Analysis of the Influence of Computer Prepress Image Color Processing Technology on Printing Quality
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Printing is a complex process composition that is a highly technical task. The pre-press computer design has special computer software, and the color operation processing technology existing in it has a vital influence on the quality of our printed products. Therefore, the more we know about pre-press knowledge and printing process, we design The work can meet the printing requirements, and the color deviation is not allowed to affect the quality of the printed products.
As an important part of the graphic design elements, color image processing technology has entered a new era. The small printing factory founded by Yachang Printing in 1993 first proposed that “printing is an industry combining culture and technology, and it is traditional printing + IT technology + culture and art”. Let us realize that the development trend of graphic art design is carried out under the basic rules of printing technology and printing technology. It requires designers not only to have superb design skills and unlimited innovative aesthetic sense, but also to be comprehensive and complete. Understand the characteristics and process flow of modern printing technology, and combine art design and printing technology to create a print with elegant style and national cultural connotation.
First, the reason why the printed image color needs to be processed
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 the limitation of factors such as pigment and manufacturing process. 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 blue ink is presented. The color performance is the worst.
Secondly, the highlight portion of the print is formed by the color of the paper, which means that the paper also participates in the formation of the color. If the whiteness of the paper is different, it will affect the color brightness and saturation of the highlights 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 red 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 for this is that due to the undesired absorption of the color light by the ink described above, that is, caused by the impureness of the ink used, in the CMYK space of the actual ink, in order to obtain the solid gray, it is necessary to increase the amount of the cyan ink. The excess cyan makes the other two colors cleaner, for example, 30% cyan, 21% 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 mixed CMYK value of the gray color is constant, that is, the gray balance ratio value of a certain ink is constant, so that after measuring the gray balance of the ink, the balance can be based on 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.
The analysis from the above two points determines that we must effectively correct the color during pre-press image processing, otherwise it will seriously affect the quality of our prints.
Second, the color correction of the prepress image
1. 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. Color correction has always been the biggest headache for computer graphics and printing, because many colors on the screen are not printed at all, or there is a serious color shift. If it can't be corrected at the design stage, it will not be too late after the finished product is printed. It is.
First. 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 set of perfect colors between these equipments. Management 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 is performed. 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.
It is reasonable to correct the color in which color mode. In Photoshop, regardless of whether the image is RGB mode or CMYK third 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 color change when expressing a certain color and its changes. In view of these aspects, the image can generally be corrected in the RGB color space, and then Fine-tuning 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 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 memory in people's minds, which is more important for professional image processing personnel. It is necessary to remember the proportion of CMYK of these colors. The more you remember, the better you can judge whether the color of the image is accurate or not.
3 should pay attention to the color shift problem of RGB to CMYK
Once again, the printing uses four colors of magenta, cyan, yellow and black. Therefore, the colors are all specified by CMYK. As long as CMYK is determined, it means that the proportion of printed ink is determined, and the color usually does not have much problem. However, the color in the computer is expressed in RGB mode. Therefore, to print the RGB color, it is necessary to go through a color conversion step. Two problems will occur here:
The first is that there is no way for the ink to print out all the RGB colors. Usually high-brightness colors, fluorescent colors, and metallic luster are not printed. At this time, the program has to be replaced with the closest CMYK. If you use Photoshop, you will find that the selected color square has a triangle exclamation mark, or an exclamation point after the CMYK value indicates that the color cannot be printed.
The second question is, what conversion standard is based on RGB to CMYK? Even if an RGB color can be converted to CMYK, it does not mean that the actual ink color will be the same as the color displayed on the screen. Usually the biggest problem is that it occurs on cyan. In the uncorrected case, the cyan (R:0 G:255 B:255) seen on the screen is a very bright color, but the actual printing cyan (C: 100% M: 0% Y: 0% K) : 0%) is a completely different feeling, with a lot of cyan colors, such as blue and green, have serious color cast problems.
4 solutions
Using CMYK to specify color Some software, such as CorelDRAW and Photoshop, will especially consider the correctness of conversion between RGB and CMYK. As long as CMYK is used directly in the software to specify the color, you can get results close to the actual printing, so Use CMYK mode as much as possible to operate. Photoshop can convert the image file into CMYK mode for editing (MENU:Image>Mode>CMYK color), so you can completely avoid using unprintable colors. The disadvantage is that most filters can't work in CMYK mode. The method has to be temporarily converted to RGB mode, use the filter and then switch back to CMYK mode.
Another way is to use the printed color table to query the actual printed color. The color table is a reference template for color values, which lists various combinations of CMYK for query. So when you want to choose a specific color, first find the desired color from the color table, and then input the CMYK value marked on the table into the software. For the average person, this is the most effective color correction method. In actual use, it is of course impossible to check all the colors appearing in the layout, as long as the focus (such as text, frame line), or large-area coloring is confirmed. There is another thing like the color chart, called the standard color standard. This thing is to name the ink used in the printing system and print the samples one by one. The color inside must be printed with special ink and can't be used in CMYK printing.
In summary, modern designers should have the corresponding pre-press knowledge, and even the best design ideas will eventually become the finished products in our hands, so color as an important element in graphic design, not only in the print The effect of the pre-image has a decisive effect, and the correctness of the pre-press correction is more related to the quality of the finished product.