Analysis and Discussion Based on Prepress Screening Technology
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Screening technology refers to the decomposition of an image into pixels, and each pixel expresses the shade of the layer with an ink dot whose ink area accounts for a percentage of the total area of the pixel. Screening technology is the most basic way to reproduce continuous-tone images in the printing field, and it is also an important technology to determine the quality of printed matter. The application of screening technology has been more than 100 years. Through the process of glass screen, contact screen, electronic screening, digital screening, etc., advanced technologies such as rationality, irrationality, supercell structure and FM screening have been formed. Nowadays, AM screening and FM screening have formed two major mainstreams of screening technology and played a leading role. With the development of technology, a number of different screening technologies have also emerged, and have been widely used in certain printing fields to print printed materials of various uses. This article is interpreted in the form of digital screening.
1. Why do you want to add a net?
From the darkest black to the brightest white, the human eye can produce vision for many different light, but in the traditional printing process, there are only two forms of expression, namely the graphic part and the blank part. Because in the traditional printing, the printing process must have the existence of the printing plate, and there are only two elements on the printing plate, and the ink is printed on the image and the ink is not printed in the blank. If there is no change in the two elements on the plate, then the printed product is only black and white, so it is impossible to transfer the rich layers and colors to the printed product.
Imagine if you can divide a plate into a number of points with different areas and sizes. By inking these points with different areas, the difference in the amount of ink will be different. Similarly, the amount of ink will also reflect the change in color. . Such "small dots" are called dots in printing. This technical technique is the screening technique. The advantage of screening technology is that it brings more vivid prints, clearer details and stable processes to the printing company.
2. Digital screening technology
The digital screening uses the electronic or digital screen to complete the formation process of the dot. The raster image processor determines the size and angle of the dot according to its requirements.
①The method of generating digital dots.
The generation of digital dots is to establish a mathematical model according to the shape of the dots, and specify the exposure order in a mesh unit. There are mainly four generation methods:
Threshold method. The method is to design a threshold matrix as a plane recording area in advance, and each recording grid of the area is set with a threshold value. When screening, the recording grid on the recording plane is made according to the pixel value read from the image. The judgment of exposure or no exposure.
When the digital mesh is generated by the threshold method, a threshold matrix needs to be designed for each layer change. In addition, different threshold matrices need to be designed for different dot shapes.
Although the threshold method can achieve good results, it requires huge resources to store the threshold matrix, and the threshold matrix must change with the change of the recording device. Even in modern times, such huge storage is still unacceptable.
Model method. This method is designed in advance by designing the various levels. For example, a 15% dot will usually be exposed at 15 small points in the center of the cell grid. Assuming the image has a grayscale of 50, a set of numbered models must be designed for each shape of the dot before exposure. . At the time of output, the output device reads out each gray value and extracts the desired dot model from the model set according to the specified dot shape, and controls exposure at the corresponding position.
The method of growing the model. Suppose a mesh unit contains 15 image points, that is, 4 horizontal and vertical lines, indicating that there are 1 5 + 1 = 16 gray levels. Suppose the image has 9 gray levels, that is, the gray value of the image ranges from 0 to 8. Since there are 15 image points and 9 levels, the two small squares correspond to one gray level. 0 For the exposure value 0, the pixel value 1 is for the exposure value 1, and each time one additional gray level is added, 2 exposure points are added, thereby obtaining a gray scale range of 0 to 8. It can be seen that regardless of the gray level, the growth model method only needs one dot model, which makes the screening process more efficient.
② Basic algorithm for digital screening.
The screening algorithms are divided into three types: rational tangent screening algorithm, irration tangent screening algorithm, and supercell screening algorithm.
Rational tangent screening algorithm. Rational tangent screening is the basis of digital screening. In color printing, adjusting the angle of the screen can avoid moiré. In the era of manual screening, the operator can rotate and adjust the screen at will, but on the electronic screening device, The number of individual angles cannot be formed. In order to solve this problem, the appropriate adjustment of the number of screen lines can also achieve the purpose of avoiding visible moiré.
The screening angle of the irrational tangent screening technique is an irrational number. In the implementation, the correction method is used to force the alignment of the corner points of the mesh to not coincide with the recording grid corner points.
Supercell screening algorithm. This method is a solution developed and improved for the deficiencies of the above two methods. This technology solves the problem of resolving power and precise approximation angle by dividing the large mesh into small meshes and setting the growth points of the dots within the range of limited output device accuracy, which not only obtains the accurate mesh angle, but also makes the digital plus The network goes beyond the traditional photographic screening, so supercell screening can ensure the improvement of copying accuracy.
3. Concluding remarks:
The purpose of screening is to convert the continuous manuscript into a mesh image. Through the understanding of the screening technology, I believe that it can enhance the understanding of the screening before the press. I believe that with the development of technology, not only the screening technology It will be more and more, and it will become more and more mature.