In addition to the technical advantages of digital proofing, remote digital proofing shortens the proof transmission time and improves work efficiency. The core technology of remote digital proofing is the color management system. This paper proposes a color management solution for remote digital proofing.
1. Remote digital proofing
1.1 The principle of remote digital proofing
Remote digital proofing is a set of software and hardware tools developed to ensure the ideal printing results at any time and any place. Remote digital proofing is a further upgrade of digital proofing. Remote digital proofing can be divided into remote soft copy proofing and remote hard copy proofing. Remote soft copy proofing refers to screen soft proofing. Screen soft proofing is used to calibrate the monitor to generate the characteristics of the monitor and printing press. The file, after editing the characteristic file of the printing machine, makes the display effect of the standard file on the display as consistent as possible with the printing effect, and realizes a true "what you see is what you get". Remote hard copy proofing is to transmit digital files remotely through the client, perform digital proofing on the server side, and realize the printer to simulate the color gamut of the printing machine, so as to achieve the color consistency of proofing and printing. This article studies remote hard copy proofing, so the following content is elaborated around remote hard copy proofing.
1.2 Features of remote digital proofing
As mentioned earlier, remote proofing is a further upgrade of digital proofing, so remote proofing has all the technical advantages of digital proofing over traditional proofing: high image resolution, good reproducibility; fast speed, low cost; stable quality and repeatability Good performance; easy operation, high reliability, etc. However, there are still problems in the transmission process of digital proofing. There is no substantial change in the transmission of proofs. Customers will take the produced documents to the printing factory for digital proofing, and then the printing factory will feedback the printed proofs. The customer signs and approves, and then the customer feeds back the information to the printing factory for plate-making and printing. In this process, if there is an error in the proofs, it is necessary to return to the customer to modify. The proofs modification process is cumbersome, resulting in unsmooth communication between the customer and the printing house. The characteristics of remote digital proofing are more prominent than digital proofing: faster speed and lower cost. Remote proofing can ensure that the proofs from the sender to the receiver are completely consistent, saving time and ensuring data security. The distance will no longer be important. Both parties can efficiently exchange opinions on the document without leaving the house, and can immediately convert the opinions into new design documents. Printers can expand their customer base nationwide or even globally. All designs can be easily confirmed online. This means time and cost savings for printers and customers. Remote digital proofing has established a real remote communication bridge between printers and users.
1.3 The core of remote digital proofing
The remote digital proofing system is composed of a digital printer and remote proofing software including color management software. To print the electronic files without using film and plate making, and print them directly with a printer, and to achieve the same effect as traditional proofing proofs, the entire system is implemented by color management through color management software. Because in remote digital proofing systems such as monitors, scanners, printers, printing machines and other equipment, different types of equipment, even the same type of different types of equipment, the performance of color is also different. For example, different monitors have different effects on the same image. Different printers produce different results on the same manuscript, not to mention the difference in color of the printed product due to the difference in ink and paper. The root cause of this difference is that each device represents color in its own color space. In other words, the definition of color is device-dependent. For example, the RGB value of a color is defined, which is suitable for a certain display of the user. When the same value is passed to another display, the color effect obtained may change. The reason is that when a color is transferred from one device to another, the conversion between color spaces does not perform a good color match. Digital proofing technology to achieve color consistency is mainly through the use of color management system, mainly for printing standard document color blocks and digital proofing equipment standard document color standard measurement, so as to measure the respective ICC format data, and then through the color management system calculation , In order to establish the characteristic standard file Profile required for digital proofing. Through CMM, digital proofing equipment and color characteristics can be converted to simulate the color characteristics of printing, so that digital proofing and printing colors can be consistent. Color gamut for printing. Remote digital proofing is a color management and matching control technology that combines traditional color control theory with modern ICC color matching theory and achieves remote proofing color consistency through a network. Therefore, the technical core of remote digital proofing is the color management system.
2. Color management
2.1 Basic concepts of color management
2.1.1 Definition of color management
Color management is the process of converting the color data of an image from one color space to another color space under the premise of minimum color distortion. Its main purpose is to realize the conversion of different color spaces to ensure that the same image color is displayed and output from input to output The appearance shown in is matched as closely as possible, and finally the color of the original and the copy is harmonized.
2.1.2 Basic mode of color management
First, select a device-independent color space as the intermediary color space; then, characterize each device of the entire system; and finally, establish a certain correspondence between the color space of each device and the standard device-independent color space .
2.1.3 Elements of color management
Calibration: A method of adjusting each device (display, scanner, printing machine, etc.) to a defined standard state to ensure that it meets or is accurate to the manufacturer's specifications. The calibration process is very important because the equipment is changing at any time, which affects the way colors are displayed and generated. All equipment must be calibrated before it can be used to ensure the normal performance of the equipment. This is like the zero calibration of the instrument. Characterisation: Since each color input device or color output device, even color materials (such as inks, dyed chemical phosphors on display screens, etc.) have their own color expression capabilities. The purpose of characterization is to establish the range of color expression of equipment or materials, and to record their characteristics in a mathematical manner for color conversion purposes. The main content is to create a color profile for each device. Conversion: On the basis of calibrating the devices in the system, using the device description file, using the standard device-independent color space as the medium, to achieve the correct conversion of the color space of each device. Since the color gamut of the output device is narrower than the color gamut of the original, scanner, and display, the color gamut needs to be compressed during color conversion. If there is no gamut compression, the colors in the gamut can be accurately matched, but the colors outside the gamut will be graded or completely lost.
2.2 Device-dependent color space and device-independent color space
Just mentioned the basic mode of color management: select a device-independent color space as the intermediate color space, first convert the color space of a device to the intermediate color space, and then to the target device color space. Color space is a model that expresses color brightness numerically. A color space usually defines one-dimensional, two-dimensional, three-dimensional, or even multi-dimensional to express color information. Each dimension, also called a color component or color channel, represents a color brightness value. For example, the RGB space is a three-dimensional color space. Color spaces can be divided into: device-dependent color spaces and device-independent color spaces according to whether they are device-dependent.
Device-dependent color space refers to the color space defined by each device with its own color definition. Different devices have different color spaces, even if their color coordinate names are the same. For example, the color spaces of monitors are all RGB color spaces, but different monitors have different RGB color spaces, that is, they have different color gamuts. Therefore, for the same color, the colors presented on different devices are not the same. The CMY color space is commonly used in various printing equipment and printers, and it describes how to use inks and pigments when printing. In theory, the same amount of yellow, magenta, and cyan inks should be overprinted, and black should be produced. Black is not needed, but the actual mixed ink often produces only a dark brown area instead of real black. Therefore, in order to produce For better printing results, black ink (K) is usually used to print on the dark brown area. The CMYK color space is related to the equipment or printing process, such as process methods, ink characteristics, paper characteristics, etc., different conditions have different printing results, so the CMYK color space is called the device-related color space. The device-independent color space defines colors in a clear way. When a set of values ​​for a certain color is given, the colors presented are consistent regardless of the device used. The device-independent color space is based on the CIE standard observer (1931, 1964). In 1931, CIE formulated the 1931CIEXYZ system, an internationally accepted colorimetric system, by defining the standard observer ’s eye color curve. On this basis, some color spaces based on Hue, Chroma, and Lightness have also been established, such as CIELAB, CIELUV. In the printing industry, CIE1976L * a * b * uniform color space is mostly used, and the color difference of colors is calculated accordingly. The national standard of "uniform color space and color difference formula GB7921-87" promulgated by the National Bureau of Standards of China in 1987 is based on the CIE1976L * a * b * and CIE1976L * u * v * uniform color space and color difference formulas.
2.3 Color space conversion and color gamut matching
The two important functions of a color management system are color space conversion and color gamut matching. Because digital printers and printing presses have different color spaces, and two different digital printers have different color gamuts in the color space, color management must be performed on different devices for color management. Color space conversion is to convert the colors represented by a certain color space to another color space, such as the mutual conversion of color spaces such as RGB, CMYK, and CIELAB. Color gamut refers to the range of colors that can be expressed by different color spaces, media, or inks in the visible spectrum. Different devices have different color gamuts. For example, RGB displays a larger color gamut than CMYK. When two color devices have When different color gamuts are needed, they need to be matched. In the remote digital proofing system, data files are transferred in different color spaces, and the color capabilities of each color space, that is, the process of input, display, and output of originals are different. When digital proofing is performed, the characteristics of the ink and paper of the inkjet printer are different from the ink, paper and working principle of the printing, so the color rendering characteristics of the two are also different. The color gamut of the printer is larger than the color gamut of the printer . Therefore, the color space of the two must be converted, and color gamut compression and color gamut matching should also be performed. Color gamut matching methods mainly include chroma matching method, saturation matching method, color vision matching method, relative color gamut matching method, hue matching method and so on. The task of color space conversion is to convert various color spaces to the CIELAB color space independent of the device, and then match these LAB spaces of different sizes with a unified coordinate system to achieve the conversion of color information between color gamuts. . When outputting, the LAB color space also needs to be converted to the color space associated with the device. This is the entire process of color space conversion and color gamut matching during color transmission. The color space conversion involved in the remote digital proofing system includes several correspondences: the conversion between the RGB color space and the device-independent CIELAB color space: the conversion between the RGB color space of the display and the CIELAB color space. Conversion between CMYK color space and device-independent CIELAB color space: CMYK color space of printer and CIELAB color space, CMYK color space of printer and CIELAB color space. Matching relationship between different CMYK color gamuts: color gamut matching between two different printers to achieve the correspondence of color coordinate points between devices of different color gamuts.
3. Color management implementation plan in remote digital proofing
The color management of remote digital proofing is carried out under the premise of ensuring the color consistency of the digital printers at both ends. After the successful color management of the local digital proofing, the data information of the original proof is transmitted to the remote end, which can ensure that the remote proof can The color accuracy is fully controlled to ensure that the final output data is consistent with the original proof. Before the data information at both ends of the remote is transmitted, the process of color management of the digital proofing by the user end is advanced from back to front. Once the parameters of the digital proofing are fixed, the printing machine only needs to be in the standard state when printing the color test block You can better restore the color of digital proofs. In the workflow of the color management system, a color characteristic parameter file that refers to the printed matter must first be generated to determine the color range of the printing press, and then a characteristic parameter file of the color characteristic of the inkjet printer is generated. Using these two ICCProfiles, you can ensure that the printout on the inkjet printer is very close to the reference print. In theory, the above method can simulate any printing method, any paper and material. The process of color management for remote digital proofing mainly includes the following basic steps: generating an ICC profile that reflects proofing or printing characteristics. That is, traditional proofing or printing is based on the IT8 target, and the target sample is measured with a spectrophotometer and related software to generate an ICCProfile reflecting the printability. Generate an ICCProfile that reflects the characteristics of the inkjet printer paper and printer. Color gamut conversion. Convert the color space data of the ICCProfile reflecting the printability and the ICCProfile of the printer through the color management module CMM, so that the sample printed by the printer and the corresponding standard sample printed by the printability can reach the same color or meet a certain accuracy Claim. Original local proofs. Call the color management data set containing the color gamut matching, print out the proofs and the remote control bar, package and transmit the measured control bar data information and color management job parameter setting information to the remote end. Remote proofing. The receiver decompresses the data information for calculation and printing, measures the color data, and calculates the color difference with the original data to obtain a satisfactory â–³ E.
Let's take BESTRemoteproof's remote proofing system as an example to see its workflow for color management of remote digital proofing.
The remote digital proofing system is established by a set of local and remote digital proofing systems through a network connection and professional remote proofing software. The digital proofing at both ends is also an application system composed of digital color management software and professional color printers. .
The software and hardware configuration of the remote digital proofing system in this example is as follows:
Local: EpsonStylusPRO4000 inkjet printer
BESTCOLORXL5.0 color management software
BESTRemoteproof software toolkit
Printopen software
ES-1000 Spectrophotometer
Xï¼RiteDTP41 Spectrophotometer
Far end: EpsonStylusPRO4000 inkjet printer
BESTCOLORXL5.0 color management software
BESTRemoteproof software toolkit
ES-1000 Spectrophotometer
Local: When the user specifies that a BestRemote file is to be created, the image file will be automatically converted to PDF format, and a JDF file will be created during job calculation. PDF files are files used for printing, and JDF files contain precise details of all job settings used to obtain the results to be printed.
After the job is calculated, the image will be automatically printed on BestRemoteproof paper. This is the original proof that the user wants to accurately copy with the exact same settings at the far end. A control bar is automatically printed in the blank space of the proof. The user uses the BestEye spectrophotometer to measure the color value of the control bar. The measurement results are displayed in the BestRemoteControl software and then added to the JDF file.
PDF files (print files) and JDF files (setting files) are now encapsulated into a single file, the so-called BestRemoteproofContainer, with the brp file extension. At this point, the remote file has been prepared and can be sent to the recipient through the network. Remote: The receiver imports BestRemoteproofContainer into the BestWindows product and decompresses the file. Then, calculate the PDF file in BestRemoteproofContainer and print it on BestRemoteproof paper, and print a control bar in the blank space. This is a remote proof. During printing, the JDF file received from the sender will be loaded into BestRemoteControl to display the color values ​​extracted from the sender's proof.
Now, the receiver uses BestEye to measure the color value of the control bar on the remote proof. The measurement results are displayed in the BestRemoteControl software together with the sender results. Then, compare the two sets of values. If the value is within the allowable tolerance range, the accuracy of the color can be guaranteed. The numerical difference will be expressed in increments of E.
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