The necessity of printing color control
Perfect color reproduction must be checked from the aspects of image acquisition, processing, color separation, and printing. Because in each link, the color information will be expressed according to the color rendering principle and color description characteristics of the currently used equipment. Different scanning and display devices will have different color expressions for the same manuscript; in addition, the color expression capabilities between the three-color RGB display and the four-color printing ink are different, and different application software will The conversion of RGB data to CMYK also lacks consistency, and different papers and inks are used, resulting in different prints. Therefore, the color reproduction on various equipment and media must be controlled through color management technology.
The bridge of color management
The establishment of the device's color profile (Profile) is the core of color management. The description file has the representative color characteristics of each device, such as the chromaticity characteristic curve, output color gamut characteristic curve, etc. The color management system uses these representative The characteristic color features enable the matching and conversion of the color space of each device.
For color management, a series of prescribed operating procedures must be followed to achieve the desired effect. Color management is divided into three steps in principle, referred to as "3C", namely calibration (Calibration), characterization (Characterization) and conversion (Conversion).
Calibration: In order to ensure the stability and sustainability of the color information transmission process, it is required to calibrate the input, display, and output devices to ensure that they are in a standard working state. Input correction includes correction of brightness, contrast, black and white fields of scanners, etc. To ensure that for the same original, no matter when it is scanned, it should be able to obtain the same image data.
By setting the brightness, contrast, color temperature of the display and the gamma value of the entire display system, it can accurately display colors. Output calibration includes the calibration of printers, imagesetters, printing presses and proofers. After correcting its characteristics, the device can be output according to the standard characteristics of the factory.
Characterization: Recording the corrected characteristics of all equipment is the process of characterization. These characteristic description files are the bridges from the device color space to the standard device-independent color space (PCS).
Conversion: Based on the calibration of the system equipment, using the device description file, using the standard device-independent color space as the medium, to achieve the correct conversion between the color space of each device. Because 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. The color gamut compression has the absolute color method, relative color method in the ICC protocol, Highlight four methods: saturation method and sensory method.
Understanding ICC
ICC file (ICC Color Profile) is the device color profile, which describes the characteristics of different devices in terms of color performance. ICC files are the basis for the implementation of color management. Having ICC files as a bridge allows software with color management functions (such as Photoshop) to accurately display the conversion and change of colors on different devices according to the color characteristics of different devices. At the same time, it also allows the color to be lost on different devices. The ICC file is to match all the color data (RGB and CMYK data) related to the device to the Lab color mode that is independent of the device. Therefore, the Lab mode is used as a transmission intermediary to maintain the consistency of the appearance of all colors, which also keeps the color unchanged when transferring between different devices.
The hardware devices in each color process including display, input and output should have their own ICC characteristic files. These files describe the color gamut characteristics of various devices in a hardware-independent color space, so that the corresponding color management The software performs color transfer and conversion between scanners, digital cameras, color displays, proofing equipment, printers, and other devices based on ICC feature files. Under the guarantee of the ICC standard, the device's characteristic files can realize accurate image display and output, that is, what you see is what you see. Due to the introduction of these color management software and good compatibility, the operator got rid of a lot of tedious manual color correction work caused by the use of non-standard mode.
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