Influence of prepress processing of the hottest im

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The influence of image pre press processing on print quality (II)

II. Digital image pre press processing

digital image pre press processing should generally be carried out from the aspects of image level, color and definition. If an image is good in these aspects, it is an image that meets the reproduction requirements from the perspective of the printed manuscript. Speaking from the requirements of printing, it is required to faithfully restore the original, but the limitations of printing tell us that it is difficult to faithfully restore, so the consistency of visual appearance effect is a good supplement to faithful restoration. However, because everyone has different visual habits and aesthetic habits, everyone has different scales when adjusting, so it can only be described qualitatively

1. Image level adjustment

level adjustment is to deal with the high-profile, middle and dark tones of the image, and reproduce all levels as much as possible. However, as mentioned earlier, in general, the density of the original can reach 3.0 or higher, while the density of the printed matter can only reach 1.8 or 2.0. The density range of the printed matter is far less than the density range of the original, and the layers must be compressed. Then how to compress the printed matter in order to restore the original better? The method we currently use is to print the powder layer by layer until it is formed, using the process compression curve, The purpose is to make the appearance effect of the printed matter basically close to the appearance effect of the original. The visual appearance effect here refers to a judgment of our eyes, not measured by density instruments. In fact, it means that the purpose is achieved if the appearance effect is close. The adjustment of image hierarchy can be analyzed from the following two aspects

① calibration of highlights and shades

highlights and shades refer to the brightest and darkest hue values on an image, also known as white field and black field. If the image to be adjusted is used for printing, it is necessary to consider how to set the values of highlights and shades. Because of the highlight of the printed image, generally, areas brighter than 3% - 5% cannot be printed, that is, 3% - 5% areas become 0%, that is, the white of the paper, In this way, the hierarchy of the high brightness area of the image will be lost. On the contrary, more than 95% of the dark areas will be printed in 100% black, and this part of the dark level will also be lost. This is the deficiency of printing. In order to compensate for the influence of this deficiency on the reproduction of image hierarchy, it is necessary to compress the image used for printing hierarchically, for example, compress 0% white to 5% gray white, and 100% black to 95% dark gray. It is not necessarily 0% or 100% here, but usually 2% or 98%

in image processing, the best way to set highlights and darkening points for printing is to use the highlight and darkening dropper in Photoshop. There are dropper tools in curves and levels in Photoshop. One of their functions is to set poles specifically, so that the tones between poles on the image will be redistributed according to the range of poles. The setting of pole color value depends on the specific printing conditions, and there is no unified standard. In most cases, the commonly used CMYK highlight pole value can be set to 5, 3, 3, 0, and the dark tone pole value can be set to 65, 53, 51, 95 or 95, 85, 85, 80. When setting, just double-click the highlight and dark dropper in the curves and levels tools, and enter the setting value, as shown in Figure 3. If it is set in RGB color mode, the RGB equivalent highlight points are 244, 244, 244, and the darkening pole values are 10, 10, 10. Set it similarly. After setting the highlight and darken dropper, all you need to do is click the highlight and darken point in the image with the highlight and darken dropper. So how to determine the highlights and dark points in the image

generally speaking, there are two kinds of printing highlights. One is the point without information, that is, 0%, which is called specular highlights. The other is the highlights with details and information, which is called scattered highlights. To determine the printed highlight is actually to find the appropriate scattered highlight in the image. This can be reported in Photoshop with the dropper tool to check the color value of the important scattered highlight, and judge whether it is within the printing range (5% - 95%). If it is within this range, no adjustment will be made. If the scattered highlight is not within the printing range, such as 2%, and this point is important, it needs to be printed, Then click the scattered highlight on the image with the defined highlight dropper. Similarly, choose the dark tone pole and define it with the dark tone dropper. In this way, after the two highlight and dark tone poles we selected are defined, the hue between the highlight and dark tone poles on the image will be redistributed according to the range of highlight and dark tone settings, that is, the level of the image will be corrected according to the characteristics of printing. After setting the highlight and dark tone poles, in order to achieve the consistency of visual effect with the original, we also need to adjust the process compression curve of the image. Next, we will analyze several typical process curves

② several typical process curves

for a given printing manuscript, after its high light and dark adjustment and calibration, its hierarchical replication can also be artificially changed to compress, expand or maintain the tone of the digital manuscript. Since the density range of the printed matter is far less than that of the original, the hierarchy must be compressed. In the compression process curve, the hierarchy of one area of the image is compressed, and the hierarchy of the corresponding other area is expanded. The purpose of this compression and expansion is to make the printed matter as close to the original as possible from the visual effect. In practical application, we should grasp the key levels in the image to be copied and emphasized according to the characteristics of the original, and give up the secondary levels accordingly. For originals with normal contrast, the low-density area can reach 0.2 ~ 0.3d, the high-density area can reach 2.1 ~ 2.9d, and the maximum density contrast can reach 2.7d. The colors are bright and the layers are rich. The highlight to the intermediate tone level of such images should be emphasized as the main body of the image. Some manuscripts, such as night scenes, backlight photography, etc., have a large area of the dark part, which is the main body of the picture. At this time, it is necessary to emphasize the dark level. In short, we should treat them differently and master them flexibly according to the characteristics of the original. Figure 4 shows several typical step adjustment process curves

curve ① shows that the highlight level is stretched and the dark level is compressed, which is suitable for originals with small dark area and need to emphasize the light level, such as originals overexposed during photography; Curve ② indicates thinning and opening the dark level, and the highlight level is compressed, which is suitable for originals with large dark area and stuffy picture. It is necessary to open the dark level to brighten the whole picture, such as originals and night scene images caused by insufficient exposure during photography; Curve ③ shows that both highlights and shades are pulled apart, and the middle tone level is flattened, which is suitable for originals with dull pictures and flat highlight levels, such as snow scenes; Curve ④ refers to the case that the highlight and dark tone levels are compressed, while the intermediate tone levels are stretched. This is a frequently used curve, which is suitable for originals with thin originals and small density contrast

from these typical process curves, we can see that in order to make the printed matter closer to the original, we must comprehensively consider various factors and choose the best gradient process curve

2. Image color correction

color reproduction refers to a complex process of color decomposition, transmission and synthesis. Color restoration is also a major aspect of printing reproduction. In the process of color reproduction, color errors are inevitable due to the influence of factors such as light source, lens, color filter, photoelectric conversion system, photosensitive materials, paper, ink and so on, In particular, the increasingly extensive use of compressed green plastic new materials at the print level in the construction market and the problem of ink have a crucial impact on the restoration of color. In order to obtain the ideal color reproduction, we must try to correct these color errors and realize the ideal color restoration

① preparation before color correction

first of all, equipment calibration and system calibration should be carried out. These equipment include scanning equipment, display equipment, output equipment and proofing equipment, which should be subject to strict professional calibration. In addition, there should be a relatively perfect color management scheme between these equipment. These are the basis of our color correction. Here we want to talk about the display device in particular. In image processing, the appearance color of the image is reproduced by the display before printing. The display is based on RGB mode, and the final product we want is the printed matter restored on the paper with ink, which is in CMYK mode. Reproducing the CMYK image with RGB display device is bound to affect the appearance effect of the color, so this conversion (color management system) in the display should be accurate, Moreover, it is necessary to maintain the consistency and stability of the lighting environment light source in order to make the screen display and proofing as consistent as possible

secondly, level correction should be carried out before color correction. According to the color rendering mechanism, the color is presented on the basis of neutral gray level, so the color correction should be carried out after the level correction, otherwise, the color will change when the level is corrected after the color correction

what color mode is more reasonable for color correction. In Photoshop, no matter whether the image is in RGB mode or CMYK mode, it can be corrected for order and color. Under which color mode, there are different advantages. The advantage of using RGB color space for correction is that the color gamut space is large, which is consistent with the color space of the display. However, because it must be converted to CMYK space when it is used for printing and output after correction, some colors cannot be displayed in CMYK color gamut, That is, the color of the image exceeds the printing color gamut, which is called overflow color. The advantage of color correction in CMYK color space is that the corrected image is directly used for printing without color overflow. In addition, because CMYK color space is a color space that conforms to people's visual habits, it is easier to grasp the change of color when representing a certain color and its change. In view of these aspects, generally, the image can be corrected in RGB color space, and then the image can be slightly adjusted in CMYK color space

② discrimination of color deviation

gray balance is a very important concept for correcting color. When we judge the color deviation of the scanned digital image, we need to use the concept of gray balance. If you know the primary color components required to generate neutral gray with various brightness, you can use the neutral gray area in the original for color correction. In Photoshop, use the screen density tool (info) to measure the color value in the digital image. If the area that should be neutral gray, but its value is not the value of gray balance, it indicates that the image has color deviation. According to the proportion of gray balance, it is easy to judge which color is more and which color is less

the scattered highlight area in the image is the best area to check neutral gray. The highlight area is not all neutral gray, but its gray component is more than that of other brightness colors. Therefore, from this inspection, we can not only judge the color deviation, but also start the calibration of the highlight pole mentioned above

in addition, there are many other experiences in distinguishing color deviation, such as memory color, such as blue sky and white clouds, green grass and green space, etc

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