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Color Management in Printing Enterprises
A manuscript containing a graphic text is often processed by many devices to obtain its copy. These devices include input devices, graphics and image processing devices, output devices, and the like. Because the digital prepress graphic information processing system is open, it is not limited to the equipment, materials, and processes used. The diversity of color rendering features of devices of various brands, types, and color characteristics increases the difficulty of accurate color reproduction. Graphic information In the transmission of these devices, it is inevitable that there will be information loss, so that the copied images and manuscripts are far from each other in terms of color, level, and saturation, and even seriously make the entire image completely different. To copy the manuscript correctly and perfectly, there must be a mechanism for controlling color conversion and transmission. This is color management.
The color management is abbreviated as the CMS (Color Mangement System), which is firstly a problem of a color space, ie, which color space is used for color control. Monitors, digital cameras, scanners, etc. all work in the RGB color space; printers, proofers, printers, etc. all work in the CMYK color space. When the same image is output on these devices, the final color effect is entirely possible. different. This is because they are in different color spaces and there are differences in color expression.
Color management needs to be based on a color space that is independent of any specific equipment, material, or process. Currently, in the color management technology, the so-called color feature connection space uses the chromaticity space of the CIE 1976 Lab, and the colors on any device can be converted to this space to form a “universal†description mode and then perform color Matching conversions. Within the computer operating system, the task of implementing color matching conversion is accomplished by the "color matching module". It is important for the color conversion whether it is reliable or not.
So, how to achieve the color in the "universal" color space transmission, to achieve the loss of color or as little loss as possible? This requires each device to generate a profile, the device's color profile. We know that different devices, materials, and processes will exhibit different characteristics when presenting and delivering colors. In color management, the colors presented on one device are presented on another device in high fidelity, which requires us to understand the color rendering characteristics of the colors on various devices. Since the device-independent color space has been selected, that is, the CIE 1976 Lab color space, the color characteristics of the device are represented by the correspondence between the description value of the device and the color value of the “general†color space. The relationship is the color description file for the device.
In the color management technology, the most common device color profile file has three types. The first category is the scanner profile , which provides Kodak, Agfa, Fujitsu's standard manuscripts and standard data for these manuscripts. These manuscripts are entered using a scanner. The difference between the scanned data and the standard manuscript data reflects the scanner. The second category is the characteristics of the display file , which provides some software, can measure the color temperature of the display, and then generate a color block on the screen, these color block information reflects the characteristics of the display; the third category for the printing device The feature file , it also provides a set of software, the software generates a graphic containing hundreds of color blocks in the computer, and then the graphics output on the output device, if the printer is directly proofed, the printing machine is the first out of the film , Proofing and then printing, measuring these output images reflect the characteristics of the printing device file information.
The generated profile, the color profile, is composed of three parts: the file header, the tag table, and the tag element data. File header, which contains the basic information of the color profile, such as file size, type of color management method, version of file format, device type, device color space, color space of profile, operating system, device manufacturer , color reproduction target, original media, light source color data, etc., file headers account for a total of 128 bytes. Tag table, which contains the name of the tag, storage location, data size information, does not contain the specific content of the tag, the tag name name occupies 4 bytes, and each item of the tag table occupies 12 bytes . Marking element data, which is based on the description of the mark table, stores various information required for color management at a predetermined position, and varies according to the complexity of the mark information and the amount of data of the mark.
For the color profile of the equipment in the printing company, there are two ways to obtain the information and information processing operators.
The first way is to purchase the device, the profile provided by the manufacturer along with the device, it can meet the device's general color management requirements, in the installation of the device's application software, profile into the system.
The second way is to use a special profile creation software, according to the actual situation of the existing equipment to generate a suitable color profile, the resulting file is usually more accurate, but also more in line with the actual situation of the user. The state of equipment, materials, and processes can change or shift with time. Therefore, it is necessary to re-create the profile at regular intervals to adapt to the color response at the time.
Now let's take a look at how colors are transmitted in various devices.
First of all , for a normal color manuscript, it is first scanned with a scanner. Due to the profile of the scanner, color (ie, red, green and blue tristimulus values) from the scanner is provided to the CIE 1976 Lab. Correspondence between chromaticity spaces, so the operating system can obtain the chromaticity value Lab of the original color according to this conversion relationship.
Second, the scanned image is displayed on the monitor screen. Because the system has mastered the correspondence between the Lab color value and the red, green, and blue drive signals on the display, the red, green, and blue color values ​​of the scanner are not directly used in the display. According to the conversion relationship given by the profile of the monitor in the Lab color value of the previous manuscript, the display driving signals of the red, green, and blue colors that can correctly display the original color on the screen are obtained, and the display is driven to display the color. This ensures that the colors displayed on the display match the colors of the original.
Finally, after the operator observes the accurate image color display, it can perform image adjustment processing according to the screen color according to the customer's request. Since the profile of the printing device is included, after the image separation, the correctness of the printing can be observed on the display. colour. When the operator is satisfied with the color of the image, the image is separated and stored. During color separation, the correct percentage of dots is obtained according to the color conversion relationship carried by the profile of the printing device. After RIP (Raster Image Processor), recording and printing, printing, proofing, and printing, a printed copy of the original can be obtained to complete the entire process (to be continued)