How to use Photoshop to correct dot gain before printing

How to use Photoshop to correct dot gain before printing

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 In printing, the dot is generally used as the basic unit. Because the dots are in the process of screening, printing and printing onto the substrate, they are always in the process of change. The change of the dot is mainly increased, and the edge portion is uniformly expanded outward, causing the image to be dark overall, the layer of the dark tone is lost, and the brightness and the middle tone are darkened. If the processing is not good, it is useless to make more adjustments to the image. It is necessary to find out the increase value of the dot during the refurbishing process to obtain the corresponding compensation value, and compensate for the increase of the dot in the printing.

The shape and characteristics of the dots

Traditional dot shapes have square dots and circular dots, and more chain points are used now. The dots of the same size have different circumferences due to their different shapes, and the dot gain rate is also different. Since the dot is enlarged outward along its edge during printing, the longer the perimeter of the dot, the more serious the dot gain.

When the dot is enlarged, there is always a part where the overlap is started. In this part, the overlap of the dots causes a sudden increase in the density of the printed matter, thereby destroying the continuity of the printed matter and causing some tonal loss. For example, gradients can cause excessive unevenness due to density jumps. In contrast, the image quality of the chain point is better because its lap joint avoids the midtone and the lap is split twice, reducing the density jump. Because of this, if the image is small and soft, such as a character-like image, a chain dot can be used; if the image contrast is large, a square or circular dot can be used.

Factor of dot gain

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

In summary, the dot gain is an unavoidable phenomenon due to specific factors in the printing process and equipment, and as a result, it is difficult to obtain an ideal dot size on the printed matter. To this end, the impact of dot gain should be compensated for in prepress computer design and color separation. The basic idea is to first reduce the dot size appropriately in the color separation, and then let it expand to the required size according to the increase rule of the dot in the printing.

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

Compensation correction for dot gain in Photoshop

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

1) Using the Curves tool in Photoshop to directly correct the data in the image is the most recalled and easiest. It is also possible to use a similar tool in the front-end scanning software to set the scan acquisition mapping curve to obtain compensated data. For example, if the dot is increased to 10%, then the control point is set to 50% of the tone curve, and the point is adjusted to 40% of the tone, then the scanned and processed image looks like the original image in the middle Brighter, but when printing or printing, 40% of the dots will be dimmed to 50%, just like the original.

2) Photoshop has two settings in the face of the dot gain: one is the standard mode and the other is the curve mode.

In the standard mode, since the dot gain has the greatest influence on the midtone effect, in the standard mode, the set dot gain value directly refers to the intermediate tone dot, that is, the 50% dot gain value. For example, setting a 20% dot gain means that the intermediate dot will be printed at 70%. In addition, in the color printer output mode, the effect of dot gain on color is also very large, and different dot, inkjet speed and printer models will bring very different dot gain. These modes are generally accurate enough.

In the curve mode, the dot gain can be set by the composite channel and the CMYK four-color channel. It can set the dot gain to be applied to at most 13 levels, and the accuracy is higher than the standard mode. The composite channel is the lower mode in the case of all the same working modes. At this time, the increase rate of each channel of CMYK is forced to be consistent, which is actually an extension of the standard mode. Expand the dot gain parameter from 50% one point to the entire gray ladder range. If the measured dot density of the original step is 20% gray, the measured dot density is 30%, then 30 can be filled in the 20% column of the dialog box. If the respective dot gain values are set using the four color channels, respectively, it can not only give the dot gain parameter of each color ink, but also can also compensate the gray balance property of the ink. To obtain the dot gain parameter, it is necessary to test the CMYK four-color gradient ruler of the test sample such as the one mentioned above, and perform the same measurement and filling work as the gray ruler.

3) Store the output transfer function wind line in EPS format, and perform correction processing by RIP at the time of output. Select File/Page Setup in Photoshop and then select Transfer Function to display a curve mapping tool similar to Curves. You can fill in the correction box of each tone value according to the different shades, and then select the EPS storage format. And click the IncludeTransfer Function button to store in EPS format. The image information does not change in this correction, 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.

Conclusion

In all printing methods, there is a phenomenon of dot gain, and each printing method and each actual printing process has a clear dot gain characteristic. In order to compensate for the inevitable dot gain in printing. In order to compensate for the inevitable dot gain in printing, it is particularly important to control and adjust the prepress work. It must be ensured that the impact of each step on the dot is stable and cannot affect the accurate copying of images in subsequent processes.