Dot shape
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The dots can have different shapes. The dot shape refers to the geometry of a single dot, that is, the edge shape of the dot. In the traditional dot technology, the dot shape is determined by the corresponding screen structure. In addition to their respective performance characteristics, the dots of different shapes have different changes in the image copying process, which will produce different copying results and affect the quality requirements of the copying results.
Different shapes of dots have different image tone transfer characteristics. In the actual plate making and printing process, the dot has a tendency to expand mechanically. Experiments show that this trend increases with the increase of the perimeter of the dot (or the sum of the perimeters). The sensitivity of the dot area ratio to the variation of the perimeter is proportional to its circumference.
The perimeter of the perimeter (the perimeter and the larger) is easier to expand, and the distortion that may occur in the text reproduction is also greater.
The shape of the dot is different, the dot area ratio of the adjacent dot is different, and the number of times of the overlap (the time when the dot is overlapped is the circumference and the maximum, so the characteristics of the dot is often used to describe the nature of the dot) Different, the state of image tone transmission is different.
The lap joint of the dots causes: the jump in density increases.
In the selection of the shape of the dot to screen the image, often dot augmentation is the primary consideration, and the trend of different shape outlets is different, which leads to different product selection of different outlets.
Traditional meshing methods use square, circular, diamond (diamond), elliptical, double-point, etc.; in modern digital screening technology, more outlets are available. The following is a description of the common dot shapes and their characteristics.
1. Square dots. When the square-shaped dot is used to copy the image, the ink color and the white color at the 50% dot are just like a checkerboard shape, and it is easy to discriminate the relative percentage of the square dot according to the dot pitch, which is sensitive to the transfer of the original layer. Figure 1 shows a 50% square dot arranged at a 90 degree angle.
The final formation of the dot shape is closely related to the plate making and printing processes. The square dot can really show its shape at the 50% dot percentage. When it exceeds 50% or less than 50%, it will be deformed at its corner point due to optical and chemical influence during dot formation. The circle in the square even becomes round. At the time of printing, the expansion of the dot area is caused by the influence of the pressure of the ink and the viscosity of the ink. The square dot area ratio is the highest compared to other shaped dots. The reason for this phenomenon is that after the area ratio of the square dots reaches 50%, the dots are connected to the four corners of the dots, and the joints are prone to blockage and blocking of the ink at the time of printing, thereby causing the dots to expand.
The above mentioned seems to be the shortcomings of the square dot, as if the square dot does not exist. In fact, in some cases, such as the case where the intermediate adjustment requirements are not particularly strict, the use of square dots can show a more distinct level.
2. Round dots. In the same area of the dot, the circumference of the circular dot is the shortest. When a circular dot is used, the highlights in the picture and the midtones are not connected to each other, and the dots can only touch each other at the dark tone. Therefore, the dot gain value below the midtone of the screen is small, and the intermediate layer can be better preserved.
The circular mesh dots have a small expansion coefficient relative to the dots of other shapes. Under normal conditions, the circular dots are connected around at 70% area ratio. Once the circular dot is connected to the circular dot, the expansion coefficient will be high, which will result in the accumulation of ink in the dark-adjusted area due to the excessive amount of ink in the dark-adjusted area, and eventually the image dark-adjusted part will lose its proper level.
Through the above description, we will find that the circular dot is inferior in performance due to its poor ability to express the dark level. Under normal circumstances, printers often avoid the use of round dots, especially when printing on offset paper. However, if there are many brighter layers in the original image to be copied, and there are fewer shadows, it is quite advantageous to use circular dots to represent the high and middle regions.
Below we give the deformation characteristics of square dots and circular dots:
The pure square dot is at 50% of the angle, and the pure circular dot is at about 78%. It can be seen that when the dot is not angled, the dot of the square dot is enlarged more than the circle.
3. Diamond-shaped mesh points. Diamond-shaped dots are also called diamond-shaped dots. Usually, the two diagonals of a diamond dot are not equal. Therefore, except for the small dots of the highlight area, which are partially independent, and the four corners of the diamond dot at the dark adjustment are connected, most of the midtone layers in the picture are connected by long axes and are not connected at the short axis. A chain, so the diamond dot is also called a chain dot. The tone of the picture represented by the diamond dot is particularly soft, and the level of reflection is also very rich, which is particularly suitable for characters and landscape pictures. Made of Photoshop's diamond-shaped (special case of diamond-shaped dots), the size is from 100% to 0%.
When the dot area ratio is about 25%, the long axis of the chain dot is transferred (referred to as the first pass); then the second pass occurs at 75%. Since the dot gain is unavoidable, the diamond dot will jump twice at 25% and 75%. However, since the intersection of diamond dots occurs only at two vertices, such a tone jump is much more moderate than when the four corners of the square dot are connected. It can be seen that when the image is reproduced by the diamond dot, the print tone curve is relatively flat, and it is particularly good in the middle range of 30%-70%. Therefore, the diamond dot is suitable for copying the original of the main scene as a character.
4. Oval dot. Such dots are similar to diamond-shaped dots that are not equal in diagonal. The difference is that the four corners are not pointed, but round, and therefore do not intersect at a 25% dot area as the diamond dots of different diagonals. In addition, there is no obvious tone jump phenomenon at the 75% dot area ratio. A gradient from 0% to 100% made with Photoshop's elliptical dots.
5. Double-point outlets. Such a dot type is commonly used in the case of multi-resolution screening, and is formed by superimposing two screens of different characteristics and dots. The composition of different size dots is: a small dot is embedded in the middle of two large dots, or a large dot is embedded in the middle of four small dots. The multi-resolution screening method seeks to weigh the minimum dot size, dynamic range, and screening specifications. In order to obtain satisfactory results under certain restrictions, the plate-making workers made a lot of suggestions, and the double-point outlets were proposed in this way. When the dots are small, they are pulled apart, while in the middle, the dots are pulled closer.
Double-point outlets are also called satellite outlets or sub-network outlets. The characteristics are that the small screens at the dark side of the screen have merged but the large outlets still exist. Although the number of outlets is less, the outlets are firm, 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 requirement for printability. However, in order to meet the needs of art reproduction, advertising and special tastes, special-shaped dots (or art netting) are often used to increase the artistic atmosphere of the picture and obtain specific artistic effects. Commonly used art reticles include concentric circular mesh, horizontal wavy mesh, vertical line mesh, cross-grain gauze, gauze mesh and brick mesh.
Different dot shapes have different effects on the dot gain produced during the printing process. The conclusion obtained through experiments is that the optimal dot shape should be a regular chain-like structure with circular dots in the highlights and shadows, and elliptical dots in the middle.
Through the introduction of different outlets above, we learned that when selecting outlets for different products, we must also consider the performance of the dot shape for different products. For example, the layered dot is far stronger than the chain dot. In the choice of special shape outlets, do not blindly choose, the first question to consider is what shape of the dot is more effective for the theme of the printed product, and whether this choice is really necessary. For example, the chain dot itself has the characteristics of being too soft in the middle, so the chain dot can be selected on many intermediate prints (especially characters); and the water wave dot has the dynamics unique to the water wave, often It will be used in special situations such as water sports; similar performances include concentric circles on vortex, brick-shaped dot-to-building performance, broken-line outlets on speeding cars, single-line dot-point performance bamboo forests, and so on.
The selection of dots of different shapes mentioned above has a very broad meaning for the production of lithographic printing.
The same is the choice of dot shape. For gravure printing (especially engraving gravure), it is not so rich. Due to the limitation of engraving process (now mainly limited by engraving knives), the dot shape used in gravure printing is often very simple, mostly Rectangle and diamond. But for gravure printing, the change in dot shape has a new role.
In the pacific version, the dot shapes of the various color groups on the gravure printing plate (plate cylinder) tend to be different. Generally, the yellow plate and the black plate adopt almost square dots, the cyan color selects the longitudinally stretched diamond dot, and the magenta plate uses the horizontally stretched diamond dot (the above is considered relative to the respective wire angle). With the change of the shape of the dot, the gravure can more effectively avoid the occurrence of moiré on the printed matter due to insufficient angular difference of the dot (the three angles of the engraved gravure are only three: 45, 30, 60).
Similarly, in the case of lithographic printing, gravure rarely considers the problem of dot gain in the printing process, but more considers the problem of ink transfer rate (inking rate in the cell), which is found during the test. The amount of ink transferred in the printing process is close to that of a circular dot. Based on this, in the development of gravure, six-sided dots and octagonal dots are constantly appearing (due to the limitations of engraving knives) It is almost impossible to produce a circular dot on the gravure plate cylinder.