Gray Balance and Dot Expansion Principles (3)

Fourth, background removal and ash replacement

(A) the role of the black version

Ideal color reproduction of the color, only need to Y, M, C tricolor ink overlay, you can combine thousands of colors and different brightness of the achromatic, which is the three primary color reproduction theory. However, the three-primary inks do not have the ideal neutral grey color and do not meet the required density (Fig. 4-4-2). The four-color prints have a higher density in the field than the three-color prints. This is due to the black version of the compensation. In the case of multi-color printing, it is in a state of wet and wet printing, and when the ink layer is thick, it is not easy to dry, and it is easy to produce a phenomenon such as overprinting. The amount of dark-colored ink must be subtracted, so black ink is needed instead.

The black version is used to compensate for the defects of the hue of the ink and to meet the needs of the multi-color high-speed printing press so that the printing can be performed smoothly and plays an important role in improving the quality of the product.

1 can enhance the density of the image contrast, according to the current printing of the conventional conditions, Y, M, C three-color ink after the density of 1.6 to 1.7 superposition, and the visual resolution can generally reach 1.8 to 1.9. Therefore, the black plate of the appropriate tone is used to enhance the absorption capacity of the paper, thereby increasing the overall image density range, increasing the dark contrast of the image, the three-dimensional sense, and improving product quality.

2 Stable tone to dark tone. Theoretically, yellow, magenta, and cyan inks according to the proportion of corresponding dots can be superimposed to the desired color, but the printing volume is large and the printing conditions are unstable, such as paper ink and other performance differences, variability of machine accuracy, etc. Dark tone, especially in the area of ​​70% to 100% dot, is restricted, so most products are dark brown and color is unstable. In fact, most of the original dark shades are darker than the requirements. The use of an appropriate black plate overlay can stabilize the color and overcome the dark tone color cast.

3 strengthen the tone to the tone level. In color reproduction, the general rule is to keep bright or slightly emphasized, to maintain the middle tone or a slight compression, dark tone level compression is more, there is a level phenomenon in the dark tone level, on the other hand, only use yellow, product The red and blue primary colors are superimposed, and the outline of the medium-to-dark tone is imaginary, and the level cannot be pulled open. The black version has clear outlines and distinct layers, and the dark tone details are clearly marked.

4 Improve printability and reduce costs. With the advent of high-speed multicolor printing presses, the per-printing time is very short, and over-thick inks cause many problems in printing. Therefore, to reduce the amount of color ink in the sexually-distributed area of ​​the screen darkening portion, and to replace it with black ink, the printing failure is reduced, and the printability is improved. At the same time, the black ink is inexpensive and the cost is reduced.

5 Solve text printing, color copying to the development of pictures and texts, such as picture descriptions, advertisement descriptions, pattern labels, calendars for calendars, all need to be copied in black copy. From the perspective of visual requirements, black characters work best. It is unrealistic to overprint text with three colors.

(B) Undercolor Removal (UCR)

Under color removal, also known as UCR (Under Color Remove), refers to a process in which the amount of ink in a shadow-composite-colored area of ​​a print is properly reduced and replaced with a black ink.

The under color removal process results from the requirements of wet-to-wet printing and the reproduction of the original dark tone levels in modern high-speed printing presses. It has the advantages of facilitating overprinting, saving expensive color inks (ie by removing a certain amount of Y, M, C ink and replacing it with a small amount of black ink) and favoring neutral gray balance and neutral gray reproduction. .

1. The principle of background removal

Theoretically speaking, if the ideal Y, M, and C inks are blended in equal amounts, the desired neutral gray color can be formed. That is, the minimum of the three colors consisting of Y, M, and C color inks is The gray ash component is shown in Figure 4-4-3. In other words, the composite color overprinted by the three primary colors of Y, M, and C has the same visual effect as the composite color of the two inks Y, M, and C superimposed on the black ink. The reason is that the black ink can correctly reproduce the neutral gray, thereby removing the color ink constituting the neutral gray in Y, M, and C and replacing it with the black ink, and achieving the same visual effect for copying the same color.

With the development of the modern desktop prepress system and image reproduction technology, the background color removal function has been continuously improved and developed, and a new process pattern has been formed in which the under color removal and the ash replacement process coexist. Ash replacement can not only complete all the work of the under color removal, but also can use the black ink to replace the Y, M, C and other color inks as much as possible, and maintain the high quality of printed copies.

2. The purpose of removal

(1) Improvement of printability and ink drying In printing, neutrality and color are generally reproduced by Y, M, and C overprinting, and in order to compensate for the lack of layered density in the intermediate tone tone tone tone tone, the blackness is increased. Version. However, for multi-color high-speed printing with fast printing speed, it is not necessary to wait until the first printing ink dries, and the ink of the next color must be overprinted, resulting in that the ink after printing cannot be completely transferred to the printing ink, resulting in overprinting. Quasi, uneven ink, even the first printed ink is carried away by the printed ink, which is particularly evident in the larger dark areas of the four-color wet-press overlay ink. Today's multi-color printing presses generally require better dark areas of printability, and the total area of ​​overprinted dots must be limited to the range of 250% to 270%. Therefore, in order to meet the requirements of the printing process and improve the above-mentioned unfavorable conditions, the under color removal process must be used to reduce the total amount of ink, to accelerate the ink drying time, and to improve the printability.

(2) Reducing Production Costs The prices of four-color inks Y, M, C, and BK used in printing are not equal. When undercolor removal is used, a total of about 50% to 80% of the dots can be removed, and this only requires about 15% increase in the black version, that is, it can replace approximately 50% to 80% of the total yellow, magenta, and blue overlap. The result is a large amount of colored ink, which is very cost-effective for large-volume printing. Generally increasing the background color removal by 50% can reduce the ink cost by 10% to 15%.

(3) Color Offset Compensation In a manuscript where a color image is copied, even if the gray balance of the image from the highlight to the midtone is well-balanced, the image is often out of balance in the dark tone portion to exhibit a certain color shift. If it is necessary to satisfy the condition that the color composition is unchanged and the color cast can be compensated, it is very effective to use the background color erasing method to solve this problem. As shown in Figure 4-4-3, there is a dark reddish manuscript. The dark tone color is composed of Y7, M9, and C6. To remove the dark tone color, the background color can be removed to make Y, If M or C is removed by an equal or unequal amount, the color composition can be changed to achieve gray balance reproduction. For example, if Y, M, and C are each removed by equal amounts, their color components become Y5, M7, and C4. Obviously, 7:9:6, 5:7:4, as long as the appropriate ratio of Y, M, and C can be determined. Correct color cast.

(4) Emphasizes that the removal of the solid background only affects the neutral region, so that the use of the undercut to emphasize the solid color can be achieved by removing the setting of the dark tone.
Usually, when the solid color is emphasized as a whole, it is better to use background color removal. If only one or two colors are emphasized, then the color correction method is better. FIG. 4-4-3 also illustrates the effect of the color removal on the solid color from another perspective. Emphasize.

(5) Keep neutral gray balance stable In ordinary color printing, Y, M, and C colors are often used to reproduce neutral gray, but due to the presence of many different ink concentrations and ink sizes during the printing process, Misprints and other uncertainties are not allowed. When copying a manuscript with the main content of a gray tone such as a machine type or a camera, the copying is difficult and the partial color cast is easily caused. Therefore, a proper amount of background color is used to remove the Y, M, and C inks from Y, M, and C, and increase the black print to make up for the lack of background color. Thus, a satisfactory effect can be obtained and the neutral gray balance can be maintained. stable.

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