Fourth, background color removal and ash replacement
(1) The role of black edition
The ideal color reproduction color, only need Y, M, C three primary color ink superimposed, you can combine thousands of colors and different brightness achromatic, this is the three primary color reproduction theory. However, the three-primary ink can not print the ideal neutral gray, and can not reach the required density (shown in Figure 4-4-2). The solid density of the four-color print is higher than that of the three-color print, which is due to the use of black plate compensation. In multi-color printing, it is in the state of wet press wet printing, and the ink layer is not easy to dry when thick, and it is easy to produce poor overprinting and other phenomena. The amount of dark-colored primary ink must be subtracted, so black ink needs to be substituted.
The black version is used to compensate for ink hue defects and to meet the needs of multi-color high-speed printers, so that printing can proceed smoothly, which plays an important role in improving product quality.
â‘ It can enhance the density contrast of the image. According to the current printing conventional conditions, the density of the Y, M, C three-color ink superimposed is 1.6 to 1.7, and the visual resolution can generally reach 1.8 to 1.9. Therefore, a black plate with a suitable tone is selected to enhance the absorption capacity of the paper, thereby increasing the overall density range of the image, increasing the contrast of the dark tone of the image, the three-dimensional feeling, and improving the quality of the product.
② Stable mid to dark colors. In theory, yellow, magenta, and cyan inks can be superimposed according to the proportion of the corresponding dots, but the printing volume is large, the printing conditions are unstable, such as the difference in performance such as paper ink, the variability of machine accuracy, etc., adjusted to The adjustment of dark tone, especially the area of ​​70% to 100% of the dots, is restricted. Therefore, in most products, the dark tone is brownish and the color is unstable. In fact, most of the manuscripts have dark colors that are not cool, which is not in line with the requirements. The use of an appropriate black version can stabilize the color and overcome the dark colors.
â‘¢Strengthen the middle tone to dark tone level. In color reproduction, the general rule is to keep the bright tone or a little emphasis, keep the middle tone or a little compression, the dark tone level is compressed more, and the middle and dark tone levels are flat. On the other hand, only yellow and The three primary colors of red and green are superimposed, and the contours from medium to dark are dull, and the layers cannot be pulled apart. The black version is used for clear contours, clear layers, and dark details.
④ Improve printability and reduce costs. With the advent of high-speed multi-color printing presses, the overprint time per color is very short, and excessively thick inks cause many failures in printing. Therefore, it is necessary to reduce the amount of color ink in the neutral area of ​​the dark part of the screen and replace it with black ink to reduce printing failures and improve printability. At the same time, black ink is cheap and the cost is reduced.
⑤Resolve the development of text printing and color copying to the combination of pictures and texts. Texts such as picture descriptions, advertising introductions, pattern labels, and calendar calendars must be copied in black. From the perspective of visual requirements, black letters work best. It is unrealistic to overprint text with three color versions.
(Two) background color removal (UCR)
Undercolor removal, also known as UCR (Under Color Remove), refers to the process of appropriately reducing the amount of ink in the dark-tone composite color area of ​​the printed matter and replacing it with black ink.
The background color removal process originates from the requirements of wet press wet printing and dark tone reproduction of the original high-speed printer. It has the advantages of rapid overprinting and saving of more expensive color inks (that is, by removing a certain amount of Y, M, and C inks and replacing them with a small amount of black ink), and is beneficial to neutral gray balance and neutral gray reproduction .
1. Principle of background color removal
In theory, if the ideal Y, M, and C inks are mixed in equal amounts, an ideal neutral gray can be formed, that is, the minimum value of the three colors composed of Y, M, and C color inks is The neutral ash component is shown in Figure 4-4-3. In other words, the composite color overprinted by the Y, M, and C primary color inks has the same visual effect as the composite color overprinted by the two color inks of Y, M, and C and the black ink. The reason is that the black ink can accurately reproduce the neutral gray, so that the color ink constituting the neutral gray in Y, M, C is removed and replaced with the black ink, and the same visual effect can be achieved by copying the same color.
With the development of modern desktop prepress systems and image copying processes, the background color removal function has been continuously improved and developed, and a new process pattern in which the background color removal and ash replacement processes coexist is formed. The ash replacement can not only complete all the work of background color removal, but also replace the Y, M, C and other color inks with black ink to the greatest extent, and maintain the high quality of the printed reproductions.
2. Purpose of background color removal
(1) Improve printing suitability and ink drying. In printing, usually neutral and color are reproduced by Y, M, C overprint. In order to compensate for the lack of gradation density in the mid-tone to dark tone parts, black is added. Version. However, for today's multi-color high-speed printing with fast printing speed, the ink of the next color must be overprinted before the ink of the first printing is dried, resulting in that the post-printing ink cannot be completely transferred to the pre-printing ink, resulting in overprinting. The ink color is uneven, and even the ink printed first is taken away by the ink printed later. They are particularly noticeable in the large dark areas of the overlapped ink volume of the four-color wet press wet overprint. Today's multi-color printing machines generally require dark toned parts with better printability. The total area of ​​the overprint 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 background color removal process must be used to reduce the overall amount of ink, speed up the ink drying time, and improve the printability.
(2) Reduce the production cost. The prices of Y, M, C, BK four-color inks used in printing vary. When the background color is removed, usually a total of about 50% to 80% of the dots can be removed, and this only needs to add about 15% of the black version, which can replace a total of about 50% to 80% of the yellow, magenta, and cyan overlays. The resulting effect saves a lot of color inks. For large-scale printing, the economic benefits are very obvious. Generally increasing the background color removal by 50% can reduce the ink cost by 10% to 15%.
(3) Color shift compensation In the original copy of the color image, even if the gray balance of the image from highlight to midtone is good, it often loses its balance and shows a certain color shift in the dark tone part. If it is necessary to meet the conditions that its color composition is unchanged and can compensate for the color cast, it is very effective to use the background color to remove the method to solve this problem. As shown in Figure 4-4-3, there is a dark reddish manuscript, the composition of the dark tone color is Y7, M9, C6. To remove the dark tone partial color, you can use the background color removal to make Y, M, C equal or unequal removal of a part, you can change its color composition to achieve gray balance reproduction. For example, if 2 of Y, M, and C are removed by the same amount, the color composition becomes Y5, M7, and C4. Obviously, 7: 9: 6 ≠5: 7: 4, as long as Y, M, and C are in proper proportion Correct the color cast.
(4) Emphasis on solid color background color removal only affects the neutral hair area. Therefore, changing the setting of dark tone through background color removal can achieve the purpose of using background color removal to emphasize solid color.
Generally, when emphasizing the solid color as a whole, it is better to use the background color removal. If only one or two colors are emphasized, it is better to use the color correction method. Figure 4-4-3 also illustrates the removal of the background color to the solid color from another angle. Emphasize.
(5) Stabilize the neutral gray balance. In common color printing, Y, M, and C colors are often used to reproduce neutral gray, but there are many such as different ink concentrations and different ink amounts in the printing process. Unstable factors such as inaccurate overprint of various colors. When the original content is reproduced in gray tones such as machines and cameras, it is difficult to copy, and it is easy to cause partial color cast. Therefore, an appropriate amount of background color is removed to remove the amount of gray Y, M, and C ink that is overprinted by Y, M, and C, and a black version is added to make up for the lack of background color, and a satisfactory effect can be obtained to maintain a neutral gray balance. stable.
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