Digital screening technology

1. Point Aggregation Jitter

The dot aggregation jitter is the most typical and commonly used screening technology. In essence, it is the same as the screening principle of the screen, and the network points grow outward in the central cell manner. The center of the dot network has a fixed spatial position, the mutual center position of each dot network remains unchanged, and the growth of the dot network is controlled by the gray value of the pixel. The dot aggregation jitter network can be represented by the four parameters of the traditional network, namely, dot area rate, dot shape, network line angle, and screen number.

Threshold matrix is ​​one of the most important influencing factors in the process of network transmission. In order to obtain high-quality halftone images, the following practical guidelines can be referenced when selecting a threshold matrix:
(1) Ensure that the edge area ratio of the outlets is the smallest. Because the noise in the halftone image mainly comes from the edge of the dot.

(2) Ensure that the displacement of the center of the network is the minimum. The size of the dot changes with the change of the gray level of the image, and when the displacement of the center of the dot is kept at a minimum, the dot growth can be ensured uniformly and the center of gravity of the dot is stable.

(3) Pay attention to the order of network growth. The growth order of digital screening networks is a major factor affecting the output image quality. The intrinsic nature of the digital grid's dispersion determines that digital dots cannot grow as evenly as traditional photo screens. Each additional level of grayscale increases the number of print pixels, thereby increasing the dot. Therefore, the dots can easily become unbalanced and asymmetric, and some visually disturbing patterns such as extra textures, rough patterns, and the like are generated when printing. If conditions permit, designing a threshold matrix for each size of dot will help avoid these disturbing patterns.
Although the point aggregation jitter technology is relatively mature, there are some unavoidable defects:

(1) Loss of detail because the distance between the dot-aggregate-ordered jitter and the screen dot is fixed, so the middle blank area will cause the image detail to be lost.
(2) Grid collision and grid generation The grid shape of the digital image of the moire and the orderly jitter of the dot aggregation state will cause interference phenomenon and the moiré appears. Moire occurs when there are regular lines over the image.
(3) Rose spots Because of the different angles of screening in four colors, when you copy colors, there will be small rose classes. If a certain color overprint is not accurate or the screening angle is incorrect, it will cause obvious rose-spot interference.
(4) Visual gradation jump In the process of increasing the gray value of a pixel, as the dot accumulation state increases the area of ​​the screen dot, the dots will eventually contact each other. This should be a gradual step. The tune will produce a jump.

2. Point Discrete State Jitter

The point-discrete-state jitter technology is one of the main development directions of today's dot-matrix technology. The dots are composed of fixed-size cells. There is no fixed spatial position in the center of the dot, and the number of cells is controlled by the gray value of the image pixels. For the relative point aggregation state jitter, the point discrete state jitter can reproduce more image details, with no moiré and rose spots, and is not limited by the dot angle. However, there are also deficiencies such as:

(1) Due to the equal size of the dots, there is a sense of graininess, especially in the highlight part and the tone of about 25% is more pronounced.
(2) During the entire production process, the latitude is smaller than that of the orderly aggregated dot-grid screening technology. Therefore, more detailed process control and monitoring technologies are needed.
(3) The dot size is too small, so many printers cannot complete dot reproduction correctly.
(4) At present, there are certain difficulties in the proofing, printing, printing, and other aspects of images that are screened using the dot-discrete-ordered dithering technique, which will seriously hamper the application of the technology.
(5) At present, the difficulty of imaging the dots on the printing plate is still relatively large and unstable.

3. Micro-aggregation network technology

From the above analysis, it can be seen that the orderly aggregation of the point aggregation state and the discrete state of the points have advantages and disadvantages. As a combination of these two technologies, the micro-aggregation network technology can shield the disadvantages of the above two technologies in different images. Regions play their respective advantages. Generally speaking, the micro-aggregation dot technique displays the characteristics of the dot discrete dots in the bright spot, and the dot aggregation dots in the midtones, and the reversal dots (ie, white dots) in the dark tone. To sum up, micro-aggregation network technology has the following advantages:

(1) Applicable to any type of digital dot grid.
(2) It is possible to generate a network with a high frequency (fine network line) and a large number of levels.
(3) The transition between different levels is relatively flat and the gradation jump is not obvious.
(4) Breaking the regularity of traditional outlets and inserting irregularities into a fixed dot pattern.
(5) The optional combination of screen number and screening angle increases.
(6) The algorithm has high efficiency and low storage requirements.

In summary, after more than 100 years of practice and improvement, the screening technology has become increasingly mature. Digital screening, which has evolved slowly from photographic screening and electronic screening, has provided us with a method of physically forming network points. Compared with the former two screening technologies, digital screening technology is superior in several aspects. It can achieve algorithms that are difficult or impossible to implement in analog screening and can achieve higher quality. Mesh tune image. Digital screening technology has become an indispensable part of current image prepress processing.



Source: Shenzhen Fengshengtai Industrial Co., Ltd.