Digital Screening Technology in Gravure Printing (I)

In ordinary printing, dots are the basic units that represent the image hierarchy and color image color hierarchy. The difference in dot coverage can produce changes in the color hierarchy. After more than 100 years of practice and improvement, the screening technology has expanded from photographic screening and electronic screening to digital screening, providing us with a method of physically forming network points. Computer digital screening is a widely used technology in the high-end networking systems of imagesetters and extensions. Electric extension screening and computer digital screening are the same in terms of technology and principle, but the implementation method is different. Now with the continuous improvement of the quality and speed of color desktop publishing systems, especially the improvement of the performance of critical parts RIP, the speed and quality of screening have been greatly improved. Compared with other screening technologies, digital screening technology has shown its superiority in many 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.

Digital screening features

Digital screening is a screening technique used in computer publishing systems. According to the distribution status of recording outlets, digital screening includes AM, FM screening, and AM/FM hybrid screening methods. The amplitude modulation screen (point aggregation mode dithering technology) uses invisible, ordered rows and columns to mesh the images. Each mesh generates mesh points with different areas according to a certain angle and number of screen lines. The AM screening technology has been developed and perfected for nearly 100 years and has been used in many printing units. In the AM screening method, the screening system used by the electronic color separation machine is generally the IS network technology developed by the German Hell Company (an irrational tangent network point ) and RT network technology (rational tangent network). The Postscript screening technology is used in the color desktop publishing system. Although the AM screening technology is relatively mature, there are some unavoidable defects. For example, the loss of details, the occurrence of moiré and rose spots, and the visual impact of the net, especially the moiré, rose spots and gradation leap (that is, before and after the beginning of the overlap point network level jump) and other issues, in the amplitude screening screen is difficult to avoid , has always affected the printing quality of the image.

FM screening (point discrete state jitter technology) is a kind of stochastic screening. It gets discretely distributed recording points. Can better avoid the defects of AM screening technology. FM screening images have a higher information capacity. There are first-order FM outlets (refer to the size of each outlet (area), only the spatial distribution of the outlets randomly changes) and the second-tier FM outlets (referring to the size of outlets (area) and spatial distribution frequency are all changing). Because frequency modulation screening technology does not have the phenomenon of frequency interference, it is not limited by the screen angle, no net corners are generated, thus no moiré and rose spots are formed, and no misuse of the network occurs; and a low-resolution image-setter may also be used. Obtain high-quality images, improve the quality of film output, improve the clarity and stability of prints, and avoid the problem of density jumps due to meshing in the midtones. This eliminates the tonal shifts in the midtones and makes the hue more natural and harmonious. . The printed color prints are more detailed and the printing effect is better. However, there are still some defects in FM screening. For example, because the dot size is equal and the random noise of FM screening cannot be filtered out, it has a rough and grainy appearance, especially in the high-profile layer and the tone of about 25% is more obvious; The perimeter of the outlets is much longer than the AM outlets, so the outlets are also enlarged, requiring high-precision and harsh process control and monitoring technologies; images with poor color separation are difficult to improve quality by adjusting the ink on the printing press; The micro-dimension is so small that many printers cannot reproduce it correctly; the stability and reliability of the dots are not ideal.

AM/FM mixed screening (micro-aggregation network technology) is a screening technology that combines the advantages of AM and FM for high-precision printing. In the highlight region and the shadow region, it is like the amplitude modulation screening, and the degree of density in the screen is represented by the density of fine mesh dots with the same size. The positions of the dots are randomly distributed and processed through special calculations so that the dots do not overlap each other and the dots do not become too large. In order to adapt to printing, another calculation method has been added, that is, using a plurality of small dots to form a larger printing dot. In the midtone region, the position of the dot is random, and at the same time, the size of the dot can be changed like the amplitude modulation and screening. Therefore, the outlets in the middle section have both the distribution characteristics of the AM sites and the tone representation of the FM sites.

Mixed screening in the high light and dark tone areas due to the special treatment of the position of the network point, so that the relative adjustment of the frequency of the particles reduced a lot, a combination of small network processing calculations to improve the replication of the reproducibility and stability. For the midtone region, because of the randomness of the dot position, the color skipping phenomenon in the amplitude modulation is avoided. Since the number of intermediate tone points is fixed, the dot density per unit area can be calculated, so that the density of halftone dots can be defined. For the printing process, the production process and equipment with conventional precision can be adopted, and the printing quality of the high-line counts can be achieved without affecting the production efficiency.

Hybrid screening can be roughly divided into two types. One is to use traditional AM screening directly in the midtones, and full FM screening in highlights and shadows. That is, true amplitude modulation is completely used in the midtone portion, and true frequency modulation is completely used in the highlight and shadow portion. Although the algorithms of FM screening and AM screening are relatively mature, how can we make a smooth transition between these two completely different screening algorithms? This is the purpose of this hybrid screening. problem lies in.

The other is to use the feature algorithm of both frequency modulation and amplitude modulation simultaneously for screening. Currently, the transition screening between FM screening and AM screening is mainly based on these two methods. There are several types of hybrid digital network screening that are currently relatively mature. (1) Samba Screen and Hybrid Screen screening. Barcom's Samba Screen and PPC's Hybrid Screen Hybrid are the latest screening methods for flexo printing. Samba uses FM-FM network for highlights and shadows, while the middle tone uses traditional AM-AM. The smooth transition from random to traditional dots (5% to 10%, from 90% to 95%) is Digital autocomputing, rather than simply interpolating the respective density interpolated. The Hybrid Screening method is a screening method in which FM and AM are overlapped in the entire density range. (2) Spekta screening. Screen's recent introduction of a new hybrid screening technology, Spekta combines the advantages of FM and AM screening in dot formation, color mixing, moiré, and ease of operation compensation on the press, resulting in the highest thermal CTP imaging machine. Good quality. (3) SambaFlex screening. Its characteristic is that in the range of midtones, the network point realizes the transition between the two types of screening by gradually increasing the amplitude modulation screening characteristics and gradually reducing the FM screening characteristics. In the transition area, it is also first to perform amplitude modulation screening, and then delete a certain number of network points from the amplitude modulation screening according to the percentage of network points. But at the same time, the outlets will shift at the center point. (4) Staccato screening. It is a new digital hybrid screen of Creo's square laser dot technology Staccato, which is closer to FM screening than Spotlight. High-fidelity images can be produced and better details can be reproduced without moiré caused by AM screening.