Gray balance replacement characteristics and background gain in grey balance

Ash Replacement (GCR) Features

Any color separation mode has its characteristics. The technicality of color separation personnel lies in the need to understand the characteristics of the color separation mode so as to fully utilize its advantages and make up for its deficiencies. It is also necessary to become familiar with the printing expertise and master the characteristics of manuscripts in order to reproduce high quality originals.

1.GCR process points

1 Make full use of the black version and make long-distance black editions, from 0% to 100%;

2 The gray and black parts of the image are mainly reproduced by black ink, and the grey components formed by the three primary colors of C, M, and Y are removed. That is to say, black ink is used to replace the gray and black color formed by the balance of three primary colors in the traditional process, so as to achieve the consistency of optical and visual effects;

3 Use black ink to replace the ash-containing complex color in the image, that is, replace the complementary color ink with black ink.

2. The advantages of GCR

1 To reduce the total amount of four-color superimposed ink, which is conducive to ink drying, and facilitate high-speed multi-color printing;
2 is conducive to achieving print gray balance, can guarantee the reproduction and stability of gray tone;
3 Replacing expensive color inks with low-priced black ink can reduce ink costs.

3. The lack of GCR

1 When black ink is used instead of the minimum primary color containing gray color, the basic color of the color is also reduced, resulting in insufficient depth of primary color saturation.

When 20% K replaces the minimum primary color C20%, the basic colors Y and M of this hue will also be reduced by 20%. Therefore, when a color separation person encounters an important deep primary color in the manuscript, he or she must use Photoshop tools to deepen this part. Shallow deep dark colors to achieve sufficient saturation.

(2) Since the black-to-black version replaces the complementary colors in the color with black, the transparency is worse than that of the complementary colors. Therefore, it has an influence on the vividness of the color and should be adjusted appropriately for darkening.

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 under color removal is for the color (background color) of the dark portion of the image to be replaced with black ink, and the gray component replacement is not only for replacing the background color with black ink, but also for the entire color area of ​​any composite color.

(2) The range of color ink removal is different
The background color reproduction technique uses black ink instead of color ink to confine the shaded neutral region, which 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) Different color ink removal
The amount of black ink used in place of the background color is generally in the range of 30% to 40%, and the amount of black ink replaced by the gray component may be in the range of 0% to 100%.

(4) Different effects of black plate

The black version of the background removal is mainly used to enhance the density contrast of the image and stabilize the midtones to the dark tone color. The black version of the gray component instead of the copy technology not only bears the reproduction of the picture gradation, but also participates in the color reproduction of the composite color, ie black. Ink also has a color effect.

Underlay 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.


Source: Bison