Color concept in printing

I. Overview

The RGB and CMYK color spaces are at least complementary in theory. Mixing the same amount of cyan, magenta, and yellow should also produce neutral ash, while the largest amount of mixing produces black (CMYK color space is also called the subtractive space. It is applied to printing technology, and prints reproduce color by reflecting the principle of light. The blend of primary colors will produce darker colors.). This effect can be simulated on the monitor but not in print. The maximum amount of CMY mixing does not produce black, but turbid brownish black. The reason is: pigments and inks can not be very pure, usually the blue ink is somewhat bluish, and the color and yellow ink are somewhat reddish. So, when you switch from RGB neutral gray to CMY in printing, it's reddish purple. This phenomenon can be remedied with black Y. However, increasing the fourth color destroys the equal conversion from RGB to CMYK, making the color correspondence between RGB and CMYK more complicated. There is no easy way to make them correspond one by one. Therefore, when converting an RGB image to a CMYK image, the total amount of black required and the range of tones you require requires a complicated algorithm. Separation parameters, such as GCR and UCR, are used to correct for excess black and can correct CMY pigment imperfections during conversion. There are such control parameters in Photoshop.

Second, master the black channel - understand black

In four-color printing, it is difficult to obtain ideal blue, purple, and green colors due to the impure ink (mainly cyan ink). Also, the amount of ink is limited in printing. For example, if you want to print black pages, you may think that: CMYK is printed at 100%, and it must be very good. However, the answer is: wrong! Four 100% color plates are printed, the paper can not stand, the ink does not dry properly, the paper will stick together, or the ink will spill out of the roller, resulting in not only not ideal black, but a blemish. 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. Do not believe? Well, 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 board, put the mouse on the screen, look at the CMYK display of info, add these four percentages, how much? 266! Far less than 300. You must pay attention to this point. Too many four-color totals are absolutely a bad thing. well, let's get back to business. Due to the restriction of the amount of ink, the ratio of black to other colors at the time of color separation is increased, and the image is not easy to be bright. How is it? (Since the total amount of ink is limited during printing, when black is added, the ratio of the other three colors must be reduced.)

The best course is of course to manage the black channel as much as possible. When converting an image into the CMYK color space, the precise creation of the black channel helps to better describe the details in areas where shadows and contrast change. Increasing the amount of detail and increasing the contrast can all enhance the color saturation of the image, which will make your eyes feel that the color of the image is brighter than the actual color.

We know that color separation with CMY tri-color inks will lack contrast and blurring, especially in tonal variation areas and dark areas. With the black channel, the image will appear clearer and the colors will be more vivid. Because black enhances the appearance of the outline effect, that is, played an emphasis on the picture.

However, simply adding black ink does not guarantee perfect color. For professionals, one must care about how much black is added, where they are added, and how to establish the best ink distribution balance between the channels. We have mentioned the term “separation parameters” in several articles. We will explain them in detail later, but now, please read the following sections. They are related.

The total amount of black ink determines the total amount of black in the image relative to other three-color printing plates. For most types of images, it is more appropriate to set a light black, which is quite dark with the outlines referred to in conventional printing, ie using only the appropriate black to increase the profile and enhance the contrast and detail. If you need to increase the amount of black in the image during color correction, you can start by increasing the black level of the image. This helps maintain the clarity of the image.

In low-key images such as night scenes and high-profile images where most details are bright, a light black or outline black can be created separately. In the night scene image, colors and details should be preserved as much as possible. However, if the black edition is too thick, color and details will be lost. For high-profile images, if you place too much black, the image will be blurred and particles appear.

When the main object is composed of neutral gray or metallic colors, such as cities or gold coins, medium or more dense black can be generated. Especially when lighter neutral gray is the center of attention, care should be taken to avoid color casts and increase the total amount of black, which helps to ensure that there is more neutral gray content in the overall color composition. Here to explain, what does it mean to increase the total amount of black? First of all, you should first know what neutral gray, the so-called neutral gray, is pure gray, that is, different degrees of black. In RGB mode, the same degree of tristimulus adds together to get neutral grey. For example, the lightness values ​​are 200 red, 200 green, and 200 blue, which are equivalent to 25% neutral gray. Note that the brightness value is not the same as the ink percentage. The 210 red, 200 green, and 200 blue colors add up to a reddish gray, which is no longer a neutral color. It is easily affected by reddish, which is the color cast. In CMYK mode, the situation is more complicated. Because of the impure ink, the same amount of CMY is added together, not neutral gray. The method is to increase the amount of cyan. See "Composition of Percentage of CMY Ink Composing Neutral Gray" below.

Percent composition of CMY inks that make up neutral gray (reference value)
Cyan Magenta Yellow Neutral Grey
5 3 3 5
10 6 6 10
20 13 13 20
25 16 16 25
30 21 21 30
40 29 29 40
50 37 37 50
60 46 46 69
75 63 63 75
80 71 71 80
90 82 82 90
95 87 87 95

Since a certain proportion of CMY inks can produce neutral gray, then, in turn, neutral ash, a certain percentage of black, can be used instead of the proportion of CMY ink components, thus increasing the total amount of black. For example: C50+M37+Y37=K50, K50 is 50% black, and the combination of a certain percentage of it and the other three inks is actually the same. So, now look at this color component: C70M42Y39K0, its total amount of ink is 151, but you look at the equation just now, can we use K50 to replace most of the CMY components? Yes, you can try it out. The result is C20+M5+Y2+K50. Black increases, but the total amount of ink is reduced.

Printed works on high-speed roll paper or gravure printing machines are more susceptible to misalignment than printing works on slower single-paper printing machines. The misalignment that appears on the black plate is less sensible, so images printed on a high-speed press should be generated using a medium degree of black.

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

Third, the separation of color parameters

In Photoshop, you can control the separation parameters to solve the different requirements for maintaining detail, color, and contrast without increasing the total amount of ink. In Photoshop 5.5, this command is in: File-Color Settings-CMYK Setup. In earlier versions of Photoshop, it was in: File-Preferences-Separation Setup. There are three main methods of color separation parameters: GCR, UCA, and UCR. Below we introduce separately.

GCR: Gray component substitution (with black ink instead of neutral ash composed of CMY)

The meaning of GCR is this: In the CMY color, it is often the two colors that play the dominant role that determine the nature of the hue, while the third color is called the gray component, which only serves to increase the black color. For example, red is mainly composed of color and yellow. If you add a certain amount of cyan, the red color will be darker. When there is too much cyan color, the red color will be dirty. Here, red is the main color and cyan is the gray component. Of course, the gray component also refers to a roughly equal amount of CMY tri-color combinations in one color, which constitute a neutral gray. With black, you can safely replace achromatic or gray components in a color. GCR color separation will replace the extra color (neutral gray component) with the appropriate black color. This method can effectively reduce the total amount of ink, and help reduce the color cast, especially for skin color, fresh fruits and vegetables, metal and sky-based images.

In most cases, UCA base gain is not used when performing GCR color separation. Black helps keep the color of the main color change area (most importantly the contour area of ​​the image) clear. Black ink will increase dramatically in dark areas because it is difficult for the eye to distinguish the color changes in the dark areas. To maintain rich and vivid colors throughout the entire tonal range of the image, using the GCR separation method is optimal.

UCA: Background Gain (Maintaining Color Saturation in Shadow Zones)

GCR is the main method we love to use in color separation. But its disadvantages are: lack of detail in the dark tone area, and black is relatively monotonous. Especially dark-tone-based images such as night scenes. Using UCA color separation technology can make up for its deficiencies. UCA color separation removes some black in darker colors, while adding CMY tris, it is the opposite of GCR processing, but it only works in the darker color areas, and the degree of adding three colors is up to you controlling. The higher the amount of UCA specified in the GCR separation, the higher the degree of replacement of shaded black with CMY.

For low-key images such as night scenes, their midtones are close to the shaded areas, but the tone of the intermediate tone is very rich and vivid. Such images should be added to the UCA so that a certain amount of CMY components in the midtones can be maintained. , that is, keep the details of the midtones. At this time, you should also pay attention to the setting of the total amount of ink. To increase the tristimulus value will inevitably increase the total amount of ink. Different printing methods, different types of paper, and restrictions on the total amount of ink are different.

UCR: Underline Removal (Aggravate Shadows)

GCR replaces CMY neutral gray in any hue range with black, and UCR is black that is replaced in dark areas. It is used to reduce total ink coverage in shadowed areas while enhancing detail. It is very suitable for high-speed printing, because the fast printing ink drying time is very short, it is easy to cause the shadow details to be blurred. The main disadvantage of UCR is that dark tone appears to be monotonous and lacks contrast, which is why dark gray tone-based dark tone images cannot achieve the best results. Therefore, when images contain rich midtones and shadows, it is best to use a combination of GCR and UCA.

Several other settings

The above is an introduction to the three separation methods. When setting separation parameters in Separation Setup, there are several parameters that are also discussed here. The first is Black Genaration (black generation). This item is only valid when GCR is selected. The popup menu specifies the degree of Photoshop generated black version. Light (mild) black version is less used, and heavy (heavy) black version is used more often. Selecting None produces no black version, and Maximum is black version. If you