The concept of color in the hottest printing

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Color concepts in printing 1. Overview RGB and CMYK color spaces are complementary in theory at least. Mixing the same amount of cyan, magenta and yellow should also produce neutral gray, while the largest amount of mixing will produce black (CMYK color space is also called subtractive space, which is applied to printing technology. Printed matter reproduces color through the principle of reflecting light. The mixing of primary colors will produce darker colors). This effect can be simulated on the display, but not in the printed matter. The largest amount of CMY mixing produces not black, but turbid brown black. The reason is that pigments and inks cannot be very pure. Usually, cyan inks tend to be blue, while magenta and yellow inks tend to be red. Therefore, when you change from RGB neutral gray to CMY in printing, it will be reddish purple. This phenomenon can be remedied by black y. However, adding the fourth color to destroy the experiment, when the pendulum is in the vertical position, the equal conversion from RGB to CMYK is realized, which makes the color correspondence between RGB and CMYK more complex. There is no simple way to match them one by one. Therefore, when converting RGB images to CMYK images, the total amount of black and the hue range you need need need a complex algorithm. Color separation parameters, such as GCR and UCR, are used to correct excess black, and can correct the impure problem of CMY pigments in the conversion process. There are such control parameters in Photoshop

II. Master the black channel - understand black

in four-color printing, it is difficult to obtain ideal blue, purple and green due to impure ink (mainly cyan ink). In addition, the ink volume is limited in printing. For example, if you want to print black page bottoms, you may think: CMYK is printed with 100% concentration, which must be very good. However, the answer is: wrong! When four 100% color plates are printed, the paper cannot bear it. If the ink cannot dry normally, the paper will stick together, or the ink will splash out of the drum. As a result, the ideal black will not be obtained, but a paste will be made. In terms of experience, the total amount of four-color ink should be less than 300, unless you have close cooperation with the output and printing departments. No? Well, please open Photoshop, ask for a new file, convert it to CMYK format, and fill it with the darkest black. In short, do your best! Then, open the info display panel, place the mouse on the screen, and see the CMYK display of info. What is the sum of these four hundred components? 266! Less than 300. You must pay attention to this point. Too much four-color total is definitely a bad thing. All right, back to business. Due to the limitation of ink quantity, the ratio of black to other colors is increased during color separation, and the image is not easy to be bright. What to do? (since the total ink volume is limited during printing, when black is increased, the proportion of other three colors will inevitably be reduced.)

of course, the best way is to manage the black channel as much as possible. When converting an image into CMYK color space, accurately establishing the black channel is helpful to better describe the details in the areas where the shadow and contrast change. Increasing the amount of detail and contrast can enhance the color saturation of the image, which will make your eyes feel that the color of the image seems brighter than the actual color

we know that image color separation with CMY tricolor ink will lack contrast and appear blurry, especially in the areas of tone change and dark areas. With the black channel, the image will be clearer and the color will be more vivid. Because black enhances the appearance outline effect, that is, it plays an important role in the picture

however, simply adding black ink does not guarantee perfect color. For professionals, they must be concerned about how much black is added, where it is added, and how to establish the best ink distribution balance between channels. We have mentioned the term "color separation parameter" in several articles, and we will explain it later. But now, please read the following parts. They are all related

the total amount of black ink determines the total amount of black in the image relative to other three color plates. For most types of images, it is more suitable to set light black, which is equivalent to the outline black in traditional printing, that is, only appropriate black is used to increase the outline and enhance the contrast and details. If you need to increase the amount of black in the image during color correction, you can start by increasing the outline black, which is conducive to maintaining the clarity of the image

in low-key images such as night view and high-key images with bright details, a light black or outline black can be created respectively. In the night scene image, the color and details should be preserved as much as possible. However, if the black version is too thick, the color and details will be lost. For high-profile images, if you put too much black, it will make the image easy to blur and appear particles

when the main object is composed of neutral gray or metallic color, such as city or gold coin, it can generate medium or thicker black. Especially when the lighter neutral gray is the center of attention, attention should be paid to avoid color deviation and increase the total amount of black, which will help to ensure that there is more neutral gray in the whole color composition. What does it mean to increase the total amount of black? First of all, you should first understand what is called neutral grey. The so-called neutral grey is pure grey, which means different degrees of black. In RGB mode, three colors of the same degree are added together to get neutral gray. For example, the lightness values of 200 red, 200 green and 200 blue are equal to 25% neutral gray. Note that the lightness value is not the same as the ink percentage. The combination of 210 red, 200 green and 200 blue will produce a reddish gray. It is no longer a neutral color. It is easily affected by the reddish color, which is called color bias. In CMYK mode, the situation is more complicated. Because the ink is impure, the same amount of CMY added together is not neutral gray. The method is to add cyan appropriately. See the "CMY ink percentage combination table for neutral gray" below

CMY ink percentage composition table of neutral grey (reference value)

Cyan Magenta yellow neutral grey

5 335

10 666 10

20 131320

25 161625

30 212130

40 2929 40

50 3737 50

60 4669

75 6363 75

80 7171 80


958787 95

since a certain proportion of CMY ink can produce neutral gray, on the contrary, You can also use neutral grey, that is, a certain percentage of black, to replace that proportion of CMY ink components, thus increasing the total amount of black. For example: c50+m37+y37=k50, K50 is 50% black, which is actually the same as a certain proportion of the other three inks. Now, let's take a look at the color component: c70m42y39k0. Its total ink volume is 151. But if you look at the equation just now, can we use K50 to replace most of the CMY components? Yes, you can try to replace it. The result is c20+m5+y2+k50. Black increased, but the total amount of ink decreased

works printed on high-speed roller paper printing machine or gravure printing machine are more prone to misalignment than works printed on slow single paper printing machine. The alignment inaccuracy on the black plate is not easy to be detected. Therefore, the image printed on the high-speed printing machine should be generated with a medium degree of black

the following describes the specific implementation method, that is, the specific setting of color separation parameters

III. color separation parameters

in Photoshop, you can control the color separation parameters to solve the different requirements of maintaining image details, color and contrast without increasing the total amount of ink. In photoshop5.5, this command is displayed in file color settings CMYK setup. In earlier versions of Photoshop, the file preferences separation setup. There are three main methods of color separation parameters, namely GCR, UCA and UCR. Let's introduce them respectively

gcr: grey component replacement (use black ink to replace neutral grey composed of CMY)

gcr means that in CMY color, two colors often play a leading role, determining the nature of hue, while the third color is called grey component, which only plays a role in adding black. For example, red is mainly composed of color and yellow. If you add a certain amount of cyan, the red will become dark. When there are too many Cyans, the red will be dirty. Here, red is the main color and cyan is the gray component. Of course, the grey component also refers to the roughly equal amount of CMY three color combinations in a color, and these three colors constitute neutral grey. Using black can safely replace the non colored or gray components in a certain color. For GCR color separation, the excess color (neutral gray component) will be replaced with appropriate black. This method can effectively reduce the total amount of ink and help to reduce color deviation, especially for skin color, fresh fruits and vegetables, metal and sky based images

in most cases, UCA background gain is not used for GCR color separation. Black helps to keep the main color change areas (most importantly, the outline area of the image) clear. Black ink will increase dramatically in the dark areas, because it is difficult for the eyes to distinguish the color changes in the dark areas. To keep rich and vivid colors in the whole hue range of an image, GCR color separation is the best way

uca: ground color gain (maintain color saturation in dark areas)

gcr is the main method that people prefer to use in color separation at present. But its disadvantage is that the dark areas lack details, and the black color is monotonous. In particular, dark tone images such as night scenes. The use of UCA color separation technology can make up for its shortcomings. UCA color separation removes some black from darker colors, and adds CMY tricolor. It is just the opposite to GCR, but it only works in darker color areas, and the degree of adding tricolor is controlled by you. The higher the UCA amount specified in GCR color separation, the higher the degree to which CMY replaces shadow black

for low-key images such as night scenes, the middle tone is close to the shadow area, but the middle tone color is very rich and vivid. UCA should be added to such images, so that a certain amount of CMY in the middle tone can be maintained, that is, the details of the middle tone can be maintained. At this time, you should also pay attention to the setting of the total amount of ink. Increasing the tricolor value will certainly increase the total amount of ink. From the perspective of future demand changes, different printing methods and different papers have different restrictions on the total amount of ink

ucr: background color removal (shading)

gcr is to replace CMY neutral gray in any hue range with black, while UCR is to replace black in dark areas. It is used to reduce total ink coverage in shaded areas while enhancing detail. It is very suitable for high-speed printing, because the ink drying time of fast printing is very short, which is easy to cause dark tone and blurred details. The main drawback of UCR is that the dark tone is relatively dull, while Zhengye International's revenue reached RMB 2.074 billion that year, which is monotonous and lack of contrast. This is the reason why the neutral gray based dark tone image can not obtain the best effect. Therefore, when the image contains rich intermediate and dark tones, it is better to use the combination of GCR and UCA

other settings

the above is an introduction to the three color separation methods. When setting the color separation parameters in the separation setup, there are several other parameters, which are also described here. first

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