An Attempt to Use CMYK Color Mode for Dye Separation (Part One)

Abstract: The CMYK four-color platemaking method was used in the printing process for the printing and dyeing process. After the printing process was transplanted into the printing and dyeing process, several problems affecting the printing and dyeing color were tested, and according to the characteristics of the printing and dyeing process, some of the best were found. CMYK four-color printing process parameters.

The color separation and platemaking methods used for printing and dyeing for a long time are different. From a craft point of view, printing is to synthesize a color picture with four colors of cyan (C), magenta (M), yellow (Y), and black (K); printing and dyeing is to represent each spot color separately The representative color. Since printing has many common features with printing and dyeing processes, people have tried to use the computer color separation process to find different ways to apply the four-color printing method to color printing and dyeing.

The author once went to a silk design company to understand how to use the printing method to make plates and to print the silk fabrics according to the requirements of the printing process. Give me an impression: There are too many mud spots on the prints, the picture is too dark, and the colors are distorted. However, one of the prints was quite satisfactory. After the analysis, it was known that the process was completely different from the conventional printing process. Instead, it used the duotone colors method called printing. And the tone curve of each color is not adjusted on the screen, but is achieved by adjusting the color tone of the dye during the printing process, which will inevitably take more time and cost. According to the analysis of the sample, it is considered that the printing process and parameters are mainly directly applied, resulting in an excessive expansion of the net point of the actual layout, and the colors are distorted. Therefore, some experiments on printing methods used for printing and dyeing are considered from the following three aspects.

The computer is used for printing plate separation, the image processing is mainly the use of Photoshop image color separation software, it needs to change the Printing Inks Setup in the File/Preferences item of the working menu for different printing inks, printing media, and printing processes. ...;Separation Setup... two subitems. The work that can't be done for Photoshop graphics software is done by special items (provided by software developers) in the RIP interpreter. The implementation is as follows.

First, the ink settings. The hue of the image on the screen is changed by printing the ink settings so that it is as consistent as possible with the dye-printed sample.
Second, separation settings. By adjusting the curve when the image RGB color mode is converted to the CMYK color mode, the printing and dyeing process requirements are satisfied.
Third, RIP compensation. The main compensation was established because the settings in Photoshop did not meet the printing and dyeing process requirements.

First, printing ink set (Print Inks Set Up)

The printing ink setting is set for the medium (satin) and dye of the printing product, and it is consistent with the tone obtained by the screen and the sample.

We understand the requirements for printing and dyeing from the following parameters.

1. Ink Colors

Swop (coated) is usually used for printing. This is a standard printing method for American reel offset printing. The chromaticity coordinates and brightness factor of the ink can be represented by a table. From the chromaticity coordinates, the x and y coordinates of the color can be found from it, and the luminance factor Y is measured upwards at the white point.

It can be seen from the table that if the printing ink CMYk chromaticity coordinates and brightness factors are different from the CMYK value of the printing and dyeing fuel, this will result in a difference between screen colors and printed colors. For this, the following two methods can be used.

a. Select an ink parameter in the Ink Colors item for printing and dyeing. If the color difference of the sample changes little, it can be corrected when the dye is formulated.

In the case that the chromaticity coordinates and brightness factors of printing and dyeing fuels are not well-known, we can start from the idea of ​​choosing a printing medium similar to the printing and dyeing medium. It is recommended to select one of the inks as the Swop (News Print) method. The main consideration is that newsprint has similar water absorption and silk properties.

b. If the difference in color difference is too large with the above method, use this scheme: In the Inks Colors item, select Other, and fill the Cursor column with the chromaticity coordinates and brightness factors of the actual dye. Specific operations can use Photoshop testprint.jpg file (in the Photoshop's Calibrat subdirectory) output film, and then dyed and then silk samples, and then use the color separation instrument can measure the reflectance curve of a spectrum and its calculations Y, X, Z Value, fill the value into the Custom column.

2. Dot Gain

Bleeding and diffusion occur when dyes are printed on silk, and this phenomenon cannot be avoided. Since the enlargement of the printed dots will have a greater impact on the midtones and shadow tone areas, the rate of enlargement of the dots will be different for different types of blankets. If the print point is enlarged, the image becomes darker. From the printed sample, the image on the sample is significantly darker, which means that the original print point gain of 20%, not enough to compensate for the actual point expansion rate, and Photoshop, the maximum point of 40% optional gain (relative increment in 50% gray scale), so the point gain can be selected to be about 35%.

3. Gray Balance

Usually the image we see on the display will have some differences in lightness and darkness compared to printed samples, and some colors will also have large differences. For minimum requirements, at least lightness and darkness are as consistent as possible. If the sample is darker than the display image, the CMYK value in Gray Balance can be reduced, and vice versa (in the case of fixed point gain). Second, if there is a bit color cast in the middle tone, you need to move the gray output slider in Images/Adjust/Lcvels to control and count this r value into the Gray Balance column.

The above three parameters are useful when converting RGB mode to CMYK color mode in Photoshop. The change of the parameter does not affect the RGB image display, only the display image of the CMYK mode on the screen is changed without changing the data of the CMYK mode. That is, seeing the screen on the screen is dark, the screen can be brightened by changing the point gain and the gray balance, and the CMYK value does not change at the same time. If this is to brighten the picture by adjusting the Images/Adjust/Levels value, the CMYK value will change