The effect of prepress processing of images on print quality(one)
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
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 curves of ideal yellow, magenta, and cyan are not achieved, that is, the true color appearance of the natural world is not achieved. As shown in the comparison in Figure 1, the actual absorption curve of the ink and the ideal absorption curve are very different, that is, they not only absorb the light in the spectral region that should be absorbed, but also absorb the light in other spectral regions, and this harmful The direct result of absorption is that the hue and saturation of the ink are far from the ideal three primary colors, and the print is to use ink to express the hue and the level. Therefore, 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. From the comparison of Figure 1, the yellow ink has the least "extra" absorption, 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, and the ink is easily absorbed by the fiber, causing a high degree of light scattering on the printed surface, which reduces the printing density, and the black area should not be black enough. Smooth, coated offset paper, the ink is adsorbed on the surface, and the light is scattered very little, making the dark tone "darker" and lacking layers.
In addition, the mesh-adjusting 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 to make the printed product have the best compromise effect. Figure 2 shows the relationship between the tone reproduction between the original and the printed matter. The 45-degree diagonal line in the figure represents the ideal print, that is, the original density is not changed by any mapping (that is, without any tone compression) Copy method. This situation is almost impossible to achieve in current offset printing methods. 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. The other two replication curves in the figure reflect the basic mapping relationship when density range compression is needed. Linear compression is a kind of compression method that is unreasonable from the visual effect. It is a tone that cannot be printed. Partially lost (ie, the tone portion with a density higher than 1.8 is completely lost), leaving only the range of tonal density that can be printed, which is obviously unreasonable. The arc is a technical compression copy curve used to enlarge the intermediate level of the image. From the visual point of view, the compression curve of this process is reasonable.
[NextPage]
2. The color cast 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 is achieved by using the color reduction method, 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 is a gray with a warmer (red component). 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, more turbid, brownish gray rather than a real gray. The reason for this 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 increase the amount of cyan ink. The extra green makes the other two colors cleaner. For example, 30% cyan, 2l% magenta, and 21% yellow are mixed to produce 30% neutral ash. If it is 30% cyan, 30% magenta, and 30% yellow, the neutral ash produced by dark yellow will be dark brown. Table 1 is a gray balance ratio table for a typical ink. 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 compensating for the defect of the ink. However, 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 increase their dot points into a nonlinear relationship. The dot gain formed by the paper and ink characteristics generally exhibits an exponential expansion. 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 due to 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. This compensation process can be applied directly during the graphics processing, or the compensation function can be added to the file first during 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, the indispensable addition of K, the reasons are many, not to repeat here, but when printing with CMYK ink, the dark area will accumulate a lot of ink, to achieve a super black appearance When, for example, the cover of books and magazines, only K is far from black, often need to use C50M50Y50K50, or even C100M100Y100K100, which is at least 200% or even 400% of ink accumulation. Since different substrates retain ink in different capacities (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-adjusted area will be pasted, thus losing the dark level. . 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 the unfavorable appearance effect caused by improper overprinting, these problems must be considered and dealt with in design and post production in the demanding color printing, which is trapping processing. For the trapping process, many people don't understand its meaning, 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.