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About offset printing points (2)

the number of points below 50% and the number of points above 50% can be complementary, that is, 40% and 60% are complementary; Three percentage points and seven percentage points complement each other; Two percent and eight percent complement each other; One percent and nine percent complement each other. That is to say, starting from 60% of the points, it is determined by how many white points of the same size can be accommodated by the spacing of white points. The identification is shown in the figure point spacing figure. The point shapes in the figure

are divided into common point shapes and special point shapes. The commonly used point shape (square point, circular point, chain point) screen is the point shape commonly used in production at present. Special point shape (child parent point, triple connection point, ripple point, concentric circle point, etc.) is a point shape specially designed to obtain special artistic effects to improve the image tone (Figure). Now, with the development of computer technology, processing technology is also constantly updated. For example, art processing, three-dimensional processing, real object processing technology and so on are constantly applied in offset plate making

graph point type

(2) the number of points and lines is also called the number of lines or frequency. It refers to the number of parallel lines per unit length, that is, the number of unidirectional transparent parallel lines per inch

because 1 inch = 2.54cm, the English and metric linear systems can be converted to each other (table)

the thinner the line, the more points per unit area. Moreover, the higher the number of points and lines, the faster the number of points per unit area. The increase in the number of dots and lines has greatly enriched the level of printed matter expression and enhanced the effect of tone reproduction

on the contrary, if the number of dots and lines decreases, the level expressed by the screen will be weakened, and the tone reproduction effect will also be reduced. However, the choice of the number of dots and lines mainly depends on the type of printed matter, the type of paper and the surface condition

table screen line count mutual calculation

line/inch line/centimeter

table line point thickness of different printed matter category sight distance paper 175 ~ 200 lines/inch fine picture books, ancient painting reproduction, fine scientific and technological illustrations, such as book covers, are closer to coated paper, and advanced whiteboard

(3) point angle point angle generally refers to the angle between the point arrangement line (eye line) and the horizontal line (datum line)

offset printing uses printing plates with different dot percentages to reproduce the original color. For a four-color overprint print, if the four printing plates have the same point angle and the corresponding points on each printing plate can be accurately overlapped during printing, the best effect will be obtained. However, this is impossible. Generally, there is a certain angle difference between the point angles of the four printing plates (Fig. Both theoretical calculation and human visual perception have found that when the point angle difference of the printing plate is 30 °, the visual effect of the printed matter is the best. The human eye can hardly see the distance b line from the top of the turtle clip arc to the bottom of the shift lever formed by interference, and the image is uniform and harmonious. When the point angle difference is 22.5 °, the visual effect of the printed matter is not as good as the former, but it also looks soft and pleasing to the eye. When the point angle difference is 15 °, the visual effect of the printed matter is the worst and it looks very uncomfortable. Therefore, in practice, the point angle difference of 30 ° or 22.5 ° should be used as much as possible to avoid the point angle difference of 15 °. However, within the range of 90 °, only three colors can be arranged with an angle difference of 30 °, and another color can only be arranged with an angle difference of 15 °. Because yellow is the lightest, it is closest to the lightness of white paper. Cyan, magenta, and black inks cause a stronger visual sensation than yellow. Therefore, cyan, magenta, and black are usually arranged at an angle difference of 30 ° and the yellow version is arranged at an angle difference of 15 °

figure point angle figure

in order to avoid the appearance of moire, through theoretical and practical exploration, people carefully arrange multi-color printing, which will directly affect the point angle of large deformation and elongation after fracture

if the four printing plates take four different angles of 90 °, 75 °, 45 ° and 15 ° respectively, the point angle difference between the second, third and fourth plates is 30 °, only the point angle difference between the first and second plates is 15 °. In terms of visual effect, the dots arranged at 45 ° have a dynamic aesthetic feeling and look particularly comfortable. The points arranged at 90 ° will appear dull. The points arranged at 15 ° or 75 ° will have an effect between the former two, such as the collision plane. In addition, in the angle arrangement, it must be considered that the strong color version (main color version) mostly adopts a 45 ° line angle

it is in the offset printing process that different forms of dots are used, that is, a certain percentage of dots, the number of dots and lines and the angle of dots are used to make the printed matter reproduce the tone of the original

source: keyin

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