Scanner, display and printer and color management
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Integrated Color Management version 2 (ICM 2.0) is a color management module built into win98 and win2000 to enable users to get stable and high quality color images.
This article describes some of the technical issues with color devices and how to implement color management on the win98 and win2000 platforms.
Color management
If the image is scanned by a scanner into a digital file for desktop publishing, the user would of course wish that the final printed or printed copy would be very similar to the original. If you have also done color correction, you want the final print color to be the color that appears on the screen when you make the correction. ICM can describe the color characteristics of an image, manage color data, and obtain high quality color reproductions.
There are more and more types of color devices and products, and the difficulty of printing high-quality color images by display displays or printers has attracted more and more attention. Color devices such as scanners, digital cameras, displays and printers are starting to enter the home, and people want to be able to make beautiful pictures such as color magazines, but many of the techniques and artistic knowledge and skills involved are not everyone. Can be easily mastered.
The goal of color management is to enable people to easily scan, photograph, display, and print high-quality color images without having to rack their brains to learn a variety of techniques that only a very skilled technician can control in the past. For color management, of course, mainly its software part, it plays a central role; but the hardware device data and its description files provide operational information, so that the software part can be truly personalized color management.
The color system shown in Figure 1 describes some typical peripherals that a user may encounter: the PC manages the communication between the peripherals, so it must have a suitable interface with different peripherals with different color rendering characteristics.
Three terms are important: calibration, characterization, and device profile profiles
After calibrating the device, the known value of the input should be able to predict the outcome of its output. For example, the reflection effect of 0.65 can be obtained by the printer printing an image with R, G, and B values of 255, 127, and 0, respectively. Characterizing a device is to enter a series of values for the device and then measure the results separately. The difference from the calibration process is that the characterization does not pre-estimate the results that may be obtained. Entering values 127, 127, and 127 for the device is generally considered to result in a neutral gray, but the result is likely to be another color. Once you've characterized it, enter a value that will give you a specific color effect for that particular device.
The device profile file can be thought of as an inverse of the characterization. For example, if you want to get a neutral gray, the profile file will tell you that the RGB values you should enter are 131, 122, 119 (here is just an example), not 127, 127, 127, because the characterization data can indicate the value of the input. The relationship between the output effects. The profile file tells you how much to get a color because of the input. Most CMS solutions today work this way.
Figure 2 depicts a simple color management workflow. Color Management Module (CMM Color Management Module) The device driver generates a signal value of the appropriate size according to the device profile. If the color device is properly characterized, then the CMM should produce the effect that the user wants. For example, with a profile file for the monitor and printer, the same image printed by the monitor and printed by the printer should be very similar.
Implement color management for color devices
The color rendering characteristics of a color device are not static. For example, after a while, the display on your desk shows the same image, the effect will be different; if the color printer changes paper or ink, the ambient humidity changes or after using it for a while, the printing effect will be different.
Characterizing the printer, the generated profile file has a certain validity period. Once the consumables such as paper are replaced or the printer is manually adjusted, the print results are likely to be unexpected. In addition to professional publishers, the average person does not use professional color measurement instruments, which are expensive and require skillful use.
The electron gun of the display has a certain service life, and different combinations of red, green and blue light will change the imaging effect of the display. There are many adjustments to the display, especially the adjustment of the cathode ray tube CRT can change the brightness contrast or even the color balance. Users can also manually set the gamma value. The aging and poor maintenance of the display will increase the difficulty of the CMS, changing the color rendering characteristics of the display, that is, changing the basic data on which the profile file depends.
Implement color management on the display
Color displays are the easiest to characterize and generate profile files. The display and graphics card communicate with each other and combine with the operating system to represent color images. Users can manually adjust the display and graphics card, but changes to brightness, contrast, and color balance must be loaded into the operating system.
Future display systems will use simple sensors to detect brightness, color balance or "white field". The display should be able to communicate in real time with the color status at the time, adjusting the offsets and errors that occur at any time. Display and graphics card vendors should enable the operating system to intervene in the imaging system to simplify the upgrade of display profile files and implement proper color management.
There is a dye-based filter behind the LCD screen. This filter is stable in nature and has a long life. The color rendering performance of the display is also quite stable. With the advent of LCDs for desktop publishing, the importance of graphics cards and operating systems has become increasingly prominent. Any adjustments made to the display should be readable by the operating system. Other types of display technologies, such as FEDs, digital mirroring devices, etc., must also reflect feedback to the operating system.
Points to note when designing your monitor:
The characteristic values of the display device, such as brightness and contrast, must be recognized and processed by the operating system for proper color management.
Adjustments to the graphics card, such as the adjustment of the gamma value, must also be recognized by the operating system.
Still, the color rendering characteristics of both the display and the graphics card should be configurable by the operating system.
Scanners, digital cameras and color management
Input devices such as scanners and digital cameras involve additional problems. Scanners usually come with software controlled imaging. But any adjustment should be able to enter the operating system and enter the color management software. The scanner's status settings should also be entered into the operating system and used by the color management software.
Digital cameras use external light sources, which may be direct sunlight, cloudy daylight, white light at home, or fluorescent lights in the office. Natural light may have many variations in spectral distribution and intensity. Various problems may affect the quality of digital images, but if you can know the approximate brightness of the environment, this process will be much easier with CMS. Perhaps every digital camera should have a simple but effective RGB sensor on the body to detect ambient brightness.
Points to note when designing a digital camera or scanner:
Two-way communication between the scanner and the operating system must enable the color management system to receive setup data for the scanner or digital camera, such as brightness and contrast.
Both digital cameras and scanners should be controllable by the operating system, and any settings that may affect image quality should be transferred to the color peripherals through the operating system.
It is more convenient to set the peripherals through the color management system. If the user knows the profile file of the scanner he wants to use, he can immediately transfer data information to the scanner through the operating system.
Printer and color management
Because of the many influencing factors, printers can be said to be the most difficult part of the color management system. The way a color is represented by a printer is quite different from how it behaves on a digital camera or scanner. This is a subtractive method. The three primary colors are C, M, and Y. They are expressed by toner and ink droplets. In order to better express the dark and dark tone and save printing costs, black ink is generally used.
There are many printing technologies, such as thermal, static, inkjet, and the like. Paper has a great impact on printing because toner, dyes, etc. interact very much with the surface of the paper. For display devices, generating profile files may require measuring dozens of hundreds of color patches; scanners and digital cameras need to measure about 200 color patches; printers need to print and measure hundreds or even thousands of color patches. This is a painful process that often requires professional equipment worth thousands of dollars, which can take hours or even hours.
Printer manufacturers should consider adding a printer's self-test feature that eliminates the need to print and measure hundreds of patches and just a few key patches. Then a more sophisticated color management system can read and process these measurements and generate new profile files. At present, the development of color management technology has taken this into consideration, and printer manufacturers should clearly recognize this and make their own efforts.
Points to note about printer design:
The printer is prepared to communicate with the user to adjust the color.
The internal monitoring capability should be able to monitor the density and spectral data of the printed colors.
The printer should be able to "download" the paper and ink characteristics to some extent for use by the operating system and advanced color management software to control the printer's output.
Developers should not expect users to be able to independently perform spectral measurements and generate profile files for color devices, which can only be done by prepress and other professional users.
to sum up
To get a high-quality color image, remember, "If you can't measure, you can't control it."
If the hardware manufacturer can provide good communication and control for its products, the operating system and color management software can compensate for the state drift of the hardware device. In the future, color management can achieve stable high-quality color reproduction with very little human participation.
Equipment manufacturers should implement two-way communication between peripherals and computer operating systems and color management software for various parameter settings that affect color reproduction.
Mature color devices should allow the color management system to obtain any device state drift information allowed by the user so that the device profile can be upgraded at any time.
The device parameters should be set by the color management system through the operating system, so that the working state based on a previous color rendering performance can be instantly achieved.