The influence of prepress processing of the hottes

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The influence of image pre-processing on the quality of printed matter (I)

I. the reasons why printed images need to be processed

the printing industry has developed into a very widely used industry. In fact, except for water and air, printing can be carried out, but it is not so easy to achieve the consistency between printed matter and the original. Take offset lithography as an example. Many of its technical limitations have not been fundamentally changed, resulting in many inevitable problems in offset printing products. These problems need to be corrected in the prepress process in order to obtain satisfactory printing products. These limitations include the following aspects

1. There is a great difference between the color and tone range of the printed matter and the color and tone range of the original

first of all, the offset printing process cannot list all the colors of the natural world in terms of the thickness, thermal performance and other indicators of vacuum insulation board, rigid polyurethane board, extruded polystyrene board, molded polystyrene board, etc., but only part of the colors, which is due to the ink used in the printing process Even if paper and printing use the same raw materials and many defects in the process. The yellow, magenta, cyan and black inks actually used in printing have defects in the range of colors, which cannot reach the ideal spectral absorption curve of yellow, magenta and cyan, that is, they cannot reach the real color appearance of nature. The actual absorption curve of the ink is very different from the ideal absorption curve, that is, they absorb not only the light in the spectral area that should be absorbed, but also the light in other spectral areas. The direct consequence of this harmful absorption is that the hue and saturation of the ink are far from the ideal three primary colors, and the printing material uses ink to express the hue and level, Therefore, the color rendering performance of the ink directly affects the fidelity of the original copy. Printing with primary color ink with additional absorption will inevitably form color deviation and greatly compress the color area of the printed matter

this defect of ink is mainly caused by the limitations of pigments, manufacturing processes and other factors. This extra absorption of yellow ink is the least, so the color rendering performance is the best, magenta ink is the second, and green ink is the worst

secondly, the highlight part of the printed matter is formed by the color of the paper, that is to say, the paper also participates in the formation of color. If the whiteness of the paper is different, it will affect the color brightness and saturation at the highlight of the picture, and then affect the color contrast of the picture. Similarly, it is also closely related to the texture of the paper. Paper is a porous material, and the ink is easily absorbed by the fiber, which causes high light scattering on the printing surface and reduces the printing density. The place that should be black is not black enough. The smooth, coated offset paper, ink adsorption on the surface, and light scattering is very little, so that the dark tone is darker, lack of hierarchy

in addition, the eye tone adding method used in printing also has an impact on the range of hues. In the process of plate making and printing, the highlights and dark tones are very easy to be polarized, that is, it is easy to lose the details of highlights and dark tones

based on the above reasons, it is almost impossible for us to make the printed matter and the original picture reach the same gradient density range. Therefore, the density correspondence between the original and the printed matter must be adjusted so that the printed matter presents the best compromise effect. The 45 degree slash represents the ideal print, that is, the reproduction method of the original density without any mapping change (that is, without any tone compression). This situation is almost impossible to achieve in the current offset copy method. In general, the density of the original can reach 3.0 or even higher, while the density of the printed product, which has won the "bioplastics Innovation Award" issued by the American Plastics Industry Association, can only reach 1.8 or 2.0, and its density range is far less than the density range of the original, so how to truly restore the original, or how to restore it better, is a pity of the current printing process, and it can only be compressed. The other two replication curves in the figure reflect the basic mapping relationship when density range compression is required. Among them, straight-line compression is a compression method that is unreasonable from the visual effect. It completely discards the gradient part that cannot be printed (that is, the gradient part with density higher than 1.8 is completely lost), leaving only the gradient density range that can be printed, which is obviously unreasonable. The arc is a technological compression reproduction curve used to expand the range of the middle tone level of the image. From the visual effect, this technological compression curve is reasonable

2. The color deviation of the image during printing

when we process the image on the display, if the display is well calibrated, the image color may be normal, but this does not mean that the color of the image is the same after printing. Because the display uses the color light addition method to realize the color, and the printing uses the color light subtraction method to realize the color, the reproduction color will be different. In fact, when printing, we should consider whether the ink used will have color deviation, that is, how to grasp the gray balance of the ink used. Briefly speaking, gray balance refers to the color combination that can produce gray color. For example, in RGB additive space, when RGB three colors with similar brightness are mixed, gray will be generated. The red, green and blue colors with brightness value of 200 are the same as 25% gray. Gray here is also called neutral gray, which is a gray tone without color components. If you mix 210 red, 200 green and 200 blue, the result will be a warmer gray (with red components). It looks like gray, but in fact it is a gray with red, and it is no longer a neutral gray. Color light is used here. In RGB additive space, the three colors only need to be added equally to produce neutral gray. However, when entering the field of CMYK printing, the situation is not so simple. Equal amounts of yellow, magenta and cyan do not produce neutral grey. They produce lighter, turbid, brownish gray, rather than real gray. The reason is that the ink described above has an unsatisfactory absorption of color light (the spectral curve of the ink is not ideal), that is, it is caused by the impurity of the ink used. In the CMYK space of the actual ink, to get the field gray, we need to increase the amount of green ink. The extra green makes the other two colors cleaner. For example, 30% cyan, 2L% magenta, and 21% yellow mix to produce 30% neutral gray. If 30% cyan, 30% magenta, and 30% yellow mix to produce neutral gray with dark brown, Table L is the gray balance ratio table of a typical ink. For a certain type of ink product, the CMY value of gray produced by mixing is constant, that is, the gray balance proportion value of a certain ink is constant. In this way, after we measure the gray balance of this ink, we can correct the image according to this balance, so that the image to be printed can accurately reproduce the color appearance after correction, so as to make up for the defect of this ink. However, in order to avoid the change of pendulum torque and strike center position, it should be noted that the gray balance data of different brands of inks are different

3. Dot increase problem

offset printing uses pressure to transfer ink. When ink is transferred to the surface of paper under pressure, a small amount of expansion will occur due to pressure and other factors. Some inks are absorbed into the paper fiber, which will also cause the dot shape to increase. Because the point size is directly related to the gradation and hue, increasing the point will make the whole image darker. Obviously, due to the above reasons, different dot enlargement effects will be produced for different printing machines and paper of different quality. In addition, the points of different sizes increase into a nonlinear relationship. The point increase formed by the characteristics of paper and ink generally presents an exponential expansion law. Other things to note are that in the process of image and text output and plate printing of plate making, due to the factors of materials and equipment, points will also increase

dot increase is an inevitable phenomenon due to specific factors in the printing process. In order to truly reproduce the color and level of the original, the influence of dot increase must be compensated in the pre press processing process. This compensation process can be directly applied in the process of image and text processing, or the compensation function can be added to the file first in the process of processing, and then compensated in the process of printing and phototypesetting

in addition to the above aspects, there are also problems such as the control of the total amount of ink and printing registration

theoretically, the three primary colors of CMY pigment can restore the color we want. However, in actual printing, K is indispensable. There are many reasons for this. I won't go into details here. But when CMYK ink is used for printing, a lot of ink will accumulate in the dark area. In order to achieve ultra black appearance effect, such as the cover of books, periodicals and pictorials, only K is far from black. C50m50y50k50 or even c100m100y100k100 are often used. The ink accumulation of this is at least 200%, or even 400%. Because different substrates have different abilities to retain ink (for example, the ability of paper to retain ink is much lower than that of high-grade coated paper), if measures are not taken to control the amount of ink applied, the darkened area will be pasted, thus losing the darkened level. Therefore, it is necessary to set the total amount of ink in prepress image processing

the registration problem is an inevitable error caused by the mechanical factors of the printing equipment, and it is also an important aspect of the printing suitability. The registration problem refers to the problem caused by the overprint position error when the four color separation plates are printed separately in color printing, especially in high-precision color printing, if you do not pay attention to solving it, it will have an adverse impact on the appearance effect of the finished product. Then in the pre press processing, in order to avoid the adverse appearance effect caused by the untimely overprint, these problems must be considered and dealt with in the design and post production in the highly demanding color printing, which is trapping processing. For trapping processing, many people don't understand its meaning, especially the majority of graphic designers, fill in some values casually when setting, which will cause major problems. It's better not to set these settings

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