In printing, dots are generally used as the basic unit. Because the network is in the course of screening, printing, and printing onto substrates, it is always changing. The dot change is mainly to increase, and the edge part spreads evenly outwards, which results in the overall darkness of the image, the loss of dark tone levels, and the darkening of the bright tone and the intermediate tone. If processing is not good, there are no more adjustments to the image. It is necessary to find out the increased value of the outlets during the renovation process to obtain the corresponding compensation value and compensate for the dot gain in printing.

The shape and characteristics of outlets

The traditional dot shapes are square dots, circular dots, and now more chain points. Because of the different shapes of meshes of the same size, their perimeters are also different, and dot gain rates are also different. Because the outlets expand outward along their edges during printing, the longer the perimeter, the more serious the outlets increase.

As the network grows, there is always a place where it starts to overlap. In this part, because the overlap of the dots will cause a sudden increase in the density of the printed matter, thus destroying the continuity of the printed matter and causing some loss of tone. For example, gradient colors cause excessive unevenness due to density jumps. In contrast, the quality of the image of the chain point is better, because its overlap site avoids the midtones, and the overlap is divided into two, weakening the density jump. Because of this, if the image is small and soft, such as a character image, a chain-shaped dot can be used; if the image contrast is large, a square or circular dot can be used.

The factors that increase the dot

In the printing process, the percentage of the final dot is often a large percentage of dots in the color film. This phenomenon is called dot gain. The reason for the increase in dot number includes the dot gain caused by the printing pressure; the dot gain caused by the lack of printing exposure and the absorption and diffusion of ink caused by the characteristics of paper and ink. That is to say, for different printing machines and different quality papers, different dot gain effects will be produced; with different sizes of dot nets, the dots will increase in a non-linear relationship. The enlargement of dots caused by the characteristics of paper and ink generally presents an expansive pattern of enlargement; in the process of plate-making graphic output and printing, due to the factors of materials and equipment, the dot gain also increases. It can be reduced by changing the printing conditions.

In summary, the dot gain is an unavoidable phenomenon due to certain factors in the printing process and equipment. As a result, it is difficult to obtain an ideal dot size on the printed matter. For this reason, it is necessary to compensate for the effect of dot gain on prepress computer design and color separation. The basic idea is to reduce the number of outlets appropriately during color separation, and then allow them to expand to the required size according to the dot gain rule in printing.

It should be emphasized that the pressure of digital printing is less than traditional printing, and dot gain should be different from general printing.

Correction of dot gain in Photoshop

The direct effect of dot gain is to make the printed and printed images darker and darker, especially in the midtones. The offset correction for dot gain is mainly used in prepress processing through the following channels:

1) Using the Curves tool in Photoshop to directly correct the data in the image is the most memorable and easiest way. It is also possible to use a similar tool in the front-end scanning software to set the scan acquisition map to obtain compensated data. For example, assuming a dot gain of 10%, set a control point for a 50% point on the tone curve and adjust the point to the 40% tone, so that the scanned and processed image will look longer than the original image in the middle tone. Brighter, but when printing or printing, 40% of the dots will darken to 50%, just like the original.

2) Photoshop has two settings in the enlarged dot area: one is the standard mode and the other is the curved mode.

In the standard mode, since the dot gain has the greatest influence on the midtone adjustment effect, in the standard mode, the set dot gain value directly refers to the midtone dot, ie, the 50% dot gain. For example, setting a 20% dot gain means that the middle dot will print 70%. In addition, under the color printer output mode, the effect of the dot gain on the color is also very large, and the different extension sheets, the ink jet speed, and the printer model will bring about very different dot gains. These modes should have sufficient accuracy under normal conditions.

In the curve mode, the dot gain can be set in two ways: composite channel and CMYK four-color channel. It can set the dot gain for up to 13 levels, and its accuracy is higher than the standard mode. Compound channel is the same as under the condition of all the same working mode. In this case, the value of the increase rate of each channel of CMYK is forced to be consistent. It is actually an extension of the standard mode. Expand the dot gain parameters from 50% to one point to the entire grayscale range. If the density of the actual measured dot after the 20% gray scale of the original ladder is printed out is 30%, fill in 30 in the dialog box 20%. If each of the four color channels is used to set the respective dot gain value, it can not only give the dot gain parameter of each color ink, but also can simultaneously compensate the gray balance characteristics of the ink. To obtain the dot gain parameter, one must test the CMYK four-color gradient rule, such as the one mentioned above, and measure and fill the form with the gray scale.

3) The output transfer function wind line is stored in the EPS format and is corrected by the RIP at the time of output. Select File/Page Setup in Photoshop and select Transfer Function Press, Curves Curve Mapping Tool appears, and fill in the correction box of each tone value according to the different shades, and then select the EPS storage format. And click IncludeTransfer Function button to store in EPS format. In this correction, the image information does not change, but the transfer function is embedded in the file for the RIP to compensate for the dot gain in the same manner as described above at the time of outputting.

Conclusion

In all printing methods, there are dot gain phenomena, and each printing method and every actual printing process has its own clear dot gain characteristics. In order to compensate for the unavoidable dot gain in printing. In order to compensate for the unavoidable dot gain in printing, it is particularly important to control and adjust the prepress work. It must be ensured that the effect of each step on the dot is stable, and the subsequent process cannot accurately reproduce the image.