Effect of Image Prepress on Print Quality

First, the reason why the printed image needs to be processed

The development of the printing industry has now become a very widely used industry. In fact, printing can be performed in addition to water and air, but it is not easy to achieve consistency between printed matter and manuscripts. Take lithographic offset printing as an example, many of its technical limitations have not changed fundamentally, and there are many unavoidable problems in offset printing products. These problems require us to correct it in the prepress process in order to expect satisfactory printing. Products, these limitations include the following aspects.

1. There is a big difference in the range of color and tonality of the printed matter and the color and tonal range of the original.

First of all, the offset printing process cannot get all the colors of nature, but only a part of the colors. This is due to many defects in the printing ink, paper and printing process. The yellow, magenta, cyan, and black inks actually used in printing have defects in the range of rendering colors, failing to achieve the spectral absorption curves of ideal yellow, magenta, and cyan, that is, they cannot reach the true color appearance in the natural world. As can be seen from the comparison of Figure 1, the actual ink absorption curve and the ideal absorption curve are very different, that is, they not only absorb light in the spectral region that should be absorbed, but also absorb light in other spectral regions. This is harmful. The direct consequence of absorption is that the hue and saturation of the ink are greatly different from the ideal three primary colors, and the printed matter uses the ink to express the hue and gradation. Therefore, the color rendering performance of the ink directly affects the fidelity of the original copying. Printing with primary inks with "extra" absorption will inevitably create a "color cast" and greatly compress the print area.

The cause of this ink defect is mainly due to the limitations of pigments and manufacturing processes and other factors. From the comparison of Figure 1, the yellow ink has the least "above" 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.

Second, the high-light part of the print is formed by the color of the paper, which means that the paper also participates in the formation of color. Then if the whiteness of the paper is different, it will affect the brightness and saturation of the highlights of the screen, and then affect the color contrast of the screen. Similarly, the texture of the paper is also very much related. Newsprint is a porous material. The ink is easily absorbed by the fibers, resulting in a high degree of light scattering on the printing surface, which reduces the printing density. The areas that should be dark are not dark enough. With a smooth, coated offset paper, the ink is absorbed on the surface and the light is scattered very little, making the dark tone “darker” and lacking layers.

In addition, the screen adjustment method used for printing has influence on the range of hue. In the process of plate making and printing, highlights and shadows are easily polarized, which means that details such as highlights and shadows are easily lost.

Based on the above reasons, it is almost impossible for us to achieve the same tone density range between the printed matter and the original picture. Therefore, the density correspondence between the original and the printed matter must be adjusted so that the printed product exhibits the best compromise effect. Figure 2 shows the relationship between the reproduction of the original and the print. The 45-degree sloping line represents the ideal print, ie, the original density does not have any mapping changes (ie, without any gradation compression). The copy method. This situation is almost impossible to achieve in the current offset printing method. 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 manuscript is far less than the density range of the manuscript. How can the printed matter actually restore the manuscript, or how to restore it better? This is a drawback of the current printing process and can only be compressed. The other two replication curves in the figure reflect the basic mapping relationship when the density range compression is needed. The straight line compression is a kind of compression method that is unreasonable in terms of visual effects. It is a non-printing adjustment method. It is clearly unreasonable that some parts are completely lost (that is, the tonal part with a density higher than 1.8 is completely lost), leaving only the range of tone density that can be printed out. The arc is a technical compression replication curve that is used to enlarge the range of the midtones of the image. From the visual effect point of view, this process compression curve is reasonable.

2. The problem of color shift in images during printing

When we process the image on the monitor, if the monitor is well calibrated, the image color may be relatively normal, but this does not mean that the image is printed with the same color, because the monitor uses the color light additive method to achieve the color, The printing uses the subtractive color method to realize the color, so the reproduced color will be different. In fact, the ink to be considered for printing will not suffer from the problem of color shift, that is, how the gray balance of the used ink is grasped. Briefly, gray balance refers to a combination of colors that can produce a gray color. For example, in the RGB additive color space, when three colors of RGB with similar brightness are mixed, a gray color is generated. The three colors of red, green, and blue with a luminance value of 200 are the same as those of 25% of gray. The grey color here is also called neutral gray, which is a gray tone that does not contain color components. If you mix 210 Red, 200 Green, and 200 Blue, the result will be a warmer (red component) gray. It looks like gray, it is actually reddish gray and no longer neutral grey. The color light is used here. In the RGB color space, three colors only need to be added in the same amount to generate neutral gray. However, when entering the CMYK printing field, the situation is not so simple. The same amount of yellow, product, and blue do not produce neutral gray. They produce a lighter, more turbid, brownish gray than true grey. The reason for this is due to the undesired absorption of colored light by ink described above (the spectral curve of the ink is not ideal), that is, caused by the impure ink used. In the CMYK space of the actual ink, in order to obtain the ground gray, it is necessary to increase the amount of the blue ink. The extra green makes the remaining two colors cleaner. For example, 30% cyan, 2l% magenta, and 21% yellow are mixed to produce 30% neutral ash, and if it is 30% cyan, 30% magenta, and 30% yellow, the resulting neutral ash will be dark brown. Table 1 is the gray balance scale of a typical ink. For a certain type of ink product, the gray CMY value of the mixture is constant, that is, the gray balance ratio of a certain ink is constant, so that after we measure the gray balance of the ink, we can use this balance. The image is corrected so that the image to be printed can accurately reproduce the color appearance after correction, thereby compensating for the defects of the ink. However, it should be noted that different brands of inks have different gray balance data.

3. Dot increase problem

Offset printing is the use of pressure to transfer the ink. When the ink is transferred to the surface of the paper under pressure, a small amount of expansion occurs due to pressure and other factors. Some inks are absorbed into the paper fibers and can also cause the dot shape to increase. Since the dot size is directly related to the tone and tone, dot gain will make the whole picture darker. Obviously, due to the above reasons, different dot gain effects will occur for different printing presses and different quality papers. In addition, the network points of different sizes increase to a non-linear relationship. The enlargement of dots formed by the characteristics of paper and ink generally presents an expansive pattern of enlargement. The other thing to note is that in the process of plate-making graphic output and printing, due to the material and equipment factors, the dot gain will also occur.

The dot gain is an unavoidable phenomenon due to certain factors in the printing process. In order to be able to faithfully reproduce the color and gradation of the original, it is necessary to compensate for the influence of dot gain in the prepress processing. This compensation process can be applied directly during the graphics processing process, or it can be added to the file first during processing, and then compensated during printing and photocopying.

In addition to the above several aspects, there are issues such as the control of the total amount of ink and printing registration.

In theory, the three primary colors of CMY pigments can restore the colors we want. However, in practical printing, K is added indispensably. The reasons for this are various, and will not be described here. However, when printing with CMYK ink, many dark inks will accumulate in the dark tone area, and a super black appearance effect will be achieved. At that time, for example, the covers of books, magazines, and pictorials, only K is far from black, and often requires the use of C50M50Y50K50, or even C100M100Y100K100, which is the amount of ink deposited at least to reach 200% or even 400%. Since the ability of different substrates to hold ink is different (for example, newsprint paper retains ink at a much lower level than advanced coated paper), if no measures are taken to control the amount of ink applied, dark areas will be pasted, losing the tone level. . Therefore, the total amount of ink needs to be set in the prepress image processing.

The registration problem is an unavoidable error caused by the mechanical factors of the printing equipment, and is an important aspect in the printability. The registration problem refers to the fact that in the color printing, the printing occurs when the four color separation layouts are printed separately. The problem, especially in high-precision color printing, will have a detrimental effect on the appearance of the finished product if it is not addressed. In pre-press processing, in order to avoid unfavorable appearance effects when overprinting is inaccurate, these problems must be considered and dealt with in design and post-production in high-demand color printing. This is trapping processing. For trapping, many people do not understand its meaning, especially the vast number of graphic designers, casually fill in some values ​​when setting, this will appear larger problems, it is better not to do these settings.

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. Not here