Gray Balance and Dot Expansion Principles (4)

3. Base color removal calculation method

In the analog electronic color separation machine, due to the use of a suitable image processing method for the under color removal, the neutral ash component of the dark tone composite color can be individually extracted, so that the under color removal function can be accurately, quickly and simply realized. .

From the analysis of Fig. 4-4-3, it can be seen that the under color removal is essentially a kind of transformation on the composite color region, replacing the color with black and reducing the amount of three-color ink, and therefore the calculation method is similar to that of the black version, but with more emphasis on color The size of the saturation, usually the removal of the ground color is limited to the composite region of the image with small saturation, and its simulation operation equation is:

R=S-L-S/K=S-Δ/K

Among them: R - represents the amount of removal of the background L - said max (Vy, Vm, Vc)

S——min (Vy, Vm, Vc)

Δ=L-S indicates the saturation of the composite color, and K indicates the degree of saturation influence on the black plate. The larger K indicates the smaller the influence of saturation on the black plate.

In the actual use of the under color removal, it is necessary to take into account both the under color removal range and the under color removal amount, and the value range thereof is determined by actual work.

The following formula is used in some electronic color separation machines to calculate the amount of under color removal.

R = removal density / replication density range × 100% = DR / Dr × 100%

Among them: DR - removal density

Dr - copy density range

In the subtractive calculation of the undercoat color, the value of K is usually taken as 5, so that the effect of the convexity on the removal amount of the under color is enhanced, and the area where the under color removal effect is significant is located at the position where the original is close to the neutral gray. The saturated color or solid color area has little or no background color removal, thereby ensuring that the original image is bright and vivid, and avoiding the phenomenon of darkening or winning due to an increase in the black version, and determining the amount of background removal. At the same time, you also need to choose a suitable background to remove the starting point.

4. Process characteristics of under color removal

When UCR separation mode is selected, its setting is only the black ink limit and the total amount of ink. The general ink amount is limited to about 260%, and the black printing start point is basically fixed at about 50%.

Adjust the black ink limit and the total amount of ink two technical indicators, UCR removal will have different changes, at the same time, UCR removal starting point will also be in accordance with the total ink limit changes.

UCR is suitable for most manuscripts and printing processes. In particular, gravure printing process and manuscripts require bright colors. Therefore, the UCR separation mode is more suitable, and the black ink limit parameter values ​​can be smaller, for example, 60%, three primary colors. Ink is more advantageous.
The disadvantage of UCR is that deeper primaries with more ash components are subject to removal and cause insufficient saturation. Therefore, when you encounter an image with an important deep primary color, you need to use Photoshop tools for local corrections to achieve sufficient saturation.

(III) Characteristics of Ash Replacement (GCR)

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 Because the use of black ink instead of the minimum primary color containing gray color, the basic color of the color will also be reduced, resulting in insufficient depth of primary color saturation, as shown in Figure 4-4-3.

From Fig. 4-4-3, it can be seen that when 20% K replaces the minimum primary color C20%, the basic colors Y and M of the color phase are also reduced by 20%. Therefore, the color separation personnel are important in encountering manuscripts. When using deep colors, use Photoshop tools to deepen this darkened dark color 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.