2. Prepress processing of digital images
The pre-press processing of digital images should generally be carried out from the aspects of image gradation, color and sharpness. If an image is good in these aspects, it is an image that meets the requirements of reproduction from the printed manuscript. Speaking of 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 the visual appearance is a good supplement to the faithful restoration, but because everyone The visual habits and aesthetic habits are different, and the scale that everyone grasps when adjusting is different, so they can only be described qualitatively.
1. Adjustment of image level
The adjustment of the layers is to deal with the high-key, middle-tone and dark-tone of the image, and reproduce as many layers as possible. But as mentioned before, under normal circumstances, the density of the original can reach 3.0 or higher, and 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, the level must be compressed, then How to compress the printed matter to restore the original manuscript better, the method we currently use is to use the process compression curve, the purpose is to make the appearance of the printed matter is basically close to the appearance of the original manuscript, the visual appearance effect here refers to a judgment of our eyes , Instead of using density instruments to measure, in fact, "looks close to the effect" even if the purpose is achieved. The adjustment of the image level can be analyzed from the following two aspects.
â‘ High light and dark calibration
Highlights and dark tones refer to the brightest and darkest tones in an image, also known as white and black points. If the image to be adjusted is used for printing, you need to consider how to set the highlight and dark tones. Due to the highlights of the printed image, areas that are generally brighter than 3% to 5% cannot be printed, that is, the area of ​​3% to 5% becomes 0%, which is the white of the paper, so the image has high brightness The level of the area will be lost. On the contrary, over 95% of the dark areas will be printed as 100% black, and this part of the dark levels will also be lost. This is the shortcoming of printing. In order to compensate for the impact of this deficiency on the gradation of the reproduced image, it is necessary to compress the gradation of the image for printing, for example, compress 0% white to 5% off-white, and 100% black Compressed to 95% dark gray. This is not necessarily 0% or 100%, usually 2% or 98%.
The best way to set the highlight and dark tone points for printing during image processing is to use the highlight and dark tone eyedroppers in Photoshop. Both the Curves and Levels tools in Photoshop have eyedropper tools. One of their functions is dedicated Used to set the poles, so that the tones between the poles on the image will be redistributed according to the range set by the poles. The setting of the pole color value depends on the specific printing conditions. There is no uniform standard. In most cases, the commonly used CMYK highlight pole values ​​can be set to 5, 3, 3, 0, and the dark tone pole values ​​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 the settings are made in the RGB color mode, the equivalent highlight points of RGB are 244, 244, and 244, and the dimming pole values ​​are 10, 10, and 10, and the same can be set. After setting the highlight and dark tone dropper, all you need to do is click on the highlight and dark tone point with the highlight and dark tone dropper in the image. So how to determine the highlight and dark points in the image?
Usually we understand the printing specular points can be divided into two types, one is the point without information, that is, 0%, called specular specular. The other is a specular highlight with details, called scatter highlight. To determine the printed specular point is actually to find the appropriate diffuse specular point in the image. This can be used in Photoshop to check the color value of the important specular spot with the eyedropper tool to determine whether it is within the printing range (5% ~ 95%), if it is not adjusted within this range, if the scattered highlight point is not within the printing range, such as 2%, and this point is very important, it needs to be printed, then use the defined highlight drops Go to click on this scattered highlight on the image. Again select the dark pole and define it with the dark dropper. In this way, after the two highlight and dark tone poles we selected are defined, the hue between the highlight and dark poles on the image will be redistributed according to the range of the highlight and dark tone settings, that is, the characteristics of the image according to the printing characteristics Corrected. After setting the highlight and dark adjustment poles, in order to achieve the consistency of the visual effect and the original, it is also necessary to adjust the process compression curve of the image. Let's analyze several typical process curves.
â‘¡Several typical process curves
For a given printed manuscript, after performing the highlight and dark tone calibration, it can also artificially change its level reproduction, and compress, expand or maintain the tone of this digital manuscript. Because the density range of the printed matter is far less than the density range of the original, the gradation must be compressed. In the compression process curve, the gradation of one area of ​​the image is compressed, and the gradation of the corresponding another area is expanded. This compression And the purpose of expansion is to make the printed matter as close to the original as possible from the visual effect. In practical applications, according to the characteristics of the original, the key layers in the image to be copied and emphasized should be grasped, and the secondary layers should be discarded 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 color is bright and the gradation is rich. It should be emphasized as the main body of the image. Some manuscripts, such as night scenes and backlit photography, have large dark areas and are the main body of the picture. At this time, the dark tone level needs to be emphasized. In short, it should be treated differently according to the characteristics of the manuscript, and flexible to master. Figure 4 shows several typical tone process curves.
Curve â‘ is that the highlight layer is pulled apart, and the dark tone layer is compressed. It is suitable for originals with a small dark tone area that needs to emphasize bright tone layers, such as documents that are overexposed during photography; curve â‘¡ represents thinning and pulling away the dark tone layer , The highlight level is compressed, suitable for dark areas with large areas, and the picture is boring. It is necessary to pull the dark levels to make the whole screen brighter. For example, the manuscripts and night images that are caused by underexposure during photography, and the curve â‘¢ Both highlight and dark tone are pulled apart, and the middle tone level is flattened. It is suitable for manuscripts with a dull picture and a relatively bright highlight level, such as a snow scene. Curve â‘£ is that the highlight and dark tone levels are compressed, while the middle tone level In the case of being brand-new, this is a curve that is often used. It is suitable for originals with thin originals and low 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 an optimal tone process curve.
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2. Image color correction
Color reproduction refers to a complex process of color decomposition, transmission, and synthesis. The restoration of color is also a major aspect of printing reproduction. In the color reproduction process, it is subject to light sources, lenses, color filters, photoelectric conversion such as scanning Due to the influence of the system, photosensitive materials, paper, ink and other factors, the occurrence of color errors is inevitable, especially the compression of the print level and the problem of ink, which have a crucial influence on the color reproduction. To obtain an ideal color reproduction , You must try to correct these color errors to achieve the ideal color reproduction.
â‘ Preparation before color correction
First of all, we must carry out equipment calibration and system calibration. These equipment include scanning equipment, display equipment, output equipment and proofing equipment. These equipment must undergo strict professional calibration. In addition, there is a relatively complete set of color management between these equipments. Program. These are the basis of our color correction. Let me talk about the display device in particular. In the image processing, the appearance color of the image is reproduced by the display before printing. The display is based on the RGB mode, and the final product we want is the printed matter restored on paper with ink. It is in CMYK mode. Using RGB display devices to reproduce CMYK images will inevitably affect the appearance of colors. Therefore, this conversion (color management system) in the display must be accurate, and the consistency and light source of the lighting environment must also be maintained. Stability can make the screen display and proofing as consistent as possible.
Secondly, the gradation correction should be carried out before the color correction. According to the color rendering mechanism, the color is presented on the basis of neutral gray levels, so the color correction should be done after the color correction is completed, otherwise, after the color correction is completed, the color will change when the level is corrected.
Then there is the reasonable color correction mode. In Photoshop, regardless of whether the image is in RGB mode or CMYK mode, tone and color correction can be carried out. Which color mode is used for correction has its merits. The advantage of using RGB color space for correction is that the color gamut space is larger, and the monitor ’s The color space is consistent, but because it must be converted to the CMYK space when it is used for print output after correction, there will be some colors that cannot be displayed in the CMYK color gamut, that is, the color of the image exceeds the printing color gamut, called overflow color . The advantage of color correction in the CMYK color space is that the corrected image is directly used for printing without color overflow. In addition, because the CMYK color space is a color space that conforms to people's visual habits, it is easier to grasp the color change when expressing a certain color and its change. In view of these aspects, in general, you can correct the image in the RGB color space, and then fine-tune the image in the CMYK color space.
â‘¡Identification of color deviation
For color correction, gray balance is a very important concept. When we judge the color shift of the scanned digital image, we need to use the concept of gray balance. If you know the components of the primary colors required to generate neutral grays of various brightness (see Table 1), you can use the neutral gray areas in the original to perform color correction. Use the Screen Density Tool (Info) in Photoshop to measure the color value in the digital image. If the area should be neutral gray, but the value is not the gray balance value, it means that the image has a color shift. According to the gray balance ratio, It is easy to determine which color is more and which color is less.
The scattered highlight area in the image is the best area for checking neutral gray. The highlight area is not all neutral gray, but compared to other brightness colors, the gray component is more, so from here, not only can check To judge the color deviation, the calibration of the highlight pole mentioned earlier also starts from here.
In addition, there are many other experiences in judging the color deviation of colors, such as memory colors, such as blue sky and white clouds, grass and green land, etc. These colors have a deep memory in the human mind. For professional image processing personnel, more It is important to remember the CMYK ratio of these colors. Remember that the more you can better judge the accuracy of the image color.
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