The effect of prepress image processing on print quality
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First, the reason why the printed image needs to be processed
The printing industry has evolved into a very widely used industry. In fact, printing can be done except for water and air, but it is not so easy to achieve consistency between prints and originals. Take lithographic offset printing, and many of its technical limitations have not changed radically. There are many unavoidable problems in offset printing products. These problems need to be corrected in the prepress process to expect satisfactory printing. Products, these limitations include the following aspects.
1. The color and tone range of the printed matter is quite different from the color and tone range of the original.
First of all, the offset process does not get all the colors of nature, but only a part of the color, which is caused by many defects in the ink, paper and printing process used in the printing process. The yellow, magenta, cyan, and black inks actually used in printing have defects in the range of color rendering, and the spectral absorption curve of ideal yellow, magenta, and cyan is not achieved, that is, the true color appearance of nature is not achieved. The actual ink not only absorbs the light in the spectral region that should be absorbed, but also absorbs the light in other spectral regions. 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 print is Ink is used to express hue and gradation, so the color rendering performance of the ink directly affects the fidelity of the original copy. Printing with primary color inks with "extra" absorption inevitably creates a "color shift" that greatly compresses the colored areas of the print. The cause of this defect in ink is mainly due to limitations of factors such as pigments and manufacturing processes. The "extra" absorption of yellow ink is the least, so the color rendering performance is the best, the magenta ink is the second, and the cyan ink has the worst color rendering performance.
Secondly, the highlight portion of the printed matter is formed by the color of the paper, that is, the paper also participates in the formation of color. Then, if the whiteness of the paper is different, it will affect the color brightness and saturation of the highlight of the picture, which will affect the color contrast of the picture. Similarly, it has a lot to do with the texture of the paper. The newsprint is a porous material. The ink is easily absorbed by the fiber, causing a high degree of light scattering on the printed surface, which reduces the printing density. The black area should not be black enough, but smooth and coated. The coated offset paper, the ink is adsorbed on the surface, and the light is scattered very little, so that the dark tone is "darker" and lacks layers.
In addition, the screening method used in printing also has an effect on the range of hue. In the process of plate making and printing, the highlights and shadows are easily polarized, that is, the details of highlights and shadows are easily lost.
For the above reasons, it is almost impossible for the printed matter and the original image to reach the same tone density range. Therefore, the density correspondence between the original and the printed matter must be adjusted, so that the printed product exhibits the best compromise effect. 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, and the density range is far less than the density range of the original, so how can the printed matter actually restore the original, or how to restore it better, This is a shortcoming of the current printing process and can only be compressed.
2. The color shift problem of the image during printing
When we process images on the display, if the display is well calibrated, the color of the image may be normal, but this does not mean that the color is the same after the image is printed. Since the display uses color and color to achieve color, Printing uses color subtraction to achieve color, so the reproduction color will be different. In fact, the ink to be used in printing should have the problem of color shift, that is, how to grasp the gray balance of the ink used. Briefly, gray balance refers to a combination of colors that produce a gray color. For example, in the RGB color-adding space, when RGB three colors with similar brightness are mixed, gray is generated, and the red, green, and blue colors having the brightness value of 200 are the same as the 25% gray. The gray here is also called neutral gray, which is a gray tone without color components. If you mix 210 red, 200 green, 200 blue, the result will be a warmer (red component) gray. It looks like gray, actually with a reddish gray, no longer a neutral gray. The color light is used here. In the RGB color adding space, the three colors only need to be added in equal amounts to produce neutral gray. However, when entering the CMYK printing field, the situation is not so simple. The same amount of yellow, yellow, and green does not produce neutral gray. They produce a lighter, turbid, brownish gray rather than a real gray. The reason is due to the undesired absorption of the color light by the ink described above (the spectral curve of the ink is not ideal), that is, caused by the impureness of the ink used, in the CMYK space of the actual ink, to obtain the solid gray, it is necessary to improve The amount of green ink. The excess cyan makes the other two colors cleaner, for example, 30% cyan, 2l% magenta, and 21% yellow mixed to produce 30% neutral ash, if it is 30% cyan, 30% magenta, and 30% yellow mixed The resulting neutral ash will have a dark brown color. For a certain type of ink product, the CMY value of the mixed gray color is constant, that is, the gray balance ratio value of a certain ink is constant, so that we can measure the gray balance of the ink, and then according to the balance The image is corrected so that the image to be printed can accurately reproduce the appearance of the color after correction, thereby making up for the defect of the ink, but it should be noted that the ink balance data of different brands of ink is different.
3. Dot increase problem
Offset printing uses the action of pressure to transfer ink. When the ink is transferred to the surface of the paper under pressure, a small amount of expansion occurs due to factors such as pressure. Some of the ink is absorbed into the paper fibers and also causes the dot shape to increase. Because the dot size is directly related to the tone and hue, the dot gain will make the entire image darker. Obviously, for the above reasons, different dot gain effects will be produced for different printers and different quality papers. In addition, the dots of different sizes have a non-linear relationship, and the dot gain formed by the characteristics of paper and ink generally exhibits an exponential expansion law. Other things to note are that in the process of graphic output and printing, the dot size increases due to material and equipment factors.
The dot gain is an inevitable phenomenon caused by a specific factor in the printing process. In order to truly reproduce the color and level of the original, it is necessary to compensate for the influence of dot gain in the prepress process. The compensation process can be applied directly during the graphic processing, or the compensation function can be added to the file first during the processing and then compensated during the printing and photographing process.
In addition to the above aspects, there are problems such as the control of the total amount of ink and the printing registration.
In theory, the three primary colors of CMY pigments can restore the color we want. However, in the actual printing, K is added indispensable. The reason is various, so I will not repeat them here. However, when printing with CMYK ink, many inks will accumulate in the dark area, and the black appearance will be achieved. When, for example, the cover of books and magazines, only K is far from black, it is often necessary to use C50M50Y50K50, or even C100M100Y100K100, which is at least 200% or even 400% of ink accumulation. Since different substrates retain inks differently (for example, newsprint retains ink much less than high-grade coated paper), if no measures are taken to control the amount of ink applied, the dark areas will be pasted and the dark level will be lost. 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 is also an important aspect in the printing suitability. The registration problem refers to the occurrence of errors in the printing position when the four color separation layouts are printed separately in color printing. The problem, especially in high-precision color printing, will not adversely affect the appearance of the finished product if it is not addressed. In the prepress processing, in order to avoid unfavorable appearance effects when the overprinting is not accurate, these problems must be considered and dealt with in design and post production in the demanding color printing, which is the trapping process. Many people don't understand the trapping process, especially the vast number of graphic designers, fill in some values when setting up, so there will be bigger problems, it is better not to make these settings.
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 these aspects, it is an image that meets the copying requirements in terms of printed originals. In terms of printing requirements, it is required to faithfully restore the manuscript, 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 each Personal visual habits and aesthetic habits are different. Everyone has different scales when adjusting. Therefore, 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 to make the original more beautiful, the method we use now 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.
1.1 high light, 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 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, not necessarily 0% or 100% here, 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 on the highlight and dark dropper in the Curves and Levels tools and enter the settings. 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 same can be set. 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 the high-light point with information with detail, called scattering highlight. To determine the highlight point of the print is actually to find the appropriate scattering highlights in the image, this can be used in Photoshop to check the color value of the important scattering highlights using the dropper tool to determine if it is in the printed 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 Click on the diffuse highlight on the image, and then select the shadow pole 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.
1.2 several typical process curves
For a given printed original, after it is subjected to highlighting and darkening, it can also artificially change its hierarchical reproduction to compress, expand or maintain the tone of the digital original. Since the density range of the printed matter is far less than the density range of the original, the level must be compressed. 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, 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 darkness and are 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.
2. Correction of image color
The reproduction of color refers to a complex 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. The influence of system, photosensitive material, paper, ink and other factors, color error is inevitable, especially the compression of the print level and the problem of ink, which has a crucial influence on the reproduction of color, in order to obtain the ideal color reproduction. You must try to correct these color errors to achieve the desired color reproduction.
2.1 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. Solutions, these are the basis for our correction of color. Here, we must pay special attention to the display device. In 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 to use the ink to restore the printed matter on the paper. In CMYK mode, using RGB display devices to reproduce CMYK images will inevitably affect the appearance of colors, so the conversion (color management system) in the display should be accurate, and the light source uniformity and stability should be maintained. Sex, in order to make the screen display and proof as consistent as possible.
Second, a level of correction is required before color correction. Because according to the color rendering mechanism, the color is presented on the basis of the neutral gray level, so the color correction should be performed after the level is corrected first. Otherwise, after the color correction is completed, the color will change when the level is corrected.
Then, in which color mode, the color correction is reasonable. 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, and the RGB color space is used. The advantage of the correction is that the color gamut space is larger and the color space of the display is the same, but since it must be converted to the CMYK space when used for printing output after correction, some colors cannot be displayed in the CMYK color gamut. That is, the color of the image is beyond the printing color gamut, called the 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, since the CMYK color space is a color space that conforms to people's visual habits, it is easier to grasp the change of color when expressing a certain color and its changes. In view of these aspects, the image can be corrected in the RGB color space in general, and then The CMYK color space fine-tunes the image.
2.2 color deviation discrimination
Gray balance is a very important concept for correcting colors. When we discriminate the color shift of the scanned digital image, we need to use the concept of gray balance. If we know the primary color components needed to generate neutral gray of various brightness, we can use the neutral in the original. The gray area is color corrected. In Photoshop, the screen density tool (Info) is used to measure the color value in the digital image. If the value is supposed to be neutral gray, the value is not the gray balance value, indicating that 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's important to remember the proportion of CMYK for these colors. The more you remember, the better you can judge the color of the image.