The dot angle refers to the angle between the arrangement line of the dot and the horizontal line. Generally, the angle measured in the counterclockwise direction prevails. The vertical and horizontal shapes of the chain dots are different, and the directions of the two columns differing by 180° are completely the same, and the dot angle thereof is 0° to 180°. However, when the square and circular dots differ by 90°, the angle is the same, so the angle is from 0° to 90°.
In general, when the angle difference between the two types of dots is 30 degrees and 60 degrees, the overall interference fringe is still more beautiful; the dot angle difference of 45 degrees is next; when the angle difference between the two dots is 15 degrees and 75 degrees At this time, the interference fringes will be very obvious, resulting in more obvious moiré (moirage), which is detrimental to the appearance of the image.
Taking a square dot as an example, from the visual effect point of view, the visual effect produced by the dot angle of 45° is the most comfortable and beautiful, and it is stable and not rigid. Therefore, for a monochrome print, the dot angle is usually a 45° angle. Although the dot angles of 15° and 75° are not very stable but not rigid, the dot angle of 0° has the worst visual effect. Although it is stable, it is very rigid. So usually the visually weakest yellow plate is used for 90°.
AM and FM sites
According to the method of screening, outlets are divided into AM screening, which is used to represent the level of the image in terms of the size of the dots, the spacing between the dots is fixed, the size of the dots is changed, and FM Screening is used to reduce the density of the dots instead of the dot size. To represent the level of the image). In the previous part of this paper, most of the descriptions of outlets were for AM sites. Here are some features of the following FM screening.
The full English name of FM screening is Frequency Modulation Screening.
The dot size of the screen dot is fixed. When the resolution of the output device is 2400 dpi, the minimum dot diameter is about 10.5 microns (ie, 1/2400 inch), which expresses the tone by controlling the denseness of the dot. The dots in the highlights are sparse, and the dots in the shadow tone are dense.
At present, the frequency modulation screening technology uses the first-order function random screening, the point size is equal, and the point distance changes randomly. Photographic image processing is based on the deposition of halogen particles (0.5-5 μm) with different sizes and distances to form a tone or a color. Therefore, there have been mechanisms to simulate the effect of photographs in principle. In other words, random placement using a second order function can change the distance and area of ​​the point randomly. Then, the second-order function screening and the original photograph are very similar in terms of the mechanism of the tone, and the copy will be much more accurate. Realistic, almost completely close to photo image level effects. Of course, the computational complexity of stochastic screening of second-order functions is even greater and more complex, and it is difficult to meet current requirements for printing equipment and conditions.
FM Screening has the advantages that no collision occurs; the resulting dots are small and can show more details; eliminating Tone jumps in the midtones; the registration problem does not affect the color balance;
Disadvantages of FM Screening: The optical level hierarchy and the peace network are prone to roughness and graininess; the density of dots in the intermediate tone is extremely high and the dot enlargement rate is difficult to control; the printing conditions for high-precision printing equipment and harshness are required; and the requirements for film output Stability, fine-point reproducibility and clarity are all higher than amplitude-modulated dots. When printing, the light position is easily lost, and the deep position is easily blocked.
At present, with the development of printing technology, there are also AM/FM hybrid networks, such as screen development technology, which will combine the advantages of conventional AM and FM screening technologies. Can avoid moire and broken lines and other issues.
In general, when the angle difference between the two types of dots is 30 degrees and 60 degrees, the overall interference fringe is still more beautiful; the dot angle difference of 45 degrees is next; when the angle difference between the two dots is 15 degrees and 75 degrees At this time, the interference fringes will be very obvious, resulting in more obvious moiré (moirage), which is detrimental to the appearance of the image.
Taking a square dot as an example, from the visual effect point of view, the visual effect produced by the dot angle of 45° is the most comfortable and beautiful, and it is stable and not rigid. Therefore, for a monochrome print, the dot angle is usually a 45° angle. Although the dot angles of 15° and 75° are not very stable but not rigid, the dot angle of 0° has the worst visual effect. Although it is stable, it is very rigid. So usually the visually weakest yellow plate is used for 90°.
AM and FM sites
According to the method of screening, outlets are divided into AM screening, which is used to represent the level of the image in terms of the size of the dots, the spacing between the dots is fixed, the size of the dots is changed, and FM Screening is used to reduce the density of the dots instead of the dot size. To represent the level of the image). In the previous part of this paper, most of the descriptions of outlets were for AM sites. Here are some features of the following FM screening.
The full English name of FM screening is Frequency Modulation Screening.
The dot size of the screen dot is fixed. When the resolution of the output device is 2400 dpi, the minimum dot diameter is about 10.5 microns (ie, 1/2400 inch), which expresses the tone by controlling the denseness of the dot. The dots in the highlights are sparse, and the dots in the shadow tone are dense.
At present, the frequency modulation screening technology uses the first-order function random screening, the point size is equal, and the point distance changes randomly. Photographic image processing is based on the deposition of halogen particles (0.5-5 μm) with different sizes and distances to form a tone or a color. Therefore, there have been mechanisms to simulate the effect of photographs in principle. In other words, random placement using a second order function can change the distance and area of ​​the point randomly. Then, the second-order function screening and the original photograph are very similar in terms of the mechanism of the tone, and the copy will be much more accurate. Realistic, almost completely close to photo image level effects. Of course, the computational complexity of stochastic screening of second-order functions is even greater and more complex, and it is difficult to meet current requirements for printing equipment and conditions.
FM Screening has the advantages that no collision occurs; the resulting dots are small and can show more details; eliminating Tone jumps in the midtones; the registration problem does not affect the color balance;
Disadvantages of FM Screening: The optical level hierarchy and the peace network are prone to roughness and graininess; the density of dots in the intermediate tone is extremely high and the dot enlargement rate is difficult to control; the printing conditions for high-precision printing equipment and harshness are required; and the requirements for film output Stability, fine-point reproducibility and clarity are all higher than amplitude-modulated dots. When printing, the light position is easily lost, and the deep position is easily blocked.
At present, with the development of printing technology, there are also AM/FM hybrid networks, such as screen development technology, which will combine the advantages of conventional AM and FM screening technologies. Can avoid moire and broken lines and other issues.
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