The influence of prepress processing of images on the quality of printed matter (two)
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Second, the pre-processing of digital images
The prepress processing of digital images should generally be carried out in terms of image hierarchy, color and sharpness. If an image is better in all of these aspects, it is an image that meets the reproduction requirements from the printed original. In terms of printing requirements, it is required to faithfully restore the original, but the limitations of printing tell us that it is difficult to carry out faithful restoration, so the consistency of the visual appearance effect is a good complement to faithful restoration, but because of everyone The visual habits and aesthetic habits are different. The scales that each person grasps when adjusting are different, so they can only be described qualitatively.
1. Image level adjustment
The level of adjustment is to deal with the high-key, mid-tone and dark-key of the image, and reproduce the levels as much as possible. However, as mentioned above, in general, 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 of the printed matter is far less than the density range of the original, and the level must be compressed. How to compress the printed matter can make the originals better. The current method we use is to use the process compression curve. The purpose is to make the appearance of the printed matter almost close to the appearance of the original. The visual appearance effect here refers to a judgment of our eyes. Instead of measuring it with a density instrument, it actually means "it looks close to the effect" even if it achieves its purpose. Image level adjustment can be analyzed from the following two aspects.
1 highlight, dark tone calibration
Highlights and shadows are the brightest and darkest tonal values on an image. They are also called white and black fields. If the image to be adjusted is used for printing, you need to consider how to set the values of highlights and shadows. Due to the highlight of the printed image, the area that is generally brighter than 3% to 5% is not printed, that is, the area of 3% to 5% becomes 0%, that is, the white color of the paper, so that the image is bright. The level of the area will be lost. Conversely, more than 95% of the dark areas will be printed as 100% black, and this part of the dark level will also be lost. This is the inadequacy of printing. In order to compensate for the effect of this deficiency on the level of the reproduced image, it is necessary to perform hierarchical compression on the image for printing, for example, compressing 0% white to 5% gray, 100% black. Compressed to 95% dark gray. This is not necessarily 0% or 100%, and is usually 2% or 98%.
The best way to set the highlights and shadows for printing during image processing is to use the highlight and dark dropper in Photoshop. There are dropper tools in the Curves and Levels tools in Photoshop. One of their functions is to specialize. It is used to set the pole so that the hue between the poles on the image will be redistributed according to the range set by the pole. 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 value can be set to 5, 3, 3, 0, and the dark key value can be set to 65, 53, 51, 95 or 95, 85, 85, 80. To set it up, simply double-click the highlight and dark dropper in the Curves and Levels tools and enter the settings as shown in Figure 3. If the setting is in the RGB color mode, the RGB equivalent highlight points are 244, 244, and 244, and the dark-adjusted pole values are 10, 10, and 10, and the settings can be made in the same manner. After setting the highlight dropper, all you need to do is click on the highlights and shadows in the image with the highlight and dark dropper. So how do you determine the highlights and shadows in the image?
Generally, we can understand that the printed highlight points can be divided into two types, one is the point without information, that is, 0%, which is called specular highlight. The other is a high-light point with information with details called diffuse highlights. To determine the highlight point of the print is actually to find the appropriate scattering highlights in the image, which can be used in Photoshop to check the color value of the important scattering highlights in the dropper tool to determine whether it is in the printing range (5% ~ 95%), if it is not adjusted within this range, if the scattering highlight is not within the printing range, such as 2%, and this is important, you need to print it, then use the defined high drop Tube to click on this diffuse highlight on the image. Also choose the dark key and define it with a dark dropper. After the two highlights and shadows that we choose are defined, the hue between the highlights and the shadows on the image will be redistributed according to the range set by the highlights and shadows, that is, the print level of the image. Corrected. After the highlights and shadows are fixed, in order to achieve the visual consistency with the original, it is necessary to adjust the image compression curve of the image. Let us analyze several typical process curves.
2 several typical process curves
For a given printed manuscript, after it has been subjected to highlighting and darkening, it is also possible to artificially change its hierarchical reproduction to compress, expand or maintain the tone of the digital manuscript. Since the density range of the printed matter is far less than the density range of the original, the level must be compressed, and in the compression process curve, the level of one area of the image is compressed, and the level of the corresponding other area is expanded. And the purpose of the extension is to make the print as close as possible to the original. In practical applications, according to the characteristics of the original, the key levels in the image to be copied and emphasized are grasped, and the secondary levels are 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 layers are rich. The highlights of such images are to the middle level. It should be emphasized as the main body of the image. Some manuscripts, such as night scenes and backlit photography works, have a large area of dark shadows, which is the main body of the picture. At this time, it is necessary to emphasize the dark level. In short, it should be treated differently according to the characteristics of the original, and be flexible. Figure 4 shows several typical tonal process curves.
Curve 1 is a high-light layer that is stretched out, and the dark-tone layer is compressed. It is suitable for small dark-adjusted areas. It is necessary to emphasize originals with bright levels, such as over-exposed originals during photography. Curve 2 indicates thinning and darkening levels. The high-light level is compressed, which is suitable for large dark-adjusted areas, and the picture is boring. It is necessary to open the dark tone level to make the whole picture brighter, such as the lack of exposure during photography, resulting in a thick original and night scene image; curve 3 Both the highlight and the dark tone are pulled apart, and the middle tone level is flattened. It is suitable for the original image with a dull picture, a flat image with a high light level, such as a snow scene, etc.; the curve 4 is a high-light dark-tone level, and the middle-tone level is compressed. In the case of being pulled, this is a curve that is often used for originals with a thin original and a small density contrast.
From these typical process curves, we can see that in order to make the printed matter closer to the original, it is necessary to comprehensively consider various factors and select an optimal tone process curve.
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2. Correction of image color
The reproduction of color refers to a complicated process of color decomposition, transmission and synthesis. The restoration of color is also a main aspect of print reproduction. In the process of color reproduction, it is subject to light source such as scanning, lens, color filter and photoelectric conversion. System, photosensitive materials, paper, ink and other factors, the production of color errors is inevitable, especially the compression of the print level and the problem of ink, which has a crucial impact on the reproduction of color, in order to obtain the desired color reproduction You must try to correct these color errors to achieve the desired color reproduction.
1 Preparation before color correction
First, equipment calibration and system calibration are required. These devices include scanning equipment, display equipment, output equipment, and proofing equipment. These equipments must undergo strict professional correction. In addition, there must be a relatively complete color management between these equipments. Program. These are the basis for our correction of color. Here, we must pay special attention to the display device. In the image processing, the appearance color of the image is reproduced by the display before the printing, the display is based on the RGB mode, and the final product we want is to use the ink to restore the printed matter on the paper. It is CMYK mode. Using RGB display device to reproduce CMYK image will inevitably affect the appearance of color, so the conversion (color management system) in the display should be accurate, and the light source environment should be kept consistent. Stability, in order to make the screen display and proof as consistent as possible.
Second, a level of correction is required before color correction. Because color is presented on the neutral gray level according to the color rendering mechanism, color correction should be performed after the gradation is corrected. Otherwise, after the color correction is completed, the color will change when the level is corrected.
Then there is a reasonable color correction in which color mode. In Photoshop, regardless of whether the image is in RGB mode or CMYK mode, the tone and color can be corrected. In which color mode, the correction is different. The advantage of using RGB color space for correction is that the color gamut space is large, and the display is The color space is the same, but since it must be converted to CMYK space when used for printing output after correction, there will be some colors that cannot be displayed in the CMYK color gamut, that is, the color of the image is beyond the printing color gamut, which is called overflow color. . The advantage of color correction in the CMYK color space is that the corrected image is used directly 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 changes. In view of these aspects, it is generally possible to correct the image in the RGB color space and then fine-tune the image in the CMYK color space.
2 color deviation discrimination
Gray balance is a very important concept for correcting colors. When we discriminate the color cast 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 of various brightness (see Table 1), 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 value is supposed to be neutral gray, the value is not the value of gray balance, then the image has a color shift, according to the proportion of gray balance. It's easy to tell which color is more and which color is less.
The diffuse highlight area in the image is the best area for checking neutral gray. The highlight area is not all neutral gray, but the gray component is more than the other brightness colors, so check it from here. Judging the color cast, the calibration of the high-light pole mentioned above is also started from here.
In addition, there are many other experiences in judging the color color deviation, such as memory color, like blue sky and white clouds, grass green space, etc. These colors have deep memories in people's minds. For professional image processing personnel, It is important to remember the proportion of CMYK for these colors. The more you remember, the better you can judge whether the color of the image is accurate or not.