Although printing can reproduce tens of thousands of colors, due to the use of subtractive coloration, the brightness of the colors is reduced, and some of the brighter colors are difficult to express in print. On the other hand, due to the technique of adding color to the screen, the screen is indeed richer than printing in terms of color expression. This is why the colors that look beautiful on the screen cannot be copied with the print, resulting in differences in the colors of the screen and the print. The solution is either to improve the ink and paper composition so that it can reproduce the fresher, more pure colors, but this is not an overnight event. Another method is to narrow the color gamut of the screen to cater for printing so that what the screen sees is printed.
The so-called color gamut is the maximum range that a device can record or copy colors. The color gamut of the human eye is all visible light. Within the wavelength range of 380 to 780, the color gamut of printing consists of paper and ink. Different paper and ink combinations have different printing color gamuts. The domain is different from book paper, Pantone's color gamut is also different from DIC. Others such as screens, scanners, printers, etc. each have their own color gamut, and it is of practical significance to grasp the color gamut of a device because a device cannot record or copy colors outside the color gamut. For example, under normal circumstances, the human eye can not see the colors in the infrared or X-ray, and some people can easily distinguish the colors, like various metallic colors, but it is not easy to record the book on the scanner. The highest quality we can get is the gamut of one device and the domain of another device. How to make people feel in the simulation process that the color gamut of the two devices is similar is the important theme of color production.
Perform color management and establish color standards
To produce color, a set of standards must be established for the representation and transmission of colors. Current popular color management systems such as LinoColor, Agfa's Phototone, etc. are all moving in this direction. Through a set of standard specifications (ICC mapping files) describing the color gamut of devices, color computing software is used to perform uniform color gamut conversion. Operation to reduce the color deviation and distortion caused by different color gamuts and specifications during the transmission of color data. To implement these color management systems, we must first find out the color gamut of the device. The most commonly used method to describe the color gamut is CIELab, a set of descriptive color data that converts the wavelength of light into luminance and hue according to the characteristics of the human eye, where L is the brightness of the description color and a represents the color deviation The degree of red greenness, b represents the extent of the color yellow to blue. In the CIELab color space, each person's eye-visible color has a position that belongs to that color. By comparing the distance between the two color positions, we can determine the approximate degree of color on both sides. Since the visible light spectrum is the basis of this set of data, it can cover the colors produced by screens and prints, as well as the color gamut of the eyes of delegations.
For example, to describe the color gamut of a printer, first print some test strips from the printer. These strips include various main colors and colors that are more difficult to reproduce. Then use the spectrometer to measure the CIELab data on the strips, and then use software to convert the data. Written as a comparison file in the ICC format. In addition to the gamut data of the device, the comparison file can also include the production characteristics of the device, such as the black version feature, dot gain value, and so on. With the contrast of the device, the color calculation software can refer to the characteristics of the two devices, and compare the color gamut of the device within the CIELab color space to obtain a better simulation effect. This technology has now reached the stage of production and application, of which the most widely used, is to print the color gamut by screen simulation, and to simulate the printing color gamut by a printer. Since the color gamut of the screen is larger than the color gamut of the print, simulation in this case is also called gamut compression simulation. The entire simulation process is based on the data in the reference file as the basis for calculation, so the generation and management of the reference file becomes the most important task.
The assumption of the color management system
Has a color management system been implemented to achieve the desired effect of color production? To answer this question, you must understand the assumptions behind the color management system. The main task of the color management system is to simulate the data of another known color gamut on the CIELab space based on a known color gamut data. Therefore, it is necessary to assume that two gamuts remain while recording gamut data. status. That is to say, the production status of the equipment-contrast file and the production status when calculating the color gamut must be the same. If the contrast file established yesterday cannot be compared with today's equipment and the production status keeps floating, the color management system cannot play the role of reducing the deviation. An unstable production process may even cause the color management system to expand the color deviation. Therefore, the color management system is more suitable for the design, color separation, proofing and printing at the same time, because it is easier to control the variables in the production process in the same factory.
Color is not only the design element, but also the standard of production receipt. Even if the customer barely accepts a printed product with unsatisfactory color, it may no longer patronize the next time. Many companies lose important customers because of color quality problems. It can be seen that mastering the laws of color presentation and controlling color quality are the technologies that must be mastered in production and production. With advanced equipment alone and without good technical cooperation, in the fierce competition in the industry, it will inevitably be eliminated.
Color principle
When it comes to paintings and images, we naturally cannot talk about color. All the patterns are composed of basic shapes and colors. Colors form an important part of our image processing. Here we will understand the principle of color. It will be our artwork. basis.
(a) The principle of three primary colors
In the middle school physics class, we may have experimented with a prism. After passing through the prism, white light is broken down into a chromatogram with gradual transitions of multiple colors. The colors are red, orange, yellow, green, cyan, blue, and violet. This is the visible spectrum. . Among them, human eyes are most sensitive to red, green and blue. Human eyes are like a system of three-color receivers. Most of the colors can be synthesized by different proportions of red, green and blue colors. The vast majority of monochromatic light can also be broken down into three shades of red, green and blue. This is the most basic principle of colorimetry, namely the principle of three primary colors. The three primary colors are independent of each other, and neither of the primary colors can have the other two colors synthesized. Red, green and blue are the three primary colors, and the three colors have the most extensive range of colors. The addition of red, green and blue primary colors in different proportions is called additive color mixing.
Red + Green = Yellow
Green + Blue = Cyan
Red + Blue = Magenta
Red + Green + Blue = White
The yellow, cyan, and magenta colors are made of a mixture of two colors and hue, so they are also called additive secondary colors. In addition:
Red + Cyan = White
Green + Magenta = White
Blue + Yellow = White
So cyan, yellow, and magenta are red, blue, and green complementary colors, respectively. Since each person's eyes have different perceptions of the same color, if we mix with three primary colors of the same intensity, assuming that the intensity of white light is 100%, then subjective feelings of people are that the green light is the brightest. Red light is the second and blue light is the weakest.
In addition to the additive color mixing method, there is a subtractive color mixing method. Under white light, the cyan pigment absorbs red and reflects cyan, the yellow pigment absorbs blue and reflects yellow, and the magenta pigment absorbs green and reflects magenta. That is:
White - Red = Cyan
White - Green = Magenta
White - Blue = Yellow
In addition, if the two pigments, cyan and yellow, are mixed, under white light irradiation, the pigment absorbs red and blue and reflects green. For the mixing of pigments, we express the following:
Pigment (yellow + cyan) = white - red - blue = green
Pigment (Magenta + Cyan) = White - Red - Green = Blue
Pigment (yellow + magenta) = white - green - blue = red
All of the above are subtractive color mixing, and subtractive color mixing is to form different colors by absorbing different proportions of three primary colors. Therefore, cyan, magenta, and yellow are called three primary colors of pigments. The color mixing of three primary colors of pigments is widely used in painting and printing. In the three basic colors of pigments, the red, green, and blue colors are called subtractive secondary colors or pigment secondary colors. In subtractive secondary colors there are:
(Cyan + Yellow + Magenta) = White - Red - Blue - Green = Black
The color mode represented by the above additive three-color primary colors is called RGB mode, and the color mode represented by the principle of subtractive primary colors is called CMYK mode, and they are widely used in the fields of painting and printing.
RGB mode is a color mode most commonly used by graphics software. In this mode, it is more convenient to process images. Moreover, RGB images stored are smaller than CMYK images, which can save memory and space.
The CMYK mode is a paint mode, so it belongs to the print mode, but it is essentially the same as the RGB mode, except that the colors are produced in different ways. RGB is the additive color mixing mode, and CMYK is the subtractive color mixing mode. For example, the display uses the RGB mode because the display is a beam of electrons that bombards the fluorescent material on the screen and emits bright light to produce color. When there is no light, it is black, and when the light is maximum, it is white. What about the printer? Its ink does not emit light by itself. Therefore, it is only necessary to use a subtractive color method to absorb the color of the other light by absorbing the specific light wave.
(B), HLS (hue, brightness, saturation) principle
HLS is Hue, Luminance, Saturation. Hue is an attribute of color, which is essentially the basic color of color, that is, we often say seven kinds of red, orange, yellow, green, blue, blue and purple, each representing a hue. The adjustment of hue is to change its color.
Brightness is the color of the graphics of a variety of colors (such as the RGB image of the original color is R, G, B or three from the various hue) of the brightness, brightness adjustment is the adjustment of brightness. The brightness ranges from 0 to 255 and is divided into 256 levels. The grayscale image we usually talk about is divided into 256 levels of brightness between pure white and pure black, that is, from white to gray, and then turn black. Similarly, in the RGB mode, it represents the shade of a primary color, that is, the brightness of the three primary colors of red, green and blue, from light to dark.
Saturation refers to the chroma of the image color. For each color there is an artificially defined standard color, and saturation is the physical quantity that describes the similarity between the color and the standard color. Adjusting the saturation is adjusting the chroma of the image. When the saturation bar of an image is zero, the image becomes a grayscale image. Everyone can try adjusting the saturation button on the TV.
Another concept is contrast. Contrast refers to the difference between different colors. The greater the contrast, the greater the difference between the two colors, and vice versa. For example, a grayscale image will increase its contrast and become more black and white. When it reaches the limit, it becomes a black and white image. On the contrary, we can get a gray canvas.
We understand the principle of color, we will not be at a loss in image processing, and can also be faster and more accurate in adjusting the color.
Reprinted from: I Love Print Forum
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