The printing process is actually the process of transferring the image information of the original image in various printing input and output devices. How to achieve the maximum compatibility between the input and output of information from the original information in the printing input and output devices Sexuality and consistency, that is to ensure the accuracy of information transmission as possible, to achieve the best copy faithful to the original.
If the printed manuscript is to be copied, the color separation must be carried out first. The current color separation technology has evolved from the traditional photo color separation and electronic color separation to the current desktop publishing system.
The color printing and printing process of the desktop publishing system is essentially the process of converting and transmitting the original graphic color information in various devices and different color spaces, so that the color can be restored. The color reproduction depends on the device that produces it, but in the integrated environment of the desktop publishing system, since each device has a different color gamut and color characteristics, the precise control and transmission of graphic color information in the desktop publishing system This is especially difficult. What you see is the ultimate goal of manuscript copying. To achieve this goal, you must implement effective color management independent of the device in the desktop publishing system to ensure the compatibility of signal input and output between devices, reduce and compensate for the equipment. The influence of the difference of color characteristics and the different color modes on the color information, so that the color distortion of the image and text color data information is converted and transferred between different color spaces and devices with different color characteristics to minimize the color distortion, to ensure that the color of the same screen The effects shown in the display to the output match as closely as possible, so that the color of the copy and the original are in harmony.
To do color management in desktop publishing systems, the key is to handle the unified color space, device system characterization, and color conversion.
1. To ensure the unity of color mode and color space
In desktop publishing systems, color reproduction often involves three color modes: RGB, CMYK, and Lab. RGB is the color mode of colored light. It consists of three channels of red, green and blue. In this mode, other colors are formed by the superposition of these three primary colors. Since all three colors have 256 brightness levels, the three-color superposition can form 16.7 million colors. In desktop publishing systems, input and display devices such as scanners, digital cameras, and monitors rely on this model to express color. When sunlight shines on an object, the object absorbs part of the light and reflects other light. The reflected light is the color of the object we see, which is a subtractive color mode. Relying on this subtractive color method, the CMYK mode suitable for printing and printing has evolved. In practice, these three colors of ink are difficult to superimpose true black, so black is introduced during printing and printing to strengthen the dark tone and deepen the dark part of the 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 imagesetters, printers, and proofers in desktop color prepress systems rely on this mode. To reproduce color. Lab mode is a color mode developed by CIE (International Commission on Lighting). Any color in nature can be expressed in Lab space, and its color space is larger than RGB space. In addition, this mode is a digital way to describe human visual perception, has nothing to do with the device, so it makes up for the lack of RGB and CMYK mode 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 that RGB and CMYK can describe can be mapped in Lab space. Therefore, in the color management of the desktop publishing system, if all the color conversion and color correction operations are completed based on the Lab space, when the color data is converted from the Lab space to the RGB or CMYK space, there will be no color caused by insufficient data. deviation. It can be seen that in the color management, to make the color space uniform, the Lab mode that is independent of the device and has a large color space should be the benchmark.
In summary, in the pre-press production process, we should store the color information obtained by the scanner and digital camera working in RGB mode in Lab mode, and perform color editing and correction in the Lab color space. Convert to CMYK space when printing or filming. This is the best solution for color processing. Because printers and imagesetters express colors in CMYK mode, in actual operations, many operators are also used to editing and correcting colors in CMYK space. This approach is undesirable because it will cause color loss, and operating in CMYK will also slow down the operation speed of the slave computer.
Second, it is necessary to describe the color characterization of the device and its system characterization
The desktop publishing system is an open system. Each device can only reproduce or display a specific range of colors. Scanners, monitors, and digital cameras generally use RGB space to express colors; while color printers and proofers , Imagesetter generally uses CMYK space to express color. In addition, due to the difference in color characteristics of different devices, as well as the impact of the use environment and device status, even the same device, its color characteristics are also relatively unstable. The result of color reproduction depends on the device that produces it. Without knowing the differences between each device, the conversion and transfer of color information will inevitably be greatly affected, so that the expected color cannot be obtained. In order to enable the sharing of color information in a multi-device environment, a cross-platform and system ICC standard was 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 into a virtual space, and then the virtual space is converted into the target device color space, thereby ensuring the correct transmission of color information in the input and output devices.
Third, pay attention to the color space conversion and the selection of common methods
Color space conversion is the conversion of color image data between different devices using a device-independent color space as a bridge. The conversion of color space involves two problems. One is the choice of color mode. The color description in the color management process should be independent of the device. The Lab color space model defined by CIE is based on a large number of color vision measurements. It is independent of the device, so it is widely used in color management systems. The second is the mapping relationship of color space conversion. The inconsistency of many color spaces will inevitably involve the compression or trade-off of color gamut. It is the requirement and goal of color conversion to make the colors displayed on the display and the colors output by color printers and imagesetters as close to the original as possible. However, because the CMYK space of the output device is narrower than the RGB space of the input and display devices such as scanners and monitors, in the color space conversion, it is necessary to establish compression or discard the reflection of the color gamut that the monitor and scanner cannot cover the ink. Relationship to ensure that the ink has as large a color gamut as possible. According to different requirements for color production and restoration, color conversion often uses the following methods, each of which is suitable for different objects.
1. Conversion of visual perception. During the image copying process, the relative relationship of the original colors is maintained, that is, the conversion ratio is adjusted according to the color gamut space of the output device, so as to achieve the mixing of colors in visual perception. This conversion is often used to restore the continuous adjustment of the original documents.
2. Chroma conversion. Chroma conversion can be divided into relative chroma conversion and absolute chroma conversion. The difference between the two lies in the way in which the color before matching matches the color beyond the matching color space. The former uses the boundary color to replace this part of the excess color, the latter compresses this part of the color into the matching color space. This conversion is often used in the adjustment of the display to ensure that the same color is obtained on different displays.
3. Best color gamut conversion. This conversion is required to produce the purest and most saturated colors in the color space limited by the output device, instead of pursuing the same color as the original or the monitor, nor does it aim at color matching between different output devices. This conversion method is often used in more creative commercial printing.
In summary, to ensure maximum compatibility among the devices in the desktop publishing system, effective color management must be implemented. First, the color space of the entire system must be unified, and the unity of the space must be independent of the device and the color. Lab space or other suitable color space with a wide range; secondly, characterize the system equipment, and rely on the color characteristic file of the equipment to enable the system to be coordinated when transmitting color information; in addition, the color space conversion should be selected It is independent of the color description language of the device, and establishes a suitable mapping relationship based on the color gamut that can be expressed by printing. At the same time, we should choose the appropriate color-space conversion method according to different situations.
In general, the specific operational implementation of color management can be divided into two types: one is the manual management method; the other is the software management method. The so-called manual management method refers to the measurement and adjustment for color control in the input and output cycle, which is basically a color management method that uses calibration and calibration as the main means and is implemented by relying on the judgment and alertness of professionals. The so-called software management method is a color management method using a color management system in a production system. The purpose of using this method is that no matter what kind of equipment and manuscript is used, CMS can make the color from the display to the proof proof to the printed matter look the same effect. The user simply 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 last proof or printed color, and manage them in a coordinated and unified manner, and finally achieve what you see is what you get. As long as the user operates according to the requirements of the color management software, the color management software automatically performs color compensation, and ultimately achieves the best color reproduction effect. Even if there is no color processing experience or no expertise, it can produce satisfactory color pictures. In the entire process of copying printed graphics, pre-press processing is the key to the whole process, giving full play to the maximum compatibility of the performance of each device in pre-press processing, and implementing the most effective color management is the most important to achieve the best quality of printed products Of the ring.
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