How to ensure the consistency of color transfer between devices
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The printing process is actually the process of transferring the image information of the original image in various printing input/output devices in various printing input/output devices, how to realize the maximum compatibility between the input and output of the information from the original information. Sex and consistency, that is, to ensure the accuracy of information transmission, to achieve the best copy of the original.
In order to be copied, the printing manuscript must first be color-separated. The current color separation technology has evolved from traditional photo 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 the color information of the original image through various devices and different color spaces to restore the color. The reproduction of color depends on the device that generates it, but in the integrated environment of desktop publishing system, because each device has different color gamut and color characteristics, the precise control and transmission of graphic color information in desktop publishing system It is especially difficult. What you see is the ultimate goal of copying the original. To achieve this goal, effective color management independent of the device must be implemented in the desktop publishing system to ensure the compatibility of signal input and output between devices, reducing and remedying the device. The difference in color characteristics and the influence of the color mode on the color information, so that the color data information of the graphic text is minimized when the color data information is transferred between different color spaces and devices with different color characteristics, so as to ensure the color of the same picture from the input. The effects displayed in the display and then in the output are matched as closely as possible to achieve a harmonious color between the reproduction and the original.
To do a good job of color management in the desktop publishing system, the key is to deal with the unified color space, device system characterization and color conversion.
First, we must guarantee the unity of color mode and its color space.
In desktop publishing systems, color reproduction often involves three color modes: RGB, CMYK, and Lab. RGB is a color mode of color light. It consists of three channels: red, green and blue. In this mode, other colors are formed by superimposing these three primary colors. Since these three colors have 256 brightness levels, the three-color overlay can form 16.7 million colors. In desktop publishing systems, input and display devices such as scanners, digital cameras, and displays rely on this mode to express colors. When the sun shines on an object, the object absorbs part of the light and reflects other light. Reflected light is the color of the object we see, which is a subtractive mode. Depending on this color reduction method, the CMYK mode suitable for printing and printing has evolved. Since the inks of these three colors are difficult to superimpose true black in practice, black is introduced during printing and printing to enhance the dark tone and deepen the dark color. Although this color mode defines a much smaller color than the color defined by RGB, that is, the color space is much smaller, the output devices such as imagesetters, printers, and proofers in the desktop color prepress system depend on this mode. To reproduce the color. Lab mode is a color mode developed by CIE (International Commission on Illumination). 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 sensing, independent of the device, so it makes up for the RGB and CMYK modes must rely on the lack of color characteristics of the device.
Because Lab's color space is larger than the color space 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, the color will not be caused by the insufficient amount of data. deviation. It can be seen that in the color management, in order to make the color space uniform, the Lab mode which is independent of the device and has a large color space should be used as a reference.
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. Since printers and imagesetters express colors in CMYK mode, in practice, many operators are also accustomed to performing in CMYK space when editing correction colors. This is not desirable because it causes color loss, and operation in CMYK can slow down the computation from the computer.
Second, 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. Like scanners, monitors, and digital cameras, RGB space is used to express colors. Color printers and proofers The photo albuming machine generally uses CMYK space to express color. In addition, because of the different color characteristics of different devices, as well as the influence of the use environment and device state, even the same device, its color characteristics are relatively unstable. The result of color reproduction depends on the device that generates it. Without understanding the difference between each device, the conversion of color information will inevitably be greatly affected, so that the expected color cannot be obtained. The system characterization process of a device is essentially the formation of a color profile of different devices in a desktop publishing system, and the process by which the entire desktop publishing system is coordinated with the file. In order to ensure the stability, reliability and continuity of the color information in the transmission process, the input, display and output devices must be systematically characterized so that the device is in a standard state. Color management is to detect the device through a series of color measurement tools, and draw the color or gamut characteristic curve of the device, and then make a color description file of the device against the color model independent of the device. These color description files are the credentials for the conversion of the color space of the respective devices with the standard, device-independent color space, ensuring the stability of the color characteristics of the device itself. In order to enable the sharing of color information in a multi-device environment, ICC has developed a cross-platform and system ICC standard. In this standard, they define 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 device.
Third, pay attention to the choice of color space conversion and its common methods
Color space conversion is a process in which color image data is bridged between different devices using a device-independent color space. The conversion of color space involves two issues. 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 in CIE is based on a large number of color vision measurements. It is independent of the device and is widely used in color management systems. The second is the mapping relationship of color space conversion. Due to the inconsistency of many color spaces, it will inevitably involve the compression or trade-off of color gamut. It is the color conversion requirement and target that makes the color displayed on the display and the color output of the color printer, imagesetter, etc. as close as possible to the original. However, since the CMYK space of the output device is narrower than the RGB space of the input and display devices of the scanner, the display, etc., in the color space conversion, it is necessary to establish a compression or subtraction of the gamut of the display and the scanner that cannot be covered by the ink. Relationships can ensure that the ink reproduces the space as large as possible. Depending on the requirements for color production and reproduction, color conversion often takes several forms, each of which applies to different objects.
1. Visual perception transformation. 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 mix the colors in visual perception. This conversion is often used to restore a continuously tuned original that requires more.
2. Chroma conversion. Chromaticity conversion can be further divided into relative chromaticity conversion and absolute chromaticity conversion. The difference between the two is that the color processing before the matching color exceeds the matching color space portion. The former replaces this part of the excess color with a border color, which compresses this part of the color into the matching color space. This conversion is often used for display adjustments to ensure the same color of chromaticity is achieved on different displays.
3. Optimal color gamut conversion. This conversion requires the production of the purest and most saturated color in the color space limited by the output device, instead of pursuing the same color as the original or display, and not targeting color matching between different output devices. This type of conversion is often used for creative commercial printing.
In summary, in order to ensure the maximum compatibility between the various devices in the desktop publishing system, it is necessary to implement effective color management. First, the color space of the whole system should be unified, and the unity of space must be independent of the device and color. A wide Lab space or other suitable color space; secondly, the system equipment is characterized, and the color profile of the device enables the system to coordinate the color information; in addition, the color space conversion should be selected Independent of the device's color description language, and based on the color gamut that can be expressed by printing, establish a suitable mapping relationship. At the same time, we should choose the appropriate color and space conversion method according to different situations.
Generally speaking, the specific operation implementation methods of color management can be divided into two types: one is manual management method; the other is 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 based on calibration and calibration implemented by 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 the CMS can make the color look the same from the display to the proof sheet to the printed matter no matter which device and original are used. 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 color of the scanner, color monitor and final proof or print, and manage it in a coordinated manner to achieve WYSIWYG. As long as the user operates according to the requirements of the color management software, by color The management software automatically performs color compensation to achieve the best color reproduction. Even with no color processing experience or expertise, a satisfactory color picture can be produced. In the whole process of printing graphic reproduction, prepress processing is the key to the whole process. Fully play the maximum compatibility of pre-press processing equipment performance, and implement the most effective color management, which is the most important to achieve good quality of printing products. A ring.