Relevant knowledge of printing and image reproduction (2)

Under normal circumstances, dot gain values ​​in offset printing average 8% to 12% in coated paper, 20% in offset paper, and more than 30% in newsprint. However, in the same type of offset printing presses, the same copperplate printing paper is used. The dot expansion value of the hard-liner and air-cushion blanket is 8%~12%; the neutral lining is more than 15%; and the soft lining is more than 20%. In the same way, the dot gain value changes due to improper control of printing pressure, ink volume, and ink balance during printing.

The phenomenon of dot enlargement is mainly reflected in three aspects in printing control. One is the minimum point of highlights, because 1% or 2% of the small dots in the printing are actually not printed, while 2% or 3% of the small dots may jump to 5% or 7% due to the effect of dot gain. The second is the dark tone of the big idea. Due to the existence of network expansion, more than 85% of outlets have become 100% on the ground (that is, regions that are not composed of outlets). Third, since the value of the network expansion at 50% is the largest and the transition from a circular network to a square network at the same time, the tone of the tone may change significantly. The presence of these conditions shifts the entire reproduction tone curve to a dark tone during the printing process. There are no white spots (absolute nets) at high light spots, and dark spots are dotted with dots, making the rich levels contained in the dark tone not be displayed correctly.

The overlap of dark spots at the high light spot and the dark spot at the dark spot are the main printing troubles in printing. The appearance of this situation indicates that the entire image copy is lost at the level of the highlight area or the shadow tone area, and the copy curve also shifts. The following figure 1 shows the appearance of high-light opacity and shadowing and paste at the same time in the image network. Figure 2 below shows the network structure of normal replication. Correctly copied images should not be screened even in the brightest areas; the darkest places should also have anti-white features.

Another factor that causes the image-reproduction curve to shift toward mid-darkness is because human vision does not feel the same for the highlights and shadows of the image. The hierarchical tones reproduced in equal density do not appear to be evenly distributed in human eyes. Therefore, the image must be brightened during the normal copying process. Although this will result in loss of the high light level, it will allow the dark tone level to be fully represented.

Although the phenomenon of dot enlargement and the offset of medium-level shadow tone are mainly manifested in the printing stage, overcoming these problems is not a solution to the printing process. The fundamental method to solve these problems is to correctly set the GAMMA value of the white field black field and the copy curve of the image copy according to the actual printing situation in the prepress production stage, especially in the image scanning stage, and enlarge the dots displayed in the printing in advance Be small. Put the problems that arise in printing before printing. This is the basic principle of data standardized print control.

Relevant knowledge of printing and image reproduction

Color images are usually printed using four colors of cyan (C), magenta (M), yellow (Y), and black (K). The principle of color synthesis is subtractive color. We know that the three primary colors of the subtractive method or the three primary colors are cyan, magenta, and yellow. However, the three theoretical colors of the three primary colors used in printing have the same theoretical three primary colors. Especially magenta and blue. For example, three-color inks used in color mesh printing, there are mainly two kinds of products based on Japanese standards or European standards. Among the three standard inks of Japan, the hue of the magenta ink is approximately C23, M69 based on the C, M, and Y ratios; C63, Y29 are blue; C3, M4, and Y94 are yellow. Obviously, only yellow is ideal in three colors. In the figure below, there are three complementary colors of R, G, and B of the additive method and Y, C, and M of the mixed color; the three primary colors of C, M, and Y of the subtractive method are R, G, and B three colors. From this we can see that the hue difference between them is still very large. Second, the transparency of the ink is not satisfactory. In multi-color printing, several colors are printed in order in a certain order.

So multi-color printing has successively printed color sequence problems. Each halftone color has a different appearance in different areas of the image in highlights, midtones, and shadows. Among them, the colored dots of the highlight region appear to be juxtaposed with each other; the intermediate tones have both the appearance of the dot juxtaposition and the overlapping of the dot dots; and the dark tone is mainly represented by the overlap of dots. Because the transparency of the ink is not ideal, the color synthesis in each area is further complicated. The left, center, and right sides of the image below show the distribution of dots in the highlight, middle, and shadow. The following figure shows the ink overprint condition. Here we first explain the principle that we can see the various colors presented by objects. When the object receives light, it reflects or passes the color light of the same hue as its own color, or it can synthesize the color of this color, and absorbs a color (complementary) that is irrelevant to itself.

The surface of a transparent object simultaneously transmits and reflects the same color light or inter-color. In the figure below, the outlets are in parallel with each other. Here take C (Cyan) as an example. The incident light white light (RGB) first reflects the inter-color (GB) of the hue of the ink on the surface of the dot, absorbs the complementary color (R), and the remaining interspersed color (GB) passes through the ink layer to reach the paper surface and is reflected again. The green (G) and green (G) and blue (B) mixtures of cyan (C) are still reduced to cyan (C). M (magenta) and yellow (Y) are the same as in the case of Cyan. At the same time there are a large number of blank areas in the highlight area. The incident white light is still reflected in white (paper). The following figure shows the situation where the outlets are both tied and overlapped. Let's look at the overlapping of yellow and blue. The incident white light first reflects the yellow color (RG) on the yellow ink surface, absorbs the complementary color of yellow, and the rest reflects the complementary color of the blue ink when it reaches the surface of the blue ink. Since the blue in the blue ink complement has been absorbed by the yellow ink, only the green remains. The rest of the green color continues to reach the paper surface through blue ink and is reflected again. Under the figure below is the stack of three layers of ink.

From the ink overprint situation, due to the ink's transparency is not ideal, plus the ink color hue is not pure, resulting in three-color ink overprint formed by the color is not real black, but dark brown. Therefore, it is necessary to add a black color in actual printing to make up for the insufficiency of the three primary color ink printing. This forms the CMYK four-color printing system. However, this four-color printing is still based on the theory of three primary color subtractive methods.

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