FM network: Irregular distribution of image density levels (on)

In the process of image reproduction, traditional screening methods are used to represent the shades of images. After more than a century of unremitting efforts, printing technicians have achieved a very high level of success. However, in this screening method, many regular rosette patterns appear in color overprinting, and even moiré sometimes appears. At the same time, due to the decomposition of the dots, with the decrease of the number of lines, the sharpness of the image is drastically reduced and the characteristics of the original are lost. In recent years, with the research of high-fidelity duplication technology, there have been superfine dot printings of more than 120 L/cm, which can accurately reproduce fine-grained and continuously tuned image effects. However, regardless of the degree of fineness, the characteristics of its outlets are the regular distribution of positive or negative points of different sizes in the image part, and there are always its fatal weaknesses. The structure of the rose pattern, that is, the pattern phenomenon caused by the screen angle It is independent of the number of network lines. At the same time, if the number of screen lines exceeds a certain limit, it will not only bring more difficulties to the copying, but also the tone value, especially the high-light part, will be more serious and the resolution will be limited. This is the biggest drawback of the traditional screening method.

FM network features

In the 70s of the 20th century, foreign countries started to study a method of FM screening. Unlike traditional screening, printing dots are calculated based on the statistics of the hue and gradation of the adjacent parts of the image, in the image area. Randomly distributed randomly and uniformly in size (mostly 7-40 μm) to represent the shades of the image. Due to the limitation of the technical conditions at the time, it was unable to satisfy a large number of computational needs. Therefore, it was postponed to the 1980s. With the development of the electronic computer, the computing capacity has greatly increased, and this screening method was realized. However, FM screening at this stage is only a random random screening of first-order functions. The dot sizes are the same. Dot pitch variation and photo image are formed by different size and distances of halogen particles (0.5-5 μm) to form a tone or color. Still different. Therefore, people continue to study that if the second order function is used for random screening, the distance and area of ​​the point can be randomly changed. Then, the second-order function screening and the original photograph are used to express the mechanism of the tone. Very similar, the replica will be more realistic and completely close to the effect of photo images. Of course, the computational complexity of stochastic screening of second-order functions is even greater and more complex, but it is believed that this will be achieved in the near future.

In fact, at the present stage, people have adopted a first-order function FM screening to show unparalleled advantages over the traditional screening methods, in order to show that the use of computer algorithms to control the arrangement of points can actually eliminate unsightly pattern periodicity. The structure is more suitable for image reproduction above the three primary colors. Because of the subtle fine tuning of the FM screen, if a 15 μm diameter dot is used, there is no doubt that a 1% -99% tone value can be reproduced. Therefore, the replica has a high resolution, a soft image tone, a rich gradation, and a realistic image. There is no Moll effect, this is why the FM screening method can get the charm of printing experts.

Marked minimum dot size
In the frequency modulation screening technology, the only key parameter is the size of the network point, which is much less than the traditional art screen printing parameters. It does not have the problems of dot network angle, screen number, and dot shape. The size of the FM spot is only related to the recording accuracy of the exposure device. Theoretically speaking, a 1200-dpi photo-recording spot size is a square point with a side length of 1/1200 in. However, the spot of light actually exposed by laser on a photographic film is a circle with a slightly smaller area than the square. 21 μm, which is equivalent to a 1.2% dot of 60 L/cm, or a 1.5% square point.

From the point of view of the output precision of the current image-setter, the size of the FM dot network that meets the printing requirements is completely guaranteed, and the problem depends on the dot size that can be printed in the printing. Therefore, it is not so much the computer hardware as FM printing and screening. technology. Therefore, the minimum printable size is a key factor in printing, but its value varies depending on the printing equipment, materials, process and technical level. According to reports, assuming a certain resolution, take a light spot as the ink-jet point, and foreign countries perform high-quality printing on glossy paper. The smallest printing dot can reach 8 μm, and 20-inch prints can also be made on rough paper. 40μm. However, in China, it is quite difficult to print less than 20μm dots. It is clear that there is a large gap.

(to be continued)