(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
The dots can have different shapes. The dot shape refers to the geometric shape of a single dot, that is, the edge shape of the dot. In traditional dot technology, the shape of the dot is determined by the corresponding screen structure. In addition to their respective performance characteristics, dots of different shapes also have different changes in the image copy process, which will produce different copy results and affect the quality requirements of the copy results.
Different shapes of dots have different image tone transfer characteristics. In the actual plate-making and printing process, there is a tendency for the dots to expand mechanically. Tests have shown that this trend increases with the increase of the dot perimeter (or the total circumference). The sensitivity of the dot area ratio to the perimeter change is proportional to its perimeter,
The dots with a large perimeter (circumference and sum) are easier to expand, and the greater the distortion that may occur when copying images.
The shape of the dots is different, the area ratio of the dots overlapped by the adjacent dots is different, and the overlap (the time when the dots overlap is the circumference and the largest, so the characteristics of the dots are often used to describe the nature of the different dots) Different conditions of image tone transfer are different.
The overlapping of the dots will cause: the jump in density increases.
When choosing the shape of the dots to screen the image, the dot gain is often the first factor to be considered. The changing trend of the dots of different shapes leads to different choices of dots for different products.
The dot shapes used by traditional screening methods are square, round, diamond (diamond), ellipse, double dot, etc .; in modern digital screening technology, more dots are available. The following is a description of commonly used dot shapes and their characteristics.
1. Square outlets. When a square-shaped dot is used to copy the image, the ink and white at 50% dots just make a checkerboard shape. It is easy to judge the relative percentage of square dots according to the dot spacing. It is more sensitive to the transmission of the original level. Figure 1 shows 50% square dots arranged at a 90-degree angle.
The above mentioned seem to be the shortcomings of square dots, it seems that square dots are no longer necessary. In fact, in some cases, such as the case where the requirements for midtones are not particularly strict, the selection of square dots can show a more vivid level.
2. Circular dots. Among the dots of the same area, the circumference of the circular dot is the shortest. When using round dots, the highlights and dots in the middle tone are not connected to each other, and only the dots in the dark tone can contact each other. Therefore, the increase value of the dots below the middle tone of the picture is small, and the middle level can be better preserved.
Compared with other shapes of dots, the expansion coefficient of circular dots is smaller. Under normal circumstances, circular dots are connected around the 70% area ratio. Once the circular dots are connected to the circular dots, the expansion coefficient will be very high, which will lead to the accumulation of dots in the dark areas due to the excessive amount of ink in the dark areas, which will eventually cause the dark parts of the image to lose its due level.
Figure 2 Gradient circular dots
Through the above description, we will find that circular dots are limited in their use due to their poor ability to express dark levels. Under normal circumstances, printing factories often avoid the use of circular dots, especially when printing with offset paper. However, if there are many bright tone levels in the original screen to be copied, and there are few dark tone parts, it is still quite advantageous to use circular dots to express the high and middle tone area levels.
Below we give the deformation characteristic curves of square dots and round dots:
Pure square dots make corners at 50% once, and pure circular dots make corners at about 78%. It can be seen that when the dots are not angled, the expansion of square dots is higher than that of circles.
Figure 3 Gradient diamond-shaped dots
When the dot area ratio is about 25%, the chain-shaped dot long axis transfer (called the first transfer) occurs; then the second transfer occurs at 75%. Since the increase of the dots is inevitable, the diamond dots will jump twice at 25% and 75%. However, since the connection of the diamond-shaped dots only occurs at two vertices, such a tone jump is more moderate than the change when the four corners of the square dots are all connected. It can be seen that the printing tone curve is relatively smooth when the image is copied with diamond dots, and it performs particularly well in the middle range of 30% -70%. Therefore, the diamond-shaped dots are suitable for copying manuscripts whose main scenes are characters.
4. Oval dots. This kind of dot is similar to the diamond dots with unequal diagonals, the difference is that the four corners are not sharp, but round, so they will not cross at 25% dot area rate like the diamond dots with unequal diagonals. In addition, there is no obvious jump phenomenon at 75% dot area ratio. Figure 4 is a gradient from 0% to 100% made with oval dots in Photoshop.
Figure 4 oval dots
5. Double-dot outlets. This type of screen dot is usually used in an occasion called multi-resolution screening, which is formed by superimposing two different screen dots with different characteristics and dots. The composition of different sizes of dots is: a small dot is embedded between two large dots, or a large dot is embedded between four small dots. The multi-resolution screening method attempts to balance the minimum dot size, dynamic range, and screening specifications. In order to obtain satisfactory results under certain constraints, the plate-making workers made a lot of suggestions, and this is how the two-dot network outlets are put forward. When the dots are smaller, the distance between them is increased, and in the midtone area, the dots are closer.
The two-dot dots are also called satellite dots or mother-daughter dots. The feature is that although the small dots in the dark tone of the screen have been merged, the large dots still exist. Although the number of dots is less, the dots are strong, smooth, complete, and high-gloss. The performance is soft and even.
6. Special shape dots. From a technical point of view, changing and selecting different dot shapes is a need for printability. However, in order to meet the needs of artwork reproduction, advertising and special interests, special shapes of dots (or artistic meshes) are often used to increase the artistic atmosphere of the picture and obtain specific artistic effects. Commonly used artistic reticulations are concentric circular reticulation, horizontal wavy reticulation, vertical line reticulation, cross cross gauze reticulation, gauze reticulation and brick wall reticulation.
Different dot shapes have different effects on dot gain during printing. The conclusion obtained through experiments is that the optimal dot shape should be a regular chain-like structure, with round dots in highlights and dark tones, and elliptical dots in midtones.
The following figures are schematic diagrams of diamond screening, linear screening, and cross screening respectively. (For the convenience of observation, they are all made with 30 lines / inch (all made with Photoshop), the actual printing effect is much better), we can easily find the different effects expressed in the three pictures in the picture.
Through the introduction of different outlets above, we understand that when selecting outlets for different products, we must also consider the performance of the shape of the outlets for different products. For example, the sense of hierarchy of square outlets is much stronger than that of chain outlets. In the selection of special-shaped dots, don't choose blindly. The first thing you should consider is what kind of dots are more effective in expressing the theme of the printed matter. Is this choice really necessary? For example, the chain-shaped dots themselves have the characteristic of excessive softness in the middle tones, so the chain-shaped dots can be used on many prints with rich midtones (especially characters); the water-wave dots have the unique dynamic of water waves, often It will be used in special situations such as water sports; performances like this include concentric circular dots to vortexes, brick-shaped dots to buildings, broken linear dots to galloping cars, single linear dots to bamboo forests, and so on.
The selection of dots of different shapes mentioned above has a very broad meaning for the production of lithography.
The same is the choice of dot shape, which is not so rich for gravure printing (especially engraving gravure). Due to the limitations of the engraving process (now mainly limited by the engraving knife), the dot shape used in gravure printing is often very simple, mostly Rectangle and rhombus. But for gravure printing, the change of dot shape has a new role.
Different from the flat version, the dot shape of each color group on the gravure printing plate (plate cylinder) is often different. The general situation is: the yellow version and the black version use almost square dots, the cyan uses longitudinally stretched diamond dots, and the magenta version uses transverse stretched diamond dots (the above is considered relative to the angle of the respective network line), With this change in dot shape, gravure printing can more effectively avoid the occurrence of moiré on printed products due to insufficient dot angle difference (there are only three kinds of dot angles for engraved gravure printing: 45, 30, 60).
The same with lithographic printing, gravure printing rarely considers the problem of dot increase during printing, and more considers the problem of ink transfer rate (the rate of ink out in the mesh). During the experiment, people found that the more The amount of ink transferred during the printing process is closer to the circular dots. Based on this, during the development of gravure, there are constantly six-row dots and octagonal dots (due to the limitation of the engraving knife) , It is almost impossible to produce round dots on the gravure printing plate cylinder).
The above article gives some introductions to the characteristics and functions of the different shapes of the outlets, and hopes that it will be helpful to readers in future work.Pets Products,Pet Supplies,Dog Supplies,Cat Supplies,Pet Store Supplies,Pet Shop Products
Ningbo Yinzhou Hengxi Winbate Household Product Manufacturer , https://www.nbwinbate.com