How to ensure the consistency of color conversion

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How to ensure the consistency of color conversion

the printing process is actually the process of transferring the image information of the original in various printing input and output devices. How to realize the maximum compatibility and consistency of the amount of information from the original input and output between printing input and output devices, that is, to ensure the accuracy of information transmission as much as possible and realize the best reproduction faithful to the original

in order to copy the printed manuscript, we must first carry out color separation. Now the color separation technology has developed from the traditional photographic color separation and electronic color separation to the current desktop publishing system

the color printing and printing process of desktop publishing system is essentially the process of restoring the color of the original text through the conversion and transmission of color information in various devices and different color spaces. The reproduction of color depends on the device that produces it. However, in the integrated environment of desktop publishing system, because each device has different color gamut and color characteristics, it is particularly difficult to accurately control and transmit graphic color information in desktop publishing system. What you see is the ultimate goal of manuscript reproduction. To achieve this goal, we must implement effective color management independent of the device in the desktop publishing system, ensure the compatibility of signal input and output between devices, and reduce and make up for the impact of differences in device color characteristics and color modes on color information, So as to minimize the color distortion when the image and text color data information is converted and transmitted between different color spaces and devices with different color characteristics, so as to ensure that the effect of the color of the same picture from input to display and then to output is matched as much as possible, so that the color of the replica and the original are harmonious and consistent

to do a good job in color management in desktop publishing system, the key is to deal with the links such as unified color space, device system characterization and color conversion

first, we should ensure the unity of color mode and its color space.

in desktop publishing system, color restoration often involves RGB, CMYK and lab color modes. RGB is the color mode of color light. It consists of red, green and blue channels. In this mode, other colors are formed by the superposition of these three primary colors. Because these three colors have 256 brightness levels, the superposition of three colors can form 16.7 million colors. In desktop publishing system, scanners, digital cameras, monitors and other input and display devices all rely on this mode to express color. When sunlight shines on an object, the object absorbs some light and reflects other light. Reflected light is the color of the object we see, which is a subtractive mode. Depending on this subtraction method, CMYK mode suitable for printing and printing has evolved. In practice, it is difficult for the inks of these three colors to superimpose true black, so black is introduced during printing and printing to strengthen the dark tone and deepen the dark color. Although the color defined by this color mode is much less than that defined by RGB, that is, the color space is much smaller, the output devices such as Imagesetter, printer and proofer in the desktop color prepress system rely on this mode to reproduce color. Lab mode is a color mode developed by CIE (International Lighting Commission). Any color in nature can be expressed in lab space, and its color space is larger than RGB space. In addition, this mode describes human visual induction in a digital way, which is independent of the device, so it makes up for the deficiency that RGB and CMYK modes must rely on the color characteristics of the device

because the color space of lab is larger than that of RGB mode and CMYK mode. This means that the color information described by RGB and CMYK can be mapped in lab space. Therefore, in the color management of desktop publishing system, if all color conversion and color correction operations are completed based on lab space, color data will not cause color deviation due to insufficient data when converting from lab space to RGB or CMYK space. Therefore, in color management, in order to unify the color space, we should take this lab mode which is independent of equipment and has a large color space as the benchmark

to sum up, in the process of prepress production, we should store the color information obtained by scanners and digital cameras working in RGB mode in lab mode, and carry out color and correction in lab color space, and then convert it to CMYK space for processing when printing or printing. This is the best scheme for color processing. Because printers and imagesetters express colors in CMYK mode, many operators are also used to correcting colors in CMYK space in actual operation. This method is not advisable, because it will cause color loss, and the operation in CMYK will also slow down the calculation speed of the slave computer

second, it is necessary to describe the color characterization of equipment and its system characterization

desktop publishing system is an open system, and each device can only reproduce or display colors within a specific range, such as scanners, displays, and digital cameras, which generally use RGB space to express colors; Color printers, proofers and imagesetters generally use CMYK space to express color. In addition, due to the different color characteristics of different devices, as well as the influence of the use environment and equipment status, even the same device, its color characteristics are relatively unstable. The result of color reproduction depends on the device that produces it. Without understanding the differences between each device, the conversion and transmission of color information will inevitably be greatly affected, so the expected color cannot be obtained. In order to share color information in multi device environment, a cross platform and system ICC Standard is developed. In this standard, they formulate the format and type of device color description files, and define a virtual device independent color space. Then according to this format, the original device color space is converted to the virtual space, and then the virtual space is converted to the target device color space, so as to ensure the correct transmission of color information in the input and output devices

third, pay attention to the choice of color space conversion and its common methods

color space conversion is the conversion of color image data between different devices using device independent color space as a bridge. The transformation of color space involves two problems. One is the choice of color mode. The description of color in the process of color management should be independent of equipment. In the technical sense, the L AB color space model defined by CIE is based on a large number of color perception measurements. It is independent of equipment, so it is widely used in color management systems. The second is the mapping relationship of color space transformation. Due to the disunity of many color spaces, it is bound to involve the compression or trade-off of color gamut. It is the requirement and goal of color conversion to make the color displayed on the display screen and the output color of color printers, imagesetters, etc. as close as possible to the original. However, since the CMYK space of the output device is smaller than the RGB space of the input and display devices such as scanners and displays, in the color space conversion, it is necessary to establish the mapping relationship between the color gamut that can not be covered by the ink by compressing or discarding the display and scanner, so as to ensure the reproduction space of the ink as large as possible. According to the different requirements of color production and restoration, color conversion often adopts the following ways, each of which is applicable to different objects

1. Visual perception conversion. In the process of image reproduction, maintain the relative relationship of the original color, that is, adjust the conversion ratio according to the color gamut space of the output device, so as to mix the colors in visual perception. This conversion is often used to restore continuous adjustment originals with high requirements

2. Chroma conversion. Chromaticity conversion can be divided into relative chromaticity conversion and absolute chromaticity conversion. The difference between the two lies in the different color processing methods for the part of the color beyond the matching color space before matching. The former uses the boundary color to replace this part of the excess color, while the latter compresses this part of the color into the matching color space. This kind of conversion is often used in the adjustment of the display to ensure that the color with the same chroma can be obtained on different displays

3. Best color gamut conversion. This conversion requires that the purest and most saturated color be generated within the color space limited by the output device, and it is no longer the pursuit of obtaining the same color as the original or the display, nor the goal of color matching between different output devices. This conversion method is often used in commercial printing with strong creativity

to sum up, in order to ensure the maximum compatibility between the devices of the desktop publishing system, it is necessary to implement effective color management. First, the color space of the whole system must be unified, and the unification of space must rely on the lab space that is independent of the device and has a wide color gamut or other appropriate color space; Secondly, the system equipment should be characterized, and the system can be coordinated and unified in the transmission of color information by virtue of the color feature file of the equipment; In addition, when converting color space, we should choose a device independent color description language, and establish a suitable mapping relationship based on the color gamut that can be represented by printing. At the same time, we should choose the appropriate color space conversion method according to different situations

generally speaking, the specific operation and implementation methods of color management can be divided into two kinds: one is manual management method; The other is software management. The so-called manual management method refers to the measurement and adjustment for color control in the input and output cycle. Basically, many companies at home and abroad have made great breakthroughs in the utilization of fiber-reinforced composite auto parts, which rely on the judgment and vigilance of professionals to implement the color management method with calibration and calibration as the main means. The so-called software management method is the color management method of using the color management system in the production system. The purpose of using this method is that no matter what equipment and originals are used, CMS can make the color look the same from display to proofing to printing. 9. The safety performance inspection products of plastic tensile testing machine. The user just presses the button, and the rest of the work is done by the computer. For example, some color management software can convert the scanner, color monitor and the color of the final proofing or printing, and manage them harmoniously, so as to finally realize WYSIWYG. As long as the user operates according to the requirements of the color management software, the color management software will automatically compensate the color, and finally achieve the best color reproduction effect. Even without color processing experience or expertise, it can produce satisfactory color pictures. In the whole process of printing image and text reproduction, prepress processing is the key to the whole process. Giving full play to the maximum compatibility of the performance of prepress processing equipment and implementing the most effective color management are the most important links to achieve good quality of printed products

source: Luo

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