4. Separation parameter settings

Gray component instead of GCR color separation can be determined in the Photoshop software black generation, black version restrictions, the total ink limit, UCA background color gain, you can adjust the "black generation" parameters, adjust the black version of the curve shape Establish the starting point and the end point of the black edition. The range of adjustment is large enough to meet the requirements of all types of original copy on the black edition.

1 Black Generation. This setting has 5 black generation function curves to choose from.
A. No black: only C, M, Y three-color version of the curve, and no black version generated. This three-color copying process is generally not used, and is used only for special effects. Because the three-color overprint gray is unstable, the density of the dark part of the image is insufficient and the contrast is not enough.

B. Light black generation: C, M, Y three-color superimposed gray, black from the middle tone to the dark tone are removed, the color ink is relatively high, the removed color part is replaced with a long tone black version, black The edition starts at 35% and the black generation curve is steep.

C. Medium black generation: C, M, Y three-tone overprint gray, black from the light to dark shadows are removed. The three-color ink has less color, and the removed color part is replaced with a long tone black plate. The black plate has a larger increase in the dark tone area, starting at 20%. This is a commonly used curve.

D. Heavy black generation: It can be seen from the color curve that the gray, black color of C, M, Y overprints have a large amount of removal from bright, medium, and dark, and the amount of C, M, Y color inks Less, and the black version is correspondingly deeper, working almost entirely on the tone, starting at 10%, and the black generation curve is closer to the 45° diagonal.

E. Maximum black generation: C, M, Y three-color superimposed gray, black area removed, no color ink, are replaced by black ink, black version of the most complete, deepest, play an important role, the starting point for At the state, the black generation curve is theoretically a 45° diagonal.

This kind of total elimination method is generally not used, because there is currently no one. % The dark ink density in the field can reach 2.2 or more, which cannot reach the 300% density value superimposed by the UCRI artist's four colors, and it can't reach the density value of 340% to 360% of the four-color superposition of the traditional craft. Black prints are poor, lack of contrast, poor visual effects and other shortcomings.

2 black ink limit. The set value of the black ink limit will affect the amount of C, M, and Y inks in the dark tone area. When the limit value of black ink increases, the amount of ink in the three colors of C, M, and Y will decrease. Conversely, when the black ink limit is reduced, the C, M, and Y color inks will increase. The default value is 100%, and most originals are set to 60% to 70%.

According to the law of dot enlargement of the current intaglio electroprinting printing process and the characteristics of the manuscript mainly for food and seeds, it is better to set the black ink limit to 60%, that is, the black ink limit value should be lower, and the tri-color ink volume should be more.

3 ink limit. The total ink limit setting is the sum of the four C, M, Y, and K color dot values ​​of the black calibration point. At present, the total amount of ink for offset printing is generally 320% to 360%, and the total amount of ink for gravure printing cylinders and plastic film printing is generally 300% to 310%.

5. The difference between GCR and UCR

Both the gray component replacement and the under color removal and copying technology use black ink instead of the color ink, so it is easy to misunderstand that the GCR is an extension of the base color removal, but the two are fundamentally different.

(1) The removal method of color ink is different. The background color removal is to replace the color (background color) of the dark portion of the image with a black ink, and the gray component substitution is to replace the background color with black ink and the entire color of any composite color. There are alternatives to colored areas.

(2) The range of color ink removal is different. The background color reproduction technology uses black ink instead of color ink to be limited to the neutral tone region of the dark tone. It acts in a narrow range near the gray axis of the color space. The gray component replacement replication method will extend the replacement of color inks to the entire colored region containing the gray component, and its range of action will extend in the direction of increasing saturation.

(3) The amount of color ink removed is different. The amount of the black ink used for the background color removal is generally between 30% and 40%, and the amount of the black ink replaced with the gray component can be changed within the range of 0% to 100%.

(4) The black version of the black version of the different background color removal is mainly used to strengthen the image density contrast, stable midtones to dark tone color; gray version of the black version of the copy replacement technology not only to assume the reproduction of the screen tone, but also participate in the composite The color reproduction of the color, ie black ink, also has a grouping effect.

(4) Background color gain (UCA)

Under color gain UCA (Undercolor Add Mon) is another plate making process, which is the opposite of the under color removal function, and is the color ink amount that increases the dark tone area.
The background gain is made along the gray axis of the color space, so the effect on neutral ash is greatest and it is limited to the neutral gray component. The background gain is controlled by the background gain strength and the starting point of the background gain (baseline gain at which level of the original is used). It can be used to adjust the gray balance of the image dark areas and to adjust the color of the dark areas. The special requirements have played a role in increasing the level of the black version of the neutral grey area.

Background color gain is an effective means of color copying for electric extensions. It can be used to eliminate the color shift caused by insufficient density in one of the three primary colors of the original image in deep darkness. It can balance the three colors C, M, and Y. .

Use the SWOP (Coated) ink paper combination, black generation function medium, median dot gain 20%, black ink limit 100%, ink total limit 400%, if you change the ground color gain from 0% to 20 %, you can get the result shown in Figure 4-4-4. It can be seen from the figure that the color ink is used in the dark tone region of the image. When the gradation value of the image is lower than 75%, the ink ratio relationship is basically unchanged.