A graphic manuscript often has to be processed by many devices before it can be copied. These devices include input devices, graphics and image processing devices, output devices, etc. The digital prepress graphic information processing system is open and not limited to the equipment, materials and processes used. The variety of color rendering characteristics of devices of various brands, types, and color characteristics increases the difficulty of accurately reproducing colors. In the transmission of graphic information in these devices, information loss will inevitably occur, and even the copied image and the original will be far apart in terms of color, gradation and saturation. In addition, pre-press processing has basically determined the general quality characteristics of the printed matter. The only adjustments that can be made during the printing process are the dot reduction characteristics and the shade of the ink color. To change the color and tone structure of the image and improve the visual effect, the most important thing is to work hard on the selection of materials before printing and the operation of scanning and color separation. It can be said that some subjective and objective factors have led to the "grade" of product quality in the initial stage of production.

This article mainly talks about several misunderstandings and solutions in pre-press processing.

Due to the uneven quality of the pictures provided by customers during printing, it is difficult to guarantee the quality of the printed matter. Without high-quality manuscripts, it is impossible to guarantee high-quality prints. This is a relationship between cause and effect. Manuscripts shot with a digital camera must be processed into A4 format color images. It is extremely difficult to meet the requirements of pre-press production if there is no high-end or professional digital camera with a precision of more than 6 million pixels and professional optical lenses. Another image downloaded from the Internet is limited by the bandwidth of the network or the accuracy of the network image itself is not high (the image available for transmission on the network, the resolution of the downloaded image is often very low, basically 72DPI), it is difficult to meet the printing needs. To meet the requirements of professional printing, the resolution should generally reach 300DPI.

In color printing operations, there are often printing quality problems such as sticky, piled ink, title, or false on the ground. On the one hand, the generation of these quality problems cannot exclude the printing paper and ink printing suitability and equipment factors, but One reason is that the prepress design is not well integrated with the actual requirements in print production.

The reason for the pre-press design is that the total tone value of the picture, that is, the total amount of ink is not set realistically, and the total ink tone value of the cold-solid sheet without drying is not more than 260%. In actual work, many people do not understand these, and only pursue the full color according to their personal preferences. They often set the sum of the tone values ​​to be very high, which causes the title to be pressed out on the ground to make the dark area of ​​the large area accumulate ink and make it false. And phenomena such as poor overprinting, reverse through-printing, and smudging of the back surface due to insufficient drying of the ink. For these problems, in the process of printing practice, you must find an effective solution: First, set the sum of the tone values ​​of the picture reasonably, and consciously make the total amount of image ink as close as possible, rather than higher than the national standard value, so that printing The resulting image can have both gradation and clarity, and at the same time can shorten the ink drying time, making the printed product not easy to stick. Second, the background color removal technology should be used reasonably, the background color should be partially removed, the neutral gray balance should be reasonably controlled, the image contrast should be increased, and the saturation should be increased. In this way, it can effectively ensure the bright colors of the picture, and it is not easy to produce ink, dull and sticky colors. Taking into account the expansion of dots in commercial rotary printing, replacing the field with about 90% of the deep web is very effective in solving the problem of the falseness or color cast of the title or the field.

Normally, black characters should be stacked, 100% black should be used, and at least 35% green should be added to a large area of ​​black, but the operator does not understand why this is necessary. Here I will briefly talk about the reasons for this with the information in my hands and my own experience.

When we are painting color paintings (gouaches), we will encounter a situation where when we paint colors with pure black, we will find that the black is gray after the picture is dry. At this time, if we add part of the blue or dark red, we will feel When the black becomes thick, the same reason, when adding at least 35% blue to the large area of ​​black in the design, you will feel the color becomes thick. Why not add red? Here we have to look at the various situations of black in software and printing.

The setting of four-color black in Photoshop is very special. As far as Photoshop 7.0 is concerned, the default black of the software is always: C75, M68, Y67, K90. This is a very difficult to understand number? In fact, this default Black, Photoshop is initially set in LAB mode, the value is L0 ､ a0 ､ b0, which is the calculated value in the software system, and the RGB colors R0, G0, B0 are the same as L0, a0, b0 , So this color is also an accurate "black" color. However, in the CMYK color, K100, which is usually called black, cannot accurately express this "system black". In the RGB (Lab) color system, which is larger than the CMYK color system, K100 is just an incomplete deep It's just gray, and this dark gray is reddish! This is a real phenomenon in the actual printing process. No matter how many expensive black (K) inks are used by printing masters, the effect of printing is always far worse than RGB (Lab) The color is black, but also reddish. Regarding the reddish point, we can look at the "gray" part of a large number of prints-after careful observation, it will be found that they are all impure gray-red.

Now we can easily understand why there is this puzzling value in Photoshop: C75, M68, Y67, K90, even if the single black is not enough black, then add more pigments, it turns out that when CMY and K are shared, It can get a fuller black effect, and at the same time, in order to correct the unpleasant "reddish tone", C, M, Y, and K colors are added with about 8 percentage points of cyan (C) ink to balance, in fact It is such a simple and effective method to make the colors of the printed products more correct.

We can also discuss the color cast of black (K) ink in more depth: when black ink is washed lightly with white printing ink (note that it is not net reduction), single black (K) ink is bluish, but As the dilute diminishes, the color cast will slowly shift to cyan-purple blue-purple-purple-red. But why do n’t you see the bluish color in the printed matter? Looking at the gray color in the printed matter with a magnifying glass, you will find that they are not produced by the dilution of the white printing ink, but with 100% black ink. By reducing the net, so they all show the darkest color cast: reddish tint.

In ink, red is a permeable color, that is, in all colors, red ink makes people feel the existence of color more. When red is added to the black ink, it will make people obviously feel that red appears, but blue does not have this situation.

All grayscale colors of single black are strictly reddish, so in Photoshop, to get the ideal grayscale color, it is best to add a certain proportion of cyan (C) color.

For text, we all know that multi-channel black can be easily printed and pasted, so when designing printed text (especially small print) in Photoshop, you must use 100% single (K) black.

If there is a gray block, the file is K20. When the film is sent out, there will be dots, and the dots cannot be seen on the display. This is mainly because the color density is controlled by the density of the dots. A color block is displayed on the screen, but the printing is realized by dots. We use a magnifying glass to see that in fact, the color of each dot is solid, but their distribution is dense and dense. How can you make a gray that is not white? It is recommended to use non-graded spot colors.

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