Analysis of ICC Profile in Color Management

In 1993, the world famous prepress software developers Adobe, Agfa, Apple, Kodak, FOGRA, Microsoft, Sun Microsystem and other companies initiated and established the International Color Consortium (ICC), which aims to establish, promote and encourage open . A cross-platform, neutral color management system, and the development of the ICC Profile specification, the latest version of the File Format for Color Profiles (Version 4.1.0) was released in January 2003.

Color management content

In the process of color image reproduction, the devices involved all work in their own unique color space. The color management technology is an unknown factor that explains the continuous changes in the replication workflow. It quantitatively analyzes them, and on this basis The corresponding color space conversion, so that the color reproduction has nothing to do with the device. Color management is divided into three steps in principle, referred to as "3C".

1. Calibration equipment (caiibration)

Establish a neutral gray environment, standardize the light source, and calibrate the input device, display device, and output device.

1) Input device calibration. Input corrections include corrections for brightness, contrast, and black and white fields. Taking the scanner calibration as an example, after the scanner is initialized to zero, the same image data can be obtained for the same document regardless of when it is scanned.

2) Display device calibration. Use the monitor to calibrate the system's brightness (including hardware and software) to the monitor. The contrast, color temperature, and gamma value of the entire display system are set.

3) Output device calibration. The gray balance parameters and black version parameters of the output device (printer, printer) are corrected. This is for correction of the printed material parameters such as paper and ink.

2. Create a characterisation file (characterisation)

The color management software is used to interpret the specific color characteristics of the device, and on this basis, the device's color profile (ICC Profile) is established against the device-independent color space. The description file is from the device color space to the device-independent color space (PCS). ) Conversion bridge.

3. Color space conversion

Based on the calibration of the equipment in the system, the device description file is used to realize the correct conversion between the color spaces of the devices using the standard device-independent color space as a medium.

ICC Profile File Format Analysis

1. The overall structure of the ICC profile

From Figure 1, we can see that the ICC Profile consists of three main parts: the file header, the tag table, and the tag element data.

(1) Profile Header

The file header is composed of 128 bytes, 4 bytes are one unit, and the file size and CMM type are recorded in turn. Version information, Profile type or device type, device color space, conversion connection space (PCS), master platform, file creation date, illumination body color, gamut compression method (including sensory method, relative colorimetry, saturation method, and Absolute colorimetry), media type and other information.

(2) Tag Table

The tag information contained in the main record file is similar to the function of retrieving the directory. The first 4 bytes are the tag counts contained in the file, followed by the description of each tag, each occupying 12 bytes, including the tag name, location and size in the label element data area.

(3) Tagged Element Data

The information of each tag is recorded in turn according to the data in the tag table. The first 4 bytes of each tag content are tag types. The following formats vary according to the tag type and have different sizes.

According to the different types of devices, the necessary tags contained in the ICC Profile are different, and for the same device, different tag combinations may be generated due to different algorithm models for generating the ICC Profile. Manufacturers of color management systems will also add their own private labels based on specific equipment conditions. When the color management system converts colors, it finds the position of the tag from the tag table according to the desired tag name, and then reads out the data of this tag to perform mathematical calculation of the color conversion. Through the ICC Profile Inspector software you can see the header information and tag information of the ICC Profile, which can be found on the official ICC website. Org is free to get.

2. Classification of ICC Profiles

The ICC Profile can be divided into four categories: Input Profile (Profile), Profile (Display Profile), Profile (Output Profile), Additional Profile (Additional Profile). These include Device Link Profile, Color Space Conversion Profile, Abstract Profile, and Named Color Profile.

Since the color spaces of the display device and the input device are all RGB, the necessary label combination of their ICC Profile files is the same, and is generally divided into three types: a single-color display (or input) device Profile, a three-dimensional display based on a matrix model (or input ) Device Profile, multidimensional display (or input) device profile based on a lookup table model. The color space of the output device is CMYK, and its label combination is more complex. There are mainly two types: a single-color output device profile and a color output device profile.

3. Common tag descriptions for the ICC Profile

The ICC defines 48 tags, a total of 30 tag types, and each tag has a certain tag type. For each ICC profile, the combination of tags it contains is not the same. Here are a few common tags.

profileDescriptionTag: general description of the profile;

copyrightTag: copy information of signature file;

mediaWhitePointTag: medium white field tristimulus value (XYZ);

mediaBlackPointTag: media black field tristimulus value (XYZ);

grayTRCTag: grayscale tone reproduction curve, generally used only for monochrome output devices;

redTRCTag: red tone reproduction curve;

greenTRCTag: green tone reproduction curve;

blueTRCTag: blue tone reproduction curve;

redMatrixColumnTag: tristimulus value of red phosphorus (XYZ);

greenMatrixColumnTag: tristimulus value of green phosphor (XYZ);

blueMatrixColumnTag. Blue phosphor tristimulus value (XYZ);

AToBOTag: conversion parameters from the device color gamut to the PCS color gamut, the adaptive color gamut compression method is the sensory method;

AToB1Tag: Conversion parameter from device color gamut to PCS color gamut. The adaptive gamut compression method is relative chroma method;

AToB2Tag: conversion parameters from the device color gamut to the PCS color gamut, and the adaptive color gamut compression method is the saturation method;

BToAOTag: conversion parameters from the PCS color gamut to the device gamut, the adaptive color gamut compression method is the sensory method;

BToA1Tag: The conversion parameter from the PCS color gamut to the device color gamut. The adaptive color gamut compression method is relative colorimetry.

BToA2Tag: The conversion parameter from the PCS color gamut to the device color gamut. The adaptive gamut compression method is the saturation method.

gamutTag: Detects whether the input color is outside the gamut.

Among them, TRCTag and MatrixColumnTag are the exclusive labels of color space space for RGB display device and input device. Since the color management system can calculate the conversion parameters adapted to the absolute color gamut compression method based on AToB1 and mediaWhitePointTag, there is no specific definition of an AToBTag tag for the absolute color gamut compression method. There are many more unlisted tags that can be selectively added depending on the algorithm requirements. The more tags, the larger the file.

The ICC color standard has been widely recognized. Most hardware devices have been calibrated at the factory and have their own ICC profile properties file. However, as the use time continues and the environment changes, the hardware parameters will change, so we need Always correct the device and re-establish the ICC Profile file that meets the characteristics of the device at the time, in order to fully utilize the device's performance.

There are many peripheral products on the market with their own color correction software, such as Agfa professional scanner color correction software COLOR TUNE; Heidelberg's color management system Scanopen (for scanners and digital cameras), Viewopen (for displays) . Printopen (for output devices); Color Sync color management system based on Apple OS; Adobe Gamma (calibration monitor) in Photoshop; and dedicated ICC tools such as Kodak's ColorFlom ICC profile Tools, including ColorFolw ICC Monitor ProfileBuilder ( For monitors, ColorFlow InputProfile Builder (for scanners and digital cameras), use these software to create a device's color profile in real time.