Talk about the working process of color management in detail

(1) ICC characteristic file

During the color printing process, it is often heard from pre-press operators that the plates we made are very good, but it is a pity that this batch of work was smashed during printing. But the printing operator has another discussion, what is the printing plate! The deep place is not deep, the shallow place is not shallow, it is almost impossible to print!

Why are there two different voices? The reason:

(1) The color on the computer screen is different from the color printed on the paper:
(2) The same contract signed by the customer does not match the printed sample;
(3) The influence of various materials and equipment on color.

It seems that color management is first and foremost a question of color space, that is, which color space is used to control color. Monitors, digital cameras, scanners, etc. all work in the RGB color space; proofers, printing machines, etc. work in the CMYK color space. When the same image is output on these devices, the final color effect may be completely different. This is because they are in different color spaces.

Since RGB and CMYK are all device-related spaces, it is necessary to realize the conversion of different device space color modes to ensure that the colors of the same image color from input, display to output match as closely as possible, and finally reach the original and replica Consistent. It needs to be established in a color space that has nothing to do with any specific equipment, materials, and processes.

In the color management technology, the color space of CIEl976Lab is adopted, and the colors on any device can be converted to this space, and then the color matching conversion is performed.

In order to illustrate the role of the CIEl976Lab color space more clearly, let's look at an example:

In actual life, we often go to the farmers ’market to buy vegetables. We will find that there is a complex weighing platform at the outlet of the farmers’ market, and it is the only one. If the buyer feels that the food he bought is not enough, he can go to the re-weighing platform to re-weigh the scale. This is a standard scale. This standard scale has nothing to do with any vendor ’s scale, and any vendor must follow this scale as standard.

In color management, to display the colors presented on one device with high fidelity on another device, this requires understanding the color rendering characteristics of colors on various devices. Since a color space irrelevant to the device has been selected, that is, CIEl976I plus chromaticity space, the color characteristics of the device are as follows: the device's color description value has a certain correspondence with the color value of the "universal" color space , Which is the color profile of the device. Or ICC characteristic file.

ICC characteristic files can be divided into: ICC characteristic files of input devices, display devices and output devices.


â‘ RGB color space

A color space composed of a collection of colors mixed with three primary colors of red, green, and blue. The colors used by the scanner and the monitor are composed of the three primary colors of red, green, and blue. The RGB color space is a color space related to the device, that is to say, the colors they produce are related to the specific device used, and the color mixing rule in the RGB color space conforms to the color light addition method.

â‘¡CMYK color space

The CMYK color space is the space formed by the color material, which is in accordance with the principle of the color material subtractive color method. The CMYK color space is also a space related to the device.

Note: CIEl976Lab chromaticity space CIE: International Illumination Commission French letter head L is to describe the brightness of the color a is to describe the degree of color reddish green.
b is to describe the degree of color yellowish blue.
Note: ICC is the International Color Consortium (Intermatconal color consoertion) [next]

(2) The three cores of the color management system (3C)

Color management must follow a series of prescribed processes to achieve the desired effect. The color management process has three elements: calibration, characterization, and color conversion. They are the three cores of color management.

1. Equipment calibration

In order to ensure the stability, reliability and sustainability of the color information transmission process, it is required to compare the input device, display device and output device to ensure that they reach the best working state.
Input correction: It is to correct the brightness, contrast, black and white field (RGB balance) of the input device. For example, the corrected scanner should obtain the same image data for the same original document whenever it is scanned.
Display calibration: enables the display card to accurately display colors on the display based on the color data of the image data.
Output calibration: The calibration of the collation machine is based on the correct image of the display; the calibration of the printing machine and the proofing machine must make the paper, ink and other printed materials used in the equipment meet the standards.

2. Characterization of equipment

As mentioned earlier, one of the cores of color management is to establish a device file, which serves as a standard and is a bridge between two color spaces (PCS).
Simply put, the color management system transfers the data file to the color space of the characteristic file according to the characteristic file of the input device, and then transfers the color information of the data file to the output device according to the characteristic file of the output device (display, proofer) Color space to ensure the consistency of color reproduction in the workflow.

3. Color conversion

The third core of color management is the color conversion module (CMM), which is used to interpret the device characteristic file and convert the color data of different devices according to the device color described in the characteristic file.
A basic principle of color conversion is that the same color is guaranteed to be the same color on different devices. In order to achieve this goal, there must be a device-independent color system to measure the colors on each device, and any color space related to the device can be represented in this color space. If different device-related colors can correspond to the same point in the device-independent color space, the conversion between them must be accurate. When performing color conversion, the color data is first converted to by the device characteristic file. The color value of PCS is then converted to the color value of other devices as needed.

The following describes the application of these three cores in color management through the specific production of input and output device ICC characteristic files:

● Creation of input device ICC characteristic file

(1) IT8 standard color code

If you want to make an ICC characteristic file for an input device, you generally use the Kodak IT8.7 / 1 IT8.7 / 2 standard color scale (hereinafter referred to as the color scale).
IT8.7 / 1 and IT8.7 / 2 color scales are suitable for color calibration of input devices (scanners and digital cameras), IT8.7 / 1
The materials used in the IT8.7 / 2 color scale are color transparent positive film and color photo paper, respectively. IT8.7 / 1 is suitable for input color correction of transmission scanners; IT8.7 / 2 is suitable for input color correction of reflection scanners.

The IT color code consists of four parts:
① Sampling color area. This part is composed of 12 × 12 color patches. It is divided into dark color block, middle color block and light color block by column, each part has four saturation changes.
â‘¡Color ladder. There are seven columns in this part, three of which represent cyan, magenta, and yellow; the other three represent red, green, and blue; and the middle column is neutral.
â‘¢ Neutral gray ladder ruler. At the bottom of the color scale are the twenty-two neutral gray scales that vary according to degree of cosmology. Mainly used for basic correction of input devices.
â‘£Optional area. Different manufacturers can arrange three columns according to their needs, such as characteristic colors. The entire color scale contains a total of 286 color patches. These color patches are the standard color patches that establish the ICC characteristic folder of the scanner. They express the entire color gamut more completely, and each color patch contains a standard colorimetric data.

(2) Preparation of scanner ICC characteristic file

The production process of the scanner ICC characteristic file is:

â‘  Calibration The purpose of calibration is to adjust equipment (such as scanners, monitors, printers, and printing presses) to a standard state to ensure that it meets production specifications. Most devices are calibrated when they leave the factory, but during the use of the device, the quality of color reproduction is also affected by factors such as environmental changes and aging of the device. Therefore, calibration is very important for reproducing colors. All equipment must be calibrated before use.
After the scanner is turned on, it warms up and then runs the scan driver software. It first scans the IT8 manuscript, and uses the gray scale at the bottom of the IT manuscript to correct the scanner's brightness, contrast, focal length, gray balance, and midtones.
For example: to ensure the first level of the gray ladder in the scanned image. The RGB value is maintained between 150 and 255, the RGB value of the twenty-second level, that is, the dark tone portion, is maintained between 0 and 5, and the RGB value of the eleventh level in the middle portion is 125 right.
If the R, G, and B values ​​of the scanned image are inconsistent, it means that the scanner has a color cast. Assume that the RGB values ​​of the eleventh level in the middle tone are actually measured as R = 118, G = 125, B = 125. Increase the intensity of red to maintain balance with green and blue. This can ensure that the scanned image has a small color cast and pure color.

â‘¡ Scan the IT8 color scale according to the reflection of the scanner; select the IT8.7 / 7 color scale or IT8.7 / 7 color scale for the transmission type. In the scan menu, cancel the screen removal, sharpening, gradation adjustment and other options related to color management, and then save the scanned RGB image.

â‘¢ There are many softwares that generate ICC characteristic files and create characteristic folders in ICC characteristic file creation software, for example: Heidelberg software and Agfa software.
Run the ICC feature file creation software, open the standard file containing the color block chromaticity data and the scanned image file in the software, the software will compare, correct and correct the RGB data in the image file with the color block standard Lab chromaticity data. Optimize to get the characteristic file of the scanner, and finally save this file to end the production of the ICC characteristic file of the input device.
In this ICC characteristic file, i 'records the basic characteristics of the color reproduction of this scanner. Different scanners have different ICC characteristic files. It can be seen that the role of the IT8 color scale in the process of making ICC characteristic files is like a ruler to measure the accuracy of color.

Production of ICC characteristic file of output equipment

(1) IT8. 7/3 digital standard color scale If you want to make an ICC characteristic file of the output device, IT 8. 7/3 digital standard color scale (hereinafter referred to as color scale) is indispensable. IT8.7.3 color standard is an electronic file, suitable for output devices that use CMYK four colors as color expression.
There are a total of 928 different color patches in the IT8.7 / 3 color scale. The color data of these color blocks are CMYK data. The coverage of CMYK dots of each color block is different. When different output devices output IT 8.7 / 3 color scale, the 928 colors printed out represent the output device Color expression ability. Devices with strong color performance can express subtle differences between excellent blocks; while devices with poor color performance have less differences between color blocks, and small differences between color blocks will be unclear. The output device's ability to reproduce the IT 8.7 / 3 color scale represents the device's accurate output capability.

On the IT8.7 / 3 color scale, there are the following different types of areas:
â‘ Dark color block area.
â‘¡Overprint color block area (check ink overprint effect j
â‘¢Saturated color block area without black.
④Saturated color patch area containing 20% ​​black.
⑤Grey area composed of CMY or CMYK (check gray balance).
â‘¥ Solid color block color area (to check solid density).
⑦CMYK ladder (check the dot coverage).

(2) The production process of the ICC characteristic file of the output device

Take printing equipment as an example: explain the production of output device ICC characteristic file.

â‘  Calibration Before officially printing the IT8.7 / 3 color scale, use the control bar tool to calibrate the printing equipment, mainly referring to the solid density and dot coverage. The environment and production conditions of the printing machine are required to be stable. For example, the field density and dot coverage in formal production must be consistent with the printing of IT 8.7 / 3 color scale. Only the ICC characteristic file generated in this way can correctly reflect this The color characteristics of the printing press.

â‘¡Print IT8.7.3 color scale Output IT8.7.3 color scale, and print to get proofs. In addition, the film output and printing process are adjusted to the standard state.

â‘¢Generate ICC characteristic file in ICC characteristic file creation software Run ICC characteristic file creation software, select measurement option, generally online automatic measurement, scanning spectrophotometer is driven, follow the prompts to carry out chromaticity measurement on IT8.7 / 3 printed samples After the measurement is completed, the software accurately corresponds the measured chromaticity data to the CMYK data of the color patches in the color scale. After the software generates the ICC characteristic file of the output device, Save this ICC feature file.
Unlike a scanner with only one ICC characteristic file, a printing machine may have multiple ICC characteristic files, because if paper, ink, dots, and environmental conditions change, it will affect the color, so it must be Create multiple ICC characteristic files for different paper and ink combinations to meet the needs of production.
If each device related to color reproduction has an ICC characteristic file, it lays the necessary foundation for the consistency of color reproduction. The following only uses screen soft proofing to illustrate the importance of ICC profile in color reproduction. A color job must undergo a proofing process before formal printing. One of the functions of proofing is to check the quality of the colors on the proofs. If errors can be found before the formal printing, the cost can be reduced. Can the preview be previewed to the actual printing through the display in advance? What about the color effect?
The color reproduction mechanism of the display and the printing is completely different. The three colors of red, green and blue are generated on the display screen, which are colored according to the color light mixing mode, and the four colors of yellow, magenta, cyan and black used in four-color printing The ink is formed according to the color mixing mode; at the same time, the color gamut of the two devices is also different. Therefore, the colors seen on the monitor are actually quite different from the colors on the final print.

The basic principle of screen soft proofing:

Display-Display (ICC)-Color Management (CMM)-Printing Machine (ICC) Printing Machine

However, if there are ICC characteristic files and color conversion module (CMM) of the display and printing equipment, the color of the final printed product can be previewed in advance through the display. Since the ICC profile contains the characteristics of the device's color reproduction, the color management software will compare the ICC profile of the two devices to determine whether the colors in the image file can be printed. If there is a super-gamut color, the color tube software will convert the super-gamut color, so that the final printed color effect can be correctly previewed on the display.

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