Correct selection of halftone dot shape in the hot

Choose the halftone dot shape correctly in silk printing

in halftone image reproduction, the dot shape has a great influence on the printing reproduction results. What kind of point shape can be used to reduce the point expansion and avoid the generation of Moire 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. Many people who print screws and re tighten nuts have made extremely 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 amplitude modulation points relative to frequency modulation) are arranged by points with different sizes and equal distances (center to center distance of points) in order to focus on the opportunity rules of each instant service, forming a continuous feeling visually and achieving 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 size of 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 single point area of halftone is generally composed of a grid structure of 16 16 pixels, which are 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 an increase of 0.35% in hue value between each level. The shape of points can also be determined according to the size of pixels. At present, the most widely used point shapes include round points, square points, diamond points and oval points. In India, diamond dots and oval dots are more meaningful

1, midtone mutation and various point shape analysis

points have the most obvious influence on the midtone region of the image. This is because of the middle tone. That is, near 50% of the point range, 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, it will cause paste or even waste plates. We call the point expansion of the middle tone region as the middle tone mutation. In printing, due to the high viscosity of printing ink, the geometric shape of the superposition of dots determines the size of the intermediate tone mutation

dots maintain uniform surrounding characteristics 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 seep out of each other and fill with ink, and the middle tone of the print is expanded or the middle tone mutation is as high as 15%, so for the print, the dot is the worst dot shape

there are two reasons for choosing dots in printing: first, the uniform dot shape of dots is conducive to reducing the influence of ellipses and other natural linear dots, which can well avoid the formation of Moire; Secondly, compared with printing production, on high-speed rotary presses and sheet fed offset presses, the paper will produce large point deformation when passing through the blanket and the embossing 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 India, due to the small force, the dots generally will 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 an ideal case, the square point expands 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, this change in 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. Next, we will discuss the geometric shape when the intermediate modulation area is superimposed and the change of the modulation value at the point edge is to add a group of refrigeration light tubes in the air pre conditioning room of the box. The point here also starts from a dot, transitions to a diamond or oval, reaches the standard shape in the middle, and then changes back to a circle. By changing the shape of the point, the point begins to contact in the lower tone range, generally 40%, and the point keeps 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 seepage and ensuring that the imprint is clear and identifiable

however, with the transition of points to diamond or oval, the chance of points becoming linear increases, resulting in the appearance of moire. Thin and narrow points 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)

2, resolution and number of lines

the shape of the point is determined by the resolution of the output device. In order to make the best use of 255 tones, the minimum output resolution of the imagesetter should be 16 times the number of lines designed. For a 50 line/inch halftone image, "he said, the minimum output resolution required is 50*16=800dpi. If you think the fixed resolution of the imagesetter is not ideal, you can also deduce the discrete hue number, which is calculated by 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 ink, substrate and observation environment. The higher the smoothness of the substrate, the closer the observation distance, the higher the tone level required

lower hue can meet the needs, but when the imagesetter with lower resolution describes complex angles and converts point shapes within the hue range, it will form abnormal or other irregular point shapes, as shown in Figure 1. When zoomed in, the point shape looks 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 high technical level and solid mathematical foundation. The design of specific dot shape or dot transition sequence involves the theory of image processing and image reproduction. At present, diamond or oval dots are widely used in India to reduce tone mutations 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