V. Cymk color model
This group of articles about color models has not been written for a long time (actually written almost). A few more netizens asked questions in the message board RGB-> Cymk's conversion problem, I pay special, I found a article written by Wu Jinhui about CYMK. Now you have a collection, it is a supplement.
I. Overview
RGB and CMYK color space are at least complementary. Mix the same amount of cyan, product, yellow should also produce neutral ash, and the maximum amount of mixed produces black (CMYK color space is also referred to as minor color space, it is applied to printing technology, the printing product is reproduced by reflecting light The mixing of the primary color will result in a darker color.). This effect can be simulated on the display, but it is not in the print. The maximum amount of CMY mix is not black, but turbid is black. The reason is that pigments and inks are unlikely to be pure, usually the cyan inks are almost blue, while character color and yellow ink are biased. So, when you transfer from the neutral ash of RGB into CMY, a red purple is slightly red. This phenomenon can be remedied with black y. However, increasing the fourth color destroys equal conversion of RGB to CMYK, making the color between RGB and CMYK becomes more complicated. There is no simple approach to enable them one by one. So, when the RGB image is converted to a CMYK image, the total amount of black required and the color range you need require a complex algorithm. The chromogenic parameters such as GCR and UCR are used to correct excess black, and can correct the incomplete problem of CMY pigments during conversion. There is such a control parameter in Photoshop.
Second, master the black channel - understand black
In the four-color printing, it is difficult to get ideal blue, purple, and green due to uncelocking ink (mainly cyan ink). Moreover, the amount of ink is limited in printing, for example, you have to print the black page, you may think: CMYK is printed with 100% concentration, must be very good. However, the answer is: wrong! The four 100% color plate printed, the paper is unbearable, the ink does not dry normally, which will stick the paper together, or the ink splash the drum, but not only the ideal black, but a paste. From the experience value, the total amount of four-color ink should be less than 300, unless you have a close cooperation with the output, the printing department. Do not believe? Well, please open Photoshop, you have a new file, turn into a CMYK format, then fill the most black black, in short, do your best! Then, open the INFO display panel, put the mouse on the screen, look at the CMYK display of INFO, add these four percentages, how much? 266! It is far less than 300. You must pay attention to this, too much four colors, absolutely bad things. well, let's get back to business. Due to the amount of ink is limited, the ratio of black relative to other colors is increased when the color is mixed, and the image is not easy. How is it good? (The total amount of ink is limited since the printing is limited. When it is increased, it will inevitably reduce the ratio of other three colors.)
The best way is of course managing black channels as much as possible. When converting images into CMYK color space, the black channel is accurately established, which helps to better describe details in areas where the shadow and contrast changes. Increase the amount of detail and increase the contrast to enhance the color saturation of the image, which will make your eyes feel that the color of the image is more bright than the actual color.
We know that image simultism with CMY tri-color ink will lack contrast, ambiguous phenomenon, especially in the tone variation region and a dark area. Plus the black channel, the image will look clearer, and the color will be more vivid. Because black enhances the outline effect, that is, it emphasizes the picture.
However, simply increasing black inks do not guarantee the perfect color. For professionals, it is necessary to care about how much black, where is added, and how to establish the best ink distribution balance between the channels. We have mentioned the word "chromoographic parameters" in several articles, and we will explain it dedicated, but now, please follow the following sections, they are all associated. The total amount of black ink determines the total amount of black relative to the other three-color printing in the image. For most types of images, it is more suitable for shallow black, which is equivalent to the contour black referred to in traditional printing, ie using appropriate black to increase the contour and enhance the contrast and details. If you need to increase the black amount of the image in color correction, you can start from increasing the contour black, which is advantageous to keep the image's sharpness.
In high-profile images like night scenes and most detailed high-profile high-profile images, you can create a shallow black or contour black. In the night view, you should keep color and detail as much as possible, but if the black version is too strong, it will lose color and detail. For high-profile images, if you put too much black, you will make images easily blurred and particles.
When the main object consists of neutral ash or metallic color, such as urban or gold coins, a medium or more concentrated black can be generated. Especially when a shallowness is a non-point of attention, it should be taken to avoid bias, increasing the total amount of black, helping to ensure that there is more neutral ash in all colors. Here to explain, what does it mean to increase the total amount of black? First of all, you should know what is called neutral ash, so-called neutral gray, it is pure gray, that is, the black is referred to. In RGB mode, the same degree of tricho is added together to get neutral ash. For example, the molar value is 200 red, 200 green, 200 blue, and 25% of neutral ash is equal. Note that the value value and the ink percentage are not one thing. And 210 red, 200 green, 200 blue adds to generate a biased gray, it is no longer neutral color, it is easy to be influenced by bias, this is a bias. In CMYK mode, the situation is more complicated. Because the ink is not pure, the isometric CMY is added together, not a neutral ash. The way is to increase cyan. See "Component CMY ink percentage forms of neutral ash" below.
Composition of neutral ash CMY ink percentage combination table (reference value) blue-colored red yellow neutral ash 5335106610201313202516162530212130402929405037375060404669756363758878795
Since a certain proportion of CMY inks can produce neutral ash, then, in turn, it can also be used in a neutral ash, which is a certain percentage of black to replace the proportion of CMY ink components, which increases the total amount of black. For example, C50 M37 Y37 = K50, K50 is 50% black, and it is actually the same as a certain proportion of the other three inks. Then, now look at this color component: C70M42Y39K0, its ink is 151, but you will look at the equation just now, can we use K50 to replace most of the CMY ingredients? Yes, you replace it, the result is C20 M5 Y2 K50. The black is increased, and the total ink is reduced.
Works printed on high-speed roller paper printers or gravure printing machines are more likely to appear to have problems than printed on a single paper printer in a single paper printer. The opposite place appeared on the black version is not allowed to be not detected, so the image printed on the high-speed printing machine should use the medium-degree black generation.
The specific implementation of the specific implementation is explained below.
Third, minute color parameters
In Photoshop, you can solve different requirements of the detail, color degree, and contrast of the image without increasing the total amount of ink without increasing the total amount of ink. In Photoshop 5.5, this command is: file-color settings-cmyk setup. In the earlier versions of Photoshop, the File-Preferences-Separation Setup. There are three main ways of chromogenic parameters, namely GCR, UCA, and UCR. Here we introduce them separately. GCR: Gray component replacement (replacing a neutral ash consisting of CMY with black ink)
The meanings of GCR is this: In the CMY color, it is often a dominant role in two colors, which determines the nature of the hue, and the third color is called a gray component, which only plays a black action. For example, red is mainly composed of character color and yellow, if you add a certain amount of cyan, red will be darker, too much cyan, red is dirty. Here, the red is the main color, the cyan is a gray component. Of course, the gray component also refers to a substantially equal amount of CMY three-color combination in a color, which is neutral ash. With black, you can safely replace the non-color or gray ingredients in a color. The GCR is performed, and the excess color (neutral ash ingredient) will be replaced with appropriate black. This approach can effectively reduce the total amount of ink and facilitate reducing color bias, especially for skin color, fresh fruit vegetables, metal and sky - oriented image.
In most cases, the GCR color gain is not used, and the UCA content gain is not used. Black helps to maintain the main color change zone (the most important thing is the contour area of the image) clear. Black ink will increase dramatically in the dark, because the eyes are difficult to distinguish between the color changes of the dark regions. It is best to maintain a rich and vivid color within the entire tone range of the image, and the GCR color is best.
UCA: Color gain (saturation of the color in the dark tone)
GCR is the main method for everyone to use in color in color. But its disadvantage is that the secret area lacks details, and black appears to be compared. In particular, the image is mainly image, such as night scene. Using UCA color color technology can make up for it. UCA Single Color removes some black in a dark color, simultaneously add CMY three colors, which is just opposite to GCR, but only in the dark color zone, and the degree of joining the three-color is by you controlling. The higher the amount of UCA specified in GCR in GCR, the higher the extent to which CMY is replaced with shadow.
For night-opt-up images, their intermediate is adjacent to the shadow area, but the middle tone is very rich and vivid, such an image should be added to UCA, which can keep a quantity of CMY ingredients in the middle adjustment That is to keep the detail of the middle tone. At this time, you should also pay attention to the setting of the total ink, increase the three-color value will increase the total amount of ink, and different printing methods, different paper, the limit of total ink is different.
UCR: Removal (aggravated shadow)
GCR is replaced with black replacement of CMY neutral ash, while UCR is replaced in a dark area. It is used to reduce the total ink coverage in the shaded area while enhances details. It is very suitable for high-speed printing, because fast-printing ink drying time is very short, easy to cause dark tone details. The main disadvantage of UCR is that the darkness is more monotonous, and lacks contrast, which is the reason for the best effects that cannot be achieved with neutral ash. Therefore, when the image contains rich intermediate tune, it is best to use the combination of GCR and UCA.
Several other settings
The above is a introduction of three different color methods. When setting a different color parameter in the Separation Setup, there are several parameters, which is also told here. The first is Black Genaration (Black Edition). This item is valid after selecting GCR, and the pop-up menu specifies the extent of Photoshop to generate black. Light (slightly) black version is less, while Heavy (heavy) has a large black version, the choice of None will not produce a black version, Maximum (Maximum) is called with a black version. If you choose Heavy or Maximum, you can add CMY's ingredients via UCA. Black INK LIMIT: The maximum black version of the black-printing can be affected by printing paper. The default is 100%. If you add a value of UCA, you should also reduce black-printing amount.
Total INK LIMIT: The total four-color ink that can be expressed in printed paper. It is used to minimize the amount of black ink to limit the total amount of CMY tri-color ink.
