The relationship between the principle of color reproduction and the limitations of four-color printing

People's recognition of natural scenery can not be separated from the shape and color. Nature scenes cannot be directly printed and reproduced. Shapes and colors must be recorded on a flat medium such as a photosensitive material by means of conversion conditions to form analog quantities of color, or recorded in a photoelectric conversion device such as a CCD, and a digital quantity of color can be directly generated. The color analog that is generated on the planar medium will eventually need to be converted into a digital quantity by means of a scanner, etc., before it can be accepted by the computer software and used in the printing and copying process.

Natural scenery printing and copying process diagram

In terms of 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 perceived by the external color to stimulate the human visual system is the combined effect of color shade and color perception. In color, the three-dimensional sense is mainly described by the neutral gray component.

The colors displayed by the natural scenery are perceived in the visible light color gamut. When 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 color digital quantity is printed and reproduced, the ink printing range color space recompresses the color space, and the color separation algorithm also changes the color tone characteristic. 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.


Due to the limitations of ink colorants and production processes, the spectral curves of C, M, Y, and K color inks used for four-color printing to print color have a large gap from the ideal color spectral curve. Ink overprint secondary colors CM, CY, MY color effect, and the ideal color difference is greater The ideal ink and the actual ink spectral reflectance curve (yellow ink, ink and blue ink) in the existing ink production process, substantially Improve the ink performance, leaving little room. 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. The reflection spectrum curves of Hexachrome C, M, Y, K, O, G color inks are as follows:

CMYKOG six-color reflectance curve


We know that the color separation technique used in four-color printing is to use the Neugebauer equation as a tool, and to use the tristimulus value of the color space of the original color space and the tristimulus value of the ink color to list the ternary equations. The ink value is Variables, using the approximation method to solve equations. The value of the ink when the values ​​on both sides of the equation are closest is the equation set solution, which is the color separation value of the original color. 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.