Method of coordinating screens and printing colors
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Although printing can replicate thousands of colors, due to the use of subtractive coloring, the brightness of the color is weakened, and some of the more vivid colors are difficult to express in a printed manner. On the other hand, the screen is indeed more print-rich in the range of color expression due to the technique of additive coloring. This is why the beautiful colors on the screen cannot be reproduced by printing, resulting in a difference in color between the screen and the print. The solution is to improve the ink and paper composition so that it can replicate the fresher, purer colors, but this is not a one-off event. Another method is to narrow the color gamut of the screen to cater to the printing, so that what the screen sees is printed.
The so-called color gamut is the maximum range in which 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 is composed of paper and ink. Different paper inks match, and there are different printing gamuts. The domain is different from the book paper, and Pantone's color gamut is also different from DIC. Others such as screens, scanners, printers, etc. also have their own color gamut. It is practical to know the color gamut of a device because a device cannot record or copy colors outside the gamut. For example, under normal circumstances, the human eye cannot see the color under infrared or X-ray, and some people can easily distinguish colors, like various metallic colors, but it is not easy to record on the scanner. The quality we can get at best is the gamut of one device, simulating the domain of another device. How to make the human eye think that the color gamut of the two devices is similar in the simulation process is an important theme of color production.
Color management and color standards
To produce color, a set of standards must be established for the representation and transmission of color. At present, the more popular color management systems such as LinoColor and Agfa's Phototone are developing in this direction. Through a set of standard specifications (ICC comparison files) describing the color gamut of devices, color calculation software is used to perform unified conversion of color gamut. Operation to reduce color deviation and distortion caused by different color gamut and specifications during color data transfer. To implement these color management systems, first find out the gamut characteristics of the device. The most common way to describe the color gamut is that CIELab is the International Lighting Association, which converts the wavelength of light into a set of descriptive color data based on human visual characteristics, where L is the color of the description color and a is the color deviation of the description. The degree of redish greenishness, b represents the degree to which the color is yellowish and bluish. In the CIELab color space, each person's visible color has a position belonging to the color. By comparing the distance between the two color positions, we can determine the approximate degree of the color of the two sides. Since the visible light spectrum is the basis of this set of data, it can cover the colors produced by the screen and printing, and can also be used to color the color of the delegation's eyes.
For example, to describe the color gamut of a printer, first print some test color strips from the printer. These color strips include various main colors and colors that are difficult to copy. Then use the spectrometer to measure the CIELab data on the color strips, and then use the software to put the data. Write the ICC format comparison file, in addition to the device's color gamut data, but also include the device's production characteristics, such as black version features, dot gain values, and so on. With the comparison file of the device, the color calculation software can refer to the characteristic data of the two devices, and compare the color gamut of the device into the CIELab color space for comparison and conversion, thereby obtaining an ideal simulation effect. The technology that has reached the stage of production application is the most widely used, which is to simulate the printing color gamut on the screen and simulate the printing color gamut with a printer. Since the color gamut of the screen is larger than the color gamut of the print, the simulation in this case is also called gamut compression simulation. The entire simulation process is based on the data in the control file, so the generation and management of the comparison file becomes the most important task.
Hypothesis of color management system
Is the color management system implemented so that the color produced can achieve the desired results? To answer this question, you must understand the assumptions behind the color management system. The main work of the color management system is to simulate the data of another known color gamut on the CIELab space according to the data of a known color gamut. Therefore, it is necessary to assume that the two color gamuts remain in the recorded gamut data. status. That is to say, the production state of the device comparison file and the production state when calculating the color gamut must be the same. If the comparison file established yesterday cannot be compared with today's equipment, the production status will continue to fluctuate, and 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 factory with design, color separation, proofing and printing, because it is easier to control the variables in the production process in the same factory.
Color is not only a design element, but also a receiving standard for production. Even if a customer reluctantly accepts a print that is not satisfactory in color, it may not be patronized next time. Many companies are losing important customers because of color quality issues. It can be seen that mastering the law of color presentation and controlling color quality is a technique that must be mastered in production. With advanced equipment and no good technical cooperation, in the fierce competition of the industry, the fate of being eliminated will inevitably be eliminated.
Color principle
When it comes to paintings and images, it is natural to talk about colors. All patterns are composed of basic shapes and colors. Colors constitute an important part of our image processing. Below we will understand the principle of color, it will be our art. basis.
(a) The principle of three primary colors
In the physics class of middle school, we may have experimented with prisms. After passing through the prism, white light is decomposed into a variety of color gradually transitioning chromatograms. The colors are red, orange, yellow, green, cyan, blue, and purple. This is the visible spectrum. . The human eye is most sensitive to red, green and blue. The human eye is like a three-color receiver system. Most of the colors can be synthesized in different proportions by red, green and blue. Similarly, most of the monochromatic light can be decomposed into 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 no one of the two primary colors can be combined with the other two colors. Red, green and blue are the three primary colors, and these three colors have the widest range of colors. The red, green and blue primary colors are added in different proportions to form a mixed color called additive color mixing.
Red + green = yellow
Green + blue = cyan
Red + blue = magenta
Red + green + blue = white
Yellow, cyan, and magenta are made up of two kinds of colors and colors, so they are also called secondary colors. In addition:
Red + cyan = white
Green + Magenta = White
Blue + yellow = white
Therefore, cyan, yellow, and magenta are complementary colors of red, blue, and green, respectively. Since each person's eyes have different feelings for the same single color, if we mix three primary colors of the same intensity, assuming that the intensity of white light is 100%, then the subjective feeling of the person is that the green light is the brightest. Red light is second, 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. 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 of cyan and yellow are mixed, under the illumination of white light, since the pigment absorbs red and blue, and reflects green, the mixing of the pigments is as follows:
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 the subtractive color mixing is to form different colors by absorbing different ratios of the three primary colors. Therefore, cyan, magenta, and yellow are called the three primary colors of the pigment. The color mixing of the three primary colors of pigments is widely used in painting and printing. Among the three primary colors of the pigment, the three colors of red, green and blue are referred to as subtractive secondary colors or pigment secondary colors. Among the subtracted secondary colors are:
(cyan + yellow + magenta) = white - red - blue - green = black
The color mode represented by the above-mentioned additive color mixing three primary colors is called RGB mode, and the color mode represented by the subtractive mixed color three primary color principle is called CMYK mode, and they are widely used in the fields of painting and printing.
RGB mode is one of the most commonly used color modes of drawing software. In this mode, it is convenient to process images, and RGB stored images are smaller than CMYK images, which can save memory and space.
CMYK mode is a paint mode, so it belongs to the print mode, but it is essentially the same as the RGB mode, but the way it produces colors is different. RGB is the additive color mixing mode, and CMYK is the subtractive color mixing mode. For example, the display adopts the RGB mode because the display is an electron beam that bombards the fluorescent material on the screen to emit light to produce color. It is black when there is no light, and white when the light is added to the maximum. And what about printers? Its ink does not emit light by itself. Therefore, it is only necessary to absorb the specific light wave and reflect the color of other light, so it needs to be solved by the subtractive color method.
(B), HLS (hue, brightness, saturation) principle
HLS is Hue, Luminance, Saturation. Hue is an attribute of color. It is essentially the basic color of color. It is the seven kinds of red, orange, yellow, green, blue, blue and purple that we often talk about, each of which represents a hue. The adjustment of the hue is to change its color.
Brightness is the brightness of the primary colors of various colors (such as the primary colors of RGB images are R, G, B or various self-colors), and the brightness adjustment is also 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 the division of 256 levels of brightness between pure white and pure black, that is, from white to gray, then black. In the same way, in the RGB mode, it represents the brightness of the primary colors, that is, the brightness of the three primary colors of red, green and blue, from shallow to deep.
Saturation refers to the chroma of the color of the image. For each color, there is an artificially defined standard color, and saturation is the physical quantity that describes how close the color is to the standard color. Adjusting the saturation is to adjust the chroma of the image. When the saturation bar of an image is zero, the image becomes a grayscale image, and you can try to adjust the saturation button on the TV.
Although printing can replicate thousands of colors, due to the use of subtractive coloring, the brightness of the color is weakened, and some of the more vivid colors are difficult to express in a printed manner. On the other hand, the screen is indeed more print-rich in the range of color expression due to the technique of additive coloring. This is why the beautiful colors on the screen cannot be reproduced by printing, resulting in a difference in color between the screen and the print. The solution is to improve the ink and paper composition so that it can replicate the fresher, purer colors, but this is not a one-off event. Another method is to narrow the color gamut of the screen to cater to the printing, so that what the screen sees is printed.
The so-called color gamut is the maximum range in which 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 is composed of paper and ink. Different paper inks match, and there are different printing gamuts. The domain is different from the book paper, and Pantone's color gamut is also different from DIC. Others such as screens, scanners, printers, etc. also have their own color gamut. It is practical to know the color gamut of a device because a device cannot record or copy colors outside the gamut. For example, under normal circumstances, the human eye cannot see the color under infrared or X-ray, and some people can easily distinguish colors, like various metallic colors, but it is not easy to record on the scanner. The quality we can get at best is the gamut of one device, simulating the domain of another device. How to make the human eye think that the color gamut of the two devices is similar in the simulation process is an important theme of color production.
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 is adjusted to the limit, it will become a black and white image. Otherwise, we can get a gray canvas.
We understand the principle of color, we will not be stunned in image processing, and it can be faster and more accurate for adjusting colors. 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 is adjusted to the limit, it will become a black and white image. Otherwise, we can get a gray canvas.
We understand the principle of color, we will not be stunned in image processing, and it can be faster and more accurate for adjusting colors.