to actual needs. For example, the Pantone Hexachrome Hi-Fi six-color separation model adds only orange and light green inks. High-fidelity color separation technology that can be used in actual production in China can design a color separation model using up to 16 primary color inks. Hexachrome and DIC are two ink series that have been introduced into the group. Each ink series has eight color separation models, including: four-color CMYK, five-color CMYKO, CMYKG, CMYKB, and six colors. CMYKOG, CMYKOB, CMYKGB three, seven-color CMYKOGB kind. And according to the needs of users, choose a few appropriate primary color ink design color separation model.

Source: Ke Yin Network