The most popular CMYK color mode is used for print

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CMYK color mode is used to try printing, dyeing, color separation and plate making (Part 2)

II. When the separation setup

is scanned, the image is generally in RGB color mode. When it is converted to CMYK mode, it needs to follow certain conversion rules, that is, it needs to have a set of color separation rules

1. Separation type

in the printing industry, GCR (gray component replacement) or UCR (under color removal) color separation processes are commonly used

r is also called "achromatic structure" process. That is to say, the achromatic components in the picture (synthesized by multicolor) are printed with black ink. However, it has no effect on monochrome and intermediate color. People call the Black Edition "long tone edition". Because the blackness of black ink is not enough, the density in the dark part of the image is low. It is necessary to use color ink to set off the dark part of the image of achromatic components. This process is called "under color

addition (UCA)

the GCR method has little effect on the multi color, dark color and neutral gray replaced by ink, while the partial picture composed of primary color and intermediate color will be biased due to the fluctuation of ink supply. Therefore, the process requirements are relatively high when using GCR

r is also called "color structure" of modern traditional printing process. The non color and color components in the picture are all composed of CMY three primary colors. Due to the lack of blackness of the achromatic components synthesized by the three primary colors, the amount of black ink is increased in the dark part, and the amount of three primary colors is reduced at the same time. People often call the black version "short tone version" or "outline version"

to sum up the two color separation methods, UCR is better than GCR in terms of printing and dyeing process, which mainly reduces the non-uniformity of plate making process and the non-uniformity of dye quantity in the printing and dyeing process. In terms of color effect, GCR is more delicate than UCR. Therefore, GCR is better on the premise that the process is relatively guaranteed. In addition, GCR is usually used for materials with coating layer, and the dye dip is relatively small. UCR is used for uncoated coating materials (most printed and dyed fabrics belong to this category). In this way, different color separation methods should be selected for different fabrics

under normal circumstances, pictures made by UCR and GCR cannot be distinguished by naked eyes. At the beginning of use, it is suggested to try UCR to obtain other process parameters

2. Black generation

this item is only valid for GCR mode. In GCR mode, black is used to replace one or all of the C, m and Y colors, and the degree of replacement is selected by this item. From the printing point of view, medium is preferred. For printing and dyeing, when the black color is large (heavy item), the chromaticity of C, m and y will be reduced, which can show the effect of soft tone and is conducive to the clarity of the picture

3. Black ink limit

controls the total amount of black ink printing. For printing, 90% ~ 100% is adopted; For printing and dyeing, the optional parameters are a little lower than those of printing (such as 70% ~ 80%). If the selected amount of black ink is found to be excessive, the black ink limit can be reduced a little

4. Total ink limit

the total ink limit indicates the total amount of C, m, y and K. If the ink has been limited, the remaining value is the total amount of C, m, y. In printing, the values of C, m, y and K are generally not less than 300%. For printing and dyeing, I think it is between 250% and 300%. Because the ink absorption of the printed silk is larger than that of the printed paper, the total amount of ink can be set lower for printing and dyeing

5. UCR amount

for the GCR mode, black ink can not meet the requirements of the dark part of the image alone (the purity of black ink is not enough). At this time, color ink must be added to the dark part of the image with non color components to set off. It only works on multicolor, but not on primary and intermediate colors. Generally, this item is not used for printing, so it is not necessarily used for printing and dyeing. It can be seen from the analysis in this chapter that the color separation setting is related to the printing and dyeing process. To make the color separation setting of Photoshop conform to the printing and dyeing process, you can make some choices on the parameters of each column of this item, and then look at the sample effect, and obtain experience and data from the small sample, so as to popularize the vulcanizer, red brick press, line bending fatigue machine, soft worry machine, electric porcelain bending and twisting machine, horizontal pulling machine, etc

III. compensation of printing and dyeing process in Rip (raster image processor) software

although the image seen on the screen has achieved satisfactory results, the actual sample is not satisfactory. In addition to the above parameter selection to meet the printing and dyeing process as much as possible, in addition, factors such as film linearity, plate making time point expansion rate and printing and dyeing tension will cause changes in printing points. Therefore, what is a digital compression and bending tester to compensate. There are many ways to compensate. The common method is: first, measure the point enlargement rate of each step on the actual sample through the test, and then add a point to the point gain in the Photoshop software mentioned above, and add a point during rip rasterization, so that the three stimulus values on the actual sample are consistent with those on the screen as much as possible. Therefore, in order to obtain the correct printing and dyeing parameters, we should start with this test

1) adjust the focus of the imagesetter to reduce the expansion or reduction of the light spot of the Imagesetter, which will affect the dot expansion rate on the film and the occurrence of interference fringes

2) adjust film linearity. Different films have different characteristics such as development and fixing, so that the same size of light spots will have different sizes when imaging on the film. In order to compensate the expansion rate of each step adjustment point on the film, the compensation parameters shall be set in rip

3) compensation for midpoint expansion in printing process. In the previous description, we have defined the point expansion as 35% ~ 40%, which only compensates for the 50% step. The compensation of each point is determined by the compensation curve set in Photoshop. In other words, this kind of compensation is different from the actual situation. In addition, if the actual point expansion rate of 50% step is greater than 40%, Photoshop will be powerless. At this time, we can only make another compensation in rip

another method is to define the point gain as 0 in Photoshop, and the point expansion of each step is compensated in rip. The compensation value is the difference between the measured value and the set value of the sample

4) rip parameter selection. In terms of printing requirements, each color is required to represent 0 ~ 255 levels, while printing and dyeing does not need such high requirements, which can generally be controlled between 64 ~ 128 levels

added mesh: for the printing and dyeing requirements, the mesh of the silk plate is about 98 ~ 118 mesh/cm (250 ~ 300 mesh/inch). For the printing and dyeing process, the number of added mesh is 24 ~ 3 "and 9 mesh/cm (60 ~ 100 mesh/inch) when ordering The requirements have been met. In order to meet a certain number of steps, the 600dpi imagesetter output machine with the purpose of adding 60 can reach 101 levels. Adding angle: in recent years, it is recommended to add angles of 15 degrees, 75 degrees, 90 degrees and 45 degrees to C, m, y and K color plates respectively. If there is a collision between silk, silk plate patterns and color plate patterns, you can choose to adjust the fabric pattern angle and mesh number of silk (generally, the mesh number should not be changed)

author/Shou Qihong

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