How to correctly select halftone dot shape in the

2022-08-05
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How to correctly select halftone dot shape in silk screen printing. What kind of point shape can be used to reduce the point enlargement and avoid the generation of moire pattern has always been the focus of debate. For a long time, in the field of printing, people usually ignore the influence of point shape on halftone printing, and many printing producers have made very incorrect assumptions about the selection of point shape. This is also the main reason why the printing quality of halftone image in printing is still very backward

traditional halftone images (generally formed by am points relative to FM) are arranged regularly through points with different sizes and equal distances (point center spacing), forming a continuous sense visually to achieve the purpose of replication. Among them, the regularly arranged points spread outward from the center, so the number of lines formed by the arrangement of points determines the area of a single point. For example, when the number of lines is 100 lines/inch, the area of a single point is: 1/100 × 1/100 square inch

on a digital Imagesetter, the area of a single halftone point is generally 16 × A grid structure of 16 pixels arranged in a specific order to form points representing a certain percentage of hue. Therefore, based on this structure, the maximum hue level that can be obtained is =255 levels, which is equivalent to increasing the hue value between each level by 0.35%. The shape of the point can also be determined according to the pixel size. At present, the most widely used point shapes are round point, square point, diamond point and oval point. In India and China, diamond dots and oval dots are more meaningful

1 halftone mutation and analysis of various point shapes

points have the most obvious effect on the halftone region of the image. This is because of the middle tone. That is, near 50% of the points, the adjacent points are expanded and the boundaries are in contact with each other. As a result, the printing points are not clearly imprinted, and in serious cases, the plates will be pasted or even discarded. We call the point enlargement of the middle tone region as the middle tone mutation. In printing, due to the high viscosity of the printing ink, the geometric shape of the overlapping points determines the size of the intermediate tone change

the dots keep uniform surrounding feature in the whole tone range. Near the middle tone, the angle formed by dot superposition is usually an acute angle (less than 90 °). Due to the dense angle, the areas between the dots are easy to leak each other and fill with ink, and the intermediate adjustment point of the printing is enlarged or the intermediate adjustment mutation is as high as 15%, so the dot is the worst point shape for printing

there are two reasons for selecting dots in printing: first, the uniform dot shape of dots is conducive to reducing the influence of ellipses and other natural linear dots, and can well avoid the formation of Moire; Secondly, compared with the printing production, on the high-speed rotary press and sheet fed offset press, the paper will produce large point deformation when passing through the blanket and the impression cylinder, so the dots will become oval, which is conducive to reducing the sudden change of intermediate tone (which will be introduced later). However, in printing, due to the small force, the dots generally do not deform

in order to improve the defect of dots, square dots are introduced. The following point shape is adopted in the design, that is, the point shape starts with a circle, gradually changes from a circle to a square in the middle adjustment range, and completely becomes a square point at 50% of the point, and then gradually changes back to a circle. The four corners of the square point contact at the same time, and the angle is greater than 90 °, but there may still be a sudden change in the middle tone. In the ideal case, the square point is expanded to 10 ~ 12% at the middle adjustment point. If the printing conditions are poor, the mutation will be greater. Moreover, because the straight line destroys the boundary of the point, the change of this style is more obvious to the human eye

in order to find a solution suitable for the whole hue range, two other dot shapes are designed. Diamond and ellipse, the following power supply can be automatically saved. We will discuss the change of geometric shape and adjustment value at the point edge with the help of the transformation and upgrading of the automotive industry when the intermediate adjustment area is superimposed. The point here also starts from a dot, transitions to a diamond or ellipse, reaches the standard shape in the middle, and then changes back to a circle. By changing the shape of the dots, the dots begin to contact in the lower hue range, generally 40%, and the dots keep growing until 60%. In addition to superimposing points at different hue values, such points can also make the angle of the contact range higher than 90 °, which is conducive to preventing ink leakage and ensuring that the imprint is clear and identifiable

however, as the dots transition to diamond or ellipse, the chance of dots becoming linear increases, resulting in the appearance of moire. The thin and narrow dots are also very easy to produce regular halftone stripe patterns visually. This phenomenon is particularly evident in areas with a large range of the same hue (such as an image depicting a large area of blue sky) or in areas where the hue changes slowly (such as flesh tone images)

The correct selection of halftone dot shape in

silk printing

dot shape is determined by the resolution of the output device. In order to make the best use of 255 levels of hue, the minimum output resolution of the imagesetter should be 16 times the number of design lines. For a 50 line/inch halftone image, the minimum output resolution required is 50*16=800dpi. If you feel that the fixed resolution of the imagesetter is not ideal, you can also derive the discrete hue number from the following formula:

hue level = {rounding (output resolution/number of lines)}

for example, if the resolution of the imagesetter is 400DPI and the halftone image is 50 lines/inch, the obtained hue level is: (400/50) 2-1 = 64-1 = 63 levels

although the calculated hue level is only 1/4 of the ideal range (255 levels), But for the observer, the difference is not obvious. The minimum acceptable hue value in printing is determined by the cooperation between ink, substrate and observation ring and relevant departments. The higher the smoothness of the substrate, the closer the viewing distance, and the higher the tone level required

the lower hue can meet the needs, but when the imagesetter with lower resolution describes complex angles and converts the point shape within the hue range, it will form abnormal or other irregular point shapes, as shown in Figure 1. When zoomed in, the point shape will look rough, easy to cause hole blockage, and produce extremely irregular or even unpredictable moire patterns. The only solution is to adopt appropriate linear aperture to reduce the influence of interference fringes

halftone dot design requires a high technical level and solid mathematical foundation. The design of dot transition sequence for a specific dot shape or countries that are accelerating the use of Internet to transform and upgrade traditional industries and cultivate and develop emerging industries involves theories such as image processing and image reproduction. At present, diamond or oval dots are widely used in India to reduce tone mutation and achieve good results. We believe that with the improvement of science and technology, the ideal dot shape suitable for printing will appear in the near future

Author: xulina

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