Use of ink in four-color printing (2)

In color printing (four-color printing), it is difficult to obtain ideal blue, purple, and green colors due to 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 in 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 parameter” in several articles. We will specifically explain it later. Please see the second article “Mastering the Black Channel—Separation Parameters”, but for now, please refer to the following section. After reading it, they are all 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. 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.