How to use Photoshop to correct the dot increase before printing

In printing, dots are generally used as the basic unit. Because the dots are always changing in the process of screen screening, printing and printing onto the substrate. The change of the dot is mainly increased, the edge part spreads out evenly, causing the overall image to be dark, the dark tone part of the layer is lost, and the bright tone and middle tone become dark. If the processing is not good, it is useless to do more tone adjustments to the image. Therefore, it is necessary to find out the increase value of the dots during the refurbishment process to obtain the corresponding compensation value to compensate for the dot increase in printing.

The shape and characteristics of the dot

The traditional dot shape has square dots and round dots, and now more chain dots are used. The dots of the same size have different circumferences due to different shapes, and the dot enlargement rate is also different. Since the dots expand outward along the edges when printing, the longer the dot circumference, the more serious the dot increase.

In the process of increasing the dots, there are always places where they start to overlap. In this part, the overlapping 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 tone loss. For example, gradient colors can cause excessive unevenness due to density jumps. In contrast, the image quality of the chain-shaped dot is better, because its overlapping part avoids the middle tone, and the overlapping is divided into two, which reduces the density jump. Because of this, if the image is small and soft, such as a person-like image, chain dots can be used; if the image contrast is large, square or round dots can be used.

Factors that increase the dots

During the printing process, the final dot percentage is often larger than the dot percentage of the color separation film. This phenomenon is called dot gain. The reasons for the increase in dots include the increase in dots caused by printing pressure; the increase in dots caused by the insufficient exposure of the printing and the ink absorption and diffusion caused by the characteristics of paper and ink. That is to say, for different printing presses and papers of different qualities, different dot gain effects will be produced; for dots of different sizes, the dot gain increases into a non-linear relationship. The increase of dots formed by the characteristics of paper and ink generally shows an exponential expansion law; in the process of graphic output and plate printing and other process processes of plate making, the dots increase due to factors of materials and equipment, this increase It can be reduced by changing the printing conditions.

In summary, dot gain is an inevitable phenomenon due to specific factors in the printing process and equipment. The result is that it is difficult to obtain the ideal dot size on the printed product. For this reason, the influence of dot gain should be compensated during pre-press computer design and color separation. The basic idea is to properly reduce the dots during color separation, and then to expand them to the required size according to the law of dot increase in printing.

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

Compensation and correction of dot increase in Photoshop

The direct effect of dot gain is to make the printed and printed images darker and darker, especially in the middle tone. Compensation and correction for the increase of dots in the pre-press processing mainly through the following channels:

1) Using Curves tool in Photoshop to directly correct and change the data in the image is the easiest and easiest way to remember. Similarly, similar tools in the front-end scanning software can be used to set the scan acquisition mapping curve to obtain compensated data. For example, assuming that the dots increase to 10%, then set a control point for the 50% point on the tone curve and adjust the point to the 40% tone, then the scanned and processed image will look more important than the original image in the middle tone Brighter, but when printing or printing, 40% of the dots will be darkened to 50%, just like the original.

2) Photoshop has two settings in the dot-increased surface: one is the standard mode, and the other is the curve mode.

In the standard mode, since the increase of the dots has the greatest effect on the midtone effect, in the standard mode, the set dot increase value directly refers to the midtone dot, that is, the increase value of the 50% dot. For example, setting a dot increase of 20% means that the middle dot will be printed as 70%. In addition, in the output mode of color printers, the effect of dot increase on color is also very large, and different contract, inkjet speed and printer model will bring very different dot increase. In general, these modes must have sufficient accuracy.

In the curve mode, you can set the dot increase in two ways: composite channel and CMYK four-color channel. It can set up the dots to be increased on up to 13 levels, and its accuracy is higher than the standard mode. The composite channel is the lower mode as in the case of all the same working methods. At this time, 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 increase parameter from 50% to the entire gray ladder range. If the measured dot density at the 20% gray scale of the original ladder is 30% after printing and outputting, then fill in 30 in the 20% column of the dialog box. If the four color channels are used to set the respective dot increase values, it can not only give the dot increase parameters of each color ink, but also can be used to compensate the gray balance characteristics of the ink. The dot gain parameter must be tested one by one, such as the CMYK four-color gradient rule that the test sample mentioned above has, and the measurement and form filling work are the same as the gray ladder ruler.

3) The output transfer function wind line is stored in EPS format and corrected by RIP when output. Select File / Page Setup in Photoshop and then select Transfer Function. A curve mapping tool similar to Curves appears, and the percentage of dots required for correction can be filled into the modification boxes of each tone value according to different tones, and then select the EPS storage format. And click the IncludeTransfer Function button to save in EPS format. In this correction, the image information will not change, but the transfer function is embedded in the file for RIP to compensate for the dot increase in the same way as above when outputting.

Conclusion

There is a phenomenon of dot increase in all printing methods, and each printing method and each actual printing process has its clear dot increase characteristics. To compensate for the inevitable increase in dots in printing. In order to compensate for the inevitable increase in dots in printing, it is particularly important to control and adjust the pre-press work. It is necessary to ensure that the impact of each step on the dots is stable, and it cannot affect the accurate copying of images in subsequent processes.

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