Digital screening technology

Screening is the basis for color printing reproduction. The color, level, and clarity of prints are reproduced by dots with certain characteristics. The reason why the digital proofing is to add screens is to copy proofs that can better preview the subsequent printing results, including whether or not there are moiré patterns, before the output of color separation film or printing and printing, and the screening process is very close to the printing process. , rose spots, dot enlargements, etc., truly WYSIWYG.

1. The location of the screening

In digital proofing workflows, the specific location of digital screening will differ depending on the post-RIP proofing and RIP proofing.

In the workflow of data proofing after receiving RIP, the digital proofing system directly receives digital screened data after RIP, and converts the extracted screened data into digital screened data for inkjet printers to control the printing equipment. The result of printing and netting equivalence. That is, the digital proofing system after RIP needs to convert and control the screening after RIP.

2. Type of screening

At present, the screening technologies used in digital proofing include several types such as AM screening, FM screening, and hybrid screening.

1) AM screening

AM screening is a traditional screening method and is referred to as AM screening. Its characteristic is that the dot interval is fixed, and the ink dot is concentrated in the center of the grid. The area size is a function of tone information. When the position and arrangement angle of the halftone dot are fixed, the depth of the image is represented by the size change of the halftone dot.

Because of the fixed frequency, the amplitude modulation network forms a fixed array of lattices, and there are limitations on the number of halftone lines and the dot angles. It is necessary to control the screen angle of two color plates at 30 degrees or more to avoid moire. AM sites have both the advantages of stable print reproduction, easy and uniform uniformity, as well as problems such as the propensity for net flowers, disturbing moire between screens, color skipping, unclear edges of text and subtle levels, and high demands. Overprint accuracy.

2) FM screening

FM screening is also called FM screening. It is a brand-new technology after AM. The feature is that randomly sized particles with the same size are randomly distributed in a unit area, and the released degree of density, ie, the spatial distribution frequency, is a function of image gradation information. The same size of the FM network, diameter between 8-30 microns, much smaller than the traditional network. The distribution of dots is uneven, and the depth and depth of the image are represented by the variation of dots within a unit area.

FM screening, as a new technology that occurs later than AM screening, has many advantages. In FM-screened images, there are only a certain number of randomly distributed element points in the unit grid, and the overall network cannot see the traditional dot. The distribution of the element points is irregular, so there is no problem with the angle of the mesh, and the interference fringes and the intermediate tone level jump phenomenon are fundamentally eliminated, and the level reproduction is even and beautiful. Since the element points are very small, the angular image power and sharpness of the image are greatly improved, and the fine resolution of the high-resolution amplitude modulation network can be obtained with a low-resolution apparatus. There is less overlap in the high-profile elements, which can increase the image saturation. Since there is no concept of screen angle, the problem of registration will not affect the color balance, and the requirement for overprinting accuracy of the equipment will be reduced, providing a possibility for mid-range equipment to duplicate high quality ultra-violet samples. In addition, the FM screening file of the sample image quality is smaller than the amplitude modulation network file, which can shorten the data backend transmission and processing time.

Because of its outstanding advantages, FM screening has received attention from people as soon as it appears, and is considered to be a better alternative to conventional AM screening. It has developed rapidly in recent years. Many foreign companies have launched their own FM screening technology, which has excellent development prospects in offset flexo printing, news printing and other fields. However, in China, the popularization and application of FM screening technology has been unsatisfactory for many reasons, especially the FM screening algorithm and other factors. The disadvantages of FM screening are:

The random distribution of the dots causes the high-light areas and the flat net areas to easily produce rough, grainy textures. Especially in the 1/4 tone area. This kind of random noise is more obvious and cannot be filtered out.

As a result, the number of dot gains increased as the number of outlets increased. The perimeter of the intermediate tone frequency modulation network is larger than the perimeter of the amplitude modulation network, and the density of the network points is extremely high, and the network point expansion is difficult to control. • Although the resulting screen file is small, due to the small network point, the calculation speed of the screening network is slower than that of the amplitude modulation. Adding network.

Due to the expansion of outlets and smaller outlets, the stability of FM outlets at the time of output is poor, and replication is now more difficult than AM screening.

High-light areas are easy to lose, dark areas are easy to paste.

When the same image is printed, FM screening consumes more ink than AM sampling.

The use of digital content samples for FM screening is not satisfactory because of the above reasons, and it is also constrained by non-self factors. Therefore, it has encountered some obstacles in the promotion process.

Limited by the traditional way of printing AM screening. In the traditional printing, it has been used to use the amplitude modulation dot, so the digital proofing that replaces the traditional proofing task must realize the seamless connection of the printing process and serve the traditional printing main process. Especially now that CTP has not yet been widely used in the transition phase, users are also required to get proofs of screening characteristics. If the digital proofing is different from the amplitude modulation netpoint frequency modulation screening, it can not accurately reflect the changes in the structure and size of the modulation dot on the printing press, and can not avoid the possible errors in the back end printing, nor reflect the actual number of lines printed, which is not conducive to the best printing conditions. The determination therefore limits the popularity of proofing in the FM network.

It is constrained by the practice of traditional offset printing and proofing. Some printing printers have contacted AM outlets for decades. After experienced workers visually examine traditional offset printing proofs, they know how to add ink and how much ink proofing the proofing network dot proofs are difficult to adapt to and instinctively resist. Many printers even rejected digital samples using FM outlets. However, after familiar with and accustomed to digital proofing and having some experience, printers are welcome to digital proofing.

In response to the international demand situation in the country, manufacturers from many color management software and color printing top digital proofing system, the company has focused on the digital proofing software research on how to meet the printing requirements in the color printer AM simulation network. For example, Blackmagic's imitation true dot technology is the final printed proof and printed matter, consistent in color and network conditions. Including the moiré that will appear in the printing process, as well as dot gain can also be pre-checked before printing. This is a hybrid screening technology.

3) Hybrid Screening

Hybrid screening is a screening technology that combines the advantages of AM and FM for high-precision printing. In the highlight area and the shadow area, it is like the FM screening, and shows the level change in the screen through 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 the printing also added a calculation method to add a? D? D using a number of small ideas to form a larger printing point.

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 an AM screen. Therefore, the middle part of the network both the distribution characteristics of FM outlets and the tone representation of the AM network.

Hybrid screening integrates both AM and FM screening structures and exhibits the following characteristics:

Due to the special treatment of the dot positions in the highlight and shadow areas, the particles are reduced in relative FM. In addition, the combination of small and medium nodes has been processed to improve the present and stability of replication.

In the midtone region, because of the randomness of the dot position, the color skipping in the AM is avoided. In addition, since the number of halftone dots is fixed, the dot density can be calculated from the number of dots per square inch, so that the halftone dot density can be defined.

For the printing process, production processes and equipment with conventional precision can be adopted to realize the printing quality of high-line-number dots and the production efficiency is not affected at the same time.

Mixed screening technology came into being with the advent of the CTP era. For example, FAIRDOT and WebPrincipal Plus, are two hybrid screening technologies for CTP that are aimed at high-precision printing. As the core of CTP technology, D?D digital proofing also introduced its own mixed screening proofing technology, imitating the real network technology. This hybrid screening technology can not only express the color vividly, but also show the exhaustion of the CTP process flow. Therefore, hybrid screening will be the best choice for future image replication and digital process inspections.