The Impact of Image Prepress on Print Quality (2)

Second, digital image prepress processing

Prepress processing of digital images should generally be carried out in terms of the level, color, and clarity of the image. If an image is better in all aspects, it is an image that meets the requirements for copying from the printed manuscript. In terms of printing requirements, faithful restoration of manuscripts is required, but the limitations of printing tell us that it is difficult to perform faithful restoration. Therefore, the consistency of visual appearance effects is a good complement to faithful restoration, but due to everyone The visual habits and aesthetic habits are different, and the scale that each individual grasps when adjusting is not the same, so they can only be described qualitatively.

1. Image level adjustment

The level of adjustment is to handle the high-profile, mid-tones, and darkness of the image, reproducing as many levels as possible. But as we have already mentioned, under normal circumstances, the density of the manuscript can reach 3.0 or even 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 manuscript, and the level is bound to be compressed. How to compress the print in order to restore the original better, we are currently using the method is to use the process compression curve, the purpose is to make the appearance of the print effect is almost close to the original appearance of the effect, where the visual appearance of the effect refers to a judgment of our eyes Instead of using a density instrument to measure, it actually means that "looks like the effect is close" even if it achieves its purpose. Image level adjustment can be analyzed from the following two aspects.

1High light, dark tone calibration

Highlights and shadows are the brightest and darkest tone values ​​on an image, also known as white and dark 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 highlights of the printed image, areas that are generally brighter than 3% to 5% are not printed, meaning that 3% to 5% of the area becomes 0%, that is, the white of the paper, so that the image is bright The level of the area will be lost. Conversely, more than 95% of dark areas will be printed as 100% black, and this dark tone level will be lost. This is the shortcoming of printing. In order to compensate for the effect of this deficiency on the level of the reproduced image, the image for printing must be subjected to gradation compression. For example, 0% white is compressed to 5% gray, and 100% black. Compressed to 95% dark gray. This is not necessarily 0% or 100%, but is usually 2% or 98%.

The best way to set highlights and dark points for printing during image processing is to use the highlights and shadows in Photoshop. There are dropper tools in the Curves and Levels tools in Photoshop. One of their functions is to It is used to set the pole so that the tone between the poles in 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 pole value can be set to 5, 3, 3, 0, and the shaded pole value can be set to 65, 53, 51, 95 or 95, 85, 85, 80. Simply double-click the Highlights and Shadows dropper in the Curves and Levels tools and enter the settings as shown in Figure 3. If it is set in the RGB color mode, the RGB equivalent highlight points are 244, 244, 244, and the dark tone pole values ​​are 10, 10, and 10, which can also be set. After setting up the high light dark tone eyedropper, all that needs to be done is to use the highlight and shadow dropper in the image to click on the determined highlight and dark tone points. So how do you determine the highlights and dark spots in the image?

Usually we understand the printing of high light spots can be divided into two kinds, one is the point without information, that is 0%, called specular highlights. The other is a highly specular highlight with information, called diffuse highlights. To determine the highlights of the printing is actually to find a suitable scattering highlights in the image, this can be used in Photoshop with a dropper tool to check the color value of the important scattering highlights to determine whether it is located in the print range (5% ~ Within 95%), if no adjustment is made within this range, if this diffuse highlight point is not within the print range, for example, 2%, and this point is very important and needs to be printed, then use the defined high-light drop Click on the scattered highlight spot on the image. Also select the dark tone pole and define it with a dark tone dropper. In this way, the two highlights and darkest poles that we have chosen define, after that, the hue between the highlights and the darkest points on the image will be redistributed according to the range of highlights and darkness settings, that is, the level of the image according to the printing characteristics. Corrected. After setting the highlight and dark tone poles, in order to achieve the consistency of the visual effect and the original, it is also necessary to adjust the image process compression curve. Here we will analyze several typical technological curves.

2 several typical process curves

For a given printed manuscript, after the high light dark tone calibration, it can also artificially change its level of copying, the digital manuscript will be compressed, expanded or remain unchanged. Since the density 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. The purpose of the expansion is to make the print as close to the original as possible from visual effects. In practical applications, it is necessary to grasp the key levels in the images to be copied and emphasized according to the features of the originals, and to reject the minor levels accordingly. For manuscripts with normal contrast, the low density area can reach 0.2-0.3D, the high density area can reach 2.1-2.9D, the maximum density contrast can reach 2.7D, the colors are bright, and the layers are rich. The high light to intermediate tone level of this kind of image is It should be emphasized as the main body of the image. Some manuscripts, such as night scenes, backlit photography, etc., have a large area of ​​shadow tone and are the subject of the picture. At this time, it is necessary to emphasize the tone levels. In short, according to the characteristics of the manuscript to be treated differently, and flexible. Figure 4 shows several typical tone modulation curves.

Curve 1 is where the highlight layer is pulled apart and the shadow levels are compressed. This applies to areas where the dark tone is small. Emphasis should be placed on highlighting the level of manuscripts, such as overexposed manuscripts during photography; curve 2 indicates thinning and levelling off the tone. The high-light level is compressed. It is suitable for dark areas, and the picture is boring. It is necessary to open the dark tone level to make the entire screen brighter. For example, the underexposure during photography leads to stuffy manuscripts and night scene images. Curve 3 Both the highlights and the dark tone are pulled apart, and the midtone level is flattened. This applies to the original with blurred pictures and relatively flat high-gloss levels, such as snow scenes; the curve 4 is compressed for high-light and dark tone levels, while the middle tone level In the case of being pulled, this is a type of curve that is often used, and is applicable to originals with thin originals and relatively low density contrast.

From these typical process curves, we can see that if we want to make prints closer to the manuscript, we must comprehensively consider various factors and choose the best one.

2. Image color correction

Color reproduction refers to a complicated process of color decomposition, transmission, and synthesis. The reduction of color is also a major aspect of printing and copying. In the process of color reproduction, the light source, lens, color filter, photoelectric conversion, such as scanning, is used. System, photosensitive material, paper, ink and other factors, the production of color error is inevitable, especially the level of print compression and ink problems, have a crucial impact on the restoration of color, in order to obtain the ideal color reproduction , you must try to correct these color errors, to achieve the ideal color reproduction.

1 Preparation before color correction

First of all, equipment calibration and system calibration are required. These equipments include scanning equipments, display equipments, output equipments and proofing equipments. These equipments must go through rigorous professional calibration, and there must be a complete set of color management between these equipments. Program. These are the basis for our color correction. Here we must particularly talk about display devices. In image processing, the appearance color of the image is reproduced by the monitor before printing. The display is based on the RGB mode, and the final product we want is printed matter that is restored to paper by ink. In CMYK mode, using RGB display devices to reproduce CMYK images will inevitably affect the appearance of the colors. Therefore, the conversion (color management system) in the display must be accurate, and the consistency of the light source in the lighting environment must be maintained. Stability can make the screen display and proofing as consistent as possible.

Secondly, level correction is required before color correction. According to the mechanism of color development, colors are presented on the basis of the neutral gray level, so the color correction should be performed after the levels have been corrected. Otherwise, after the color correction is completed, the colors will change when the levels are corrected.
Then there is a reasonable color correction in what color mode. In Photoshop, whether the image is RGB mode or CMYK mode, you can perform the adjustment of the tone and color, in which color mode to correct each time. The advantage of using RGB color space for correction is the larger color space, and the display The color space is the same, but it must be converted to CMYK space when used for printout after correction. This is because some colors cannot be displayed in the CMYK color space. That is, the color of the image exceeds 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 causing color spillage. In addition, since the CMYK color space is a color space that conforms to people's visual habits, it is easier to grasp color changes when expressing a certain color and its changes. Given 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

Gray balance is a very important concept for correcting colors. We use the concept of gray balance when judging the color shift of the scanned digital image. If the primary color components necessary for generating neutral ash of various brightnesses are known, the neutral gray region in the original can be used for color correction. In the Photoshop screen density tool (Info) to measure the color value of the digital image, if the area should be neutral gray, the value is not the value of the gray balance, then the image has a color cast, according to the proportion of gray balance, It is easy to judge which color is more or less.

Scattered highlights in the image are the best areas for checking neutral grays. Specular areas are not all neutral grey, but they have more gray components than other luma colors, so check from here not only Judging the color cast, the calibration of the high light poles mentioned earlier is also started here.

In addition, there are many other experiences in distinguishing color deviations, such as memory colors, blue sky and white clouds, green grass and so on. These colors have deep memory in people's minds. For professional image processing personnel, they are more It is important to remember the proportion of CMYK for these colors. The more you remember, the better you can determine the color of the image.