PART I - CHAPTER VI
ICC PROFILES

Presentation of International Color Consortium

          The ICC, started in 1993, its goal is to promote a standardization and the development of an interplatform architecture, corresponding with an open system of the color management. The principle is that each application must be in direct relationship with the operating system (MacOs system, Windows, UNIX, etc.), which involves all the transformations of colors. This must entrust this information to a specific module called CMM (Color Management Module). If no CMM reference is used, the operating system employs one CMM by default. Certain applications such as Adobe Photoshop makes it possible to use several types of CMM (Adobe, Heidelberg, Kodak etc.).

Definition

          An ICC profile is a file containing the data describing the characteristics of colors for the given peripheral. This numeric file can be used via a Color Management Software (CMS). It is able to maintain the constancy of the colors on the graphic chain. The profile characterizes the peripheral and its support.

Structure of an ICC Profile

          An ICC profile comprises a descriptive part concerning the profiles from different peripherals, colorimetric spaces, and spaces of connection of profiles. Also the architecture of an ICC profile contains descriptions, that concerns with the formats of profiles, sequence of the peripherals and conversions from one color space to another.




Diagram 14: Structure generic of a profile ICC

          The whole characterization of the photographic chain must be carried out via the use of these profiles. It is thus necessary to distinguish the profiles from different peripherals of input, visualization, output, sequence of peripherals and also as well as the conversion of profiles of colorimetric spaces.

Description of a profile

           Header part Profile

          The header provides information about total size of the profile, the type of CMM used by default, the version of the profile (for example V.2.2.0), the type of peripheral which this profile is associated, the type of colorimetric space to which it refers, its origin, its creator, Rendering Intent and other indications.
The following examples of signatures represent the peripherals of input, monitor and output: scnr, mntr and prtr respectively. In the preceeding case, the type of colorimetric space in which the profile is defined under the same type of signature. In the spaces such as the CIELAB or the CIELUV, the signature of space of reference of the profile (called PCS or Profiles Connection Space) is clearly defined by a code. Two spaces arise for the PCS: space CIEYxy and Space CIELAB.
The platform of work is determined by a signature dedicated to APPLE, Microsoft, Silicon Graphics or Sun Microsystems. Header Profiles makes it possible to get information about the type of Render Intent selected. Each four mode has a specific code.

          The parts Tag Table and Tag Element Data supplements the total structure of an ICC profile. The colorimetric indications of the profile are contained in header profiles, whereas precise descriptions of the peripherals, models of conversion of colorimetric spaces (algorithms) are described in the other two parts.


What is PCS ?

         A PCS (Profiles Connection Space) is an intermediate space or an interface which provides a bridge between an input and an output. From a technical point of view, this space is based on the characteristics of standard observer CIE1931. This space is supposed to simplify the exchanges, and indicated like reference in system ICC.
In general the Profile of peripheral uses LAB like PCS and colorimetric Space like ColorMatch RGB, our sRGB uses XYZ like PCS.






Here is a scanner profile. Its colorimetric space is RGB, the space
of connection is LAB, the rendering intent is perceptive.



Here is a printer profile CMYK. Its colorimetric space is CMYK, the space
of connection is LAB, the rendering intent is perceptive.
Use of ICC Profiles

          What is CMM?

          CMM means Color Management Module . The goal of the CMM is to map the colors of one colorimetric space to another. Companies such as Heidelberg, Adobe, APPLE, Kodak, etc provide their own CMM, thus the user can choose his engine of conversion. Perfect correspondence between two peripherals and with the two different color spaces does not exist. The colorimetric space of the scanner is in general larger than that of the screen, and even larger than that of the printer. That means that the colors that the scanner sees, all cannot be display on the screen, and the colors which the screen posts, cannot all be printed. Which makes the colors out of gamut? This is why it is N essaire to make choices. Four modes of calculations are possible. They are also called Render Intent.



Mapping

The mapping is the transfer of colors from one colorimetric space to another space..
Colors RGB of the scanner or the digital camera are initially transformed in space PCS (Profile Connection Space, which is in general called space LAB) then they are transformed into space RGB of the screen.
According to the rendering intent, the colorimetric space of the screen is smaller than that of the scanner the colors will be cut or modified (compressed) when return to smaller space. If the image is then printed on a printer CMYK, colors RGB are again transformed into space PCS and then transformed into CMYK. Still the space of destination is smaller than that of the source, then according to the rendering the colors will be cut or modified.
The transformation of color CMYK of the machine offset towards the space of the screen (for posting) is less complicated, because space RGB of the screen is larger than the space of the machine offset. It is the same thing for the numerical proof, i.e proofer spaces is larger than that of the machine offset.




          The perceptuel mode:

          It is also named as photographic mode. It chooses to privilege the relation which exists between the colors. Above all, they are the internal modulations between two images, and those must be most homogeneous. It is made up of two images and must be coherent between them. It is a mode which is not normalized, and which can thus present discordances according to the various sources of files. Practically, this mode uses a homothetic projection of one space to another, which makes it possible to consider the relations between the colors. Its major disadvantage is that it causes color loss, in the case of source space is larger than that of Destination.

This method is thus based on the fact that the human is much more sensitive to the relations between the colors than with their respective wavelength.
In Clear: In order to reproduce colors which are out of gamut, the colors inside the gamut are modified in order to make it enter into colors which are out of gamut.
It is recommended to use this mode for images for which it is more important to keep the dynamics of the image i.e the exact correspondence of the colors inside the gamut.

          The saturation mode:



          This mode is primarily based on the axis of saturation. It preserves promptnesses of the colors, contrary to the preceding case. The relation between the colors is not considered here any more. Especially It acts on the average values in the surroundings of 50% in saturation, and rise these to some points. The result is not very close to the original.

          Relative colorimetric mode:

          This mode is one which transform one space to another, with an obligatory loss of information when source space A is smaller than target space B. This loss is accompanied by a total desaturation of the final image compared to the original. This mode seeks possible color that is most identical to that of origin, and thus sees itself being more precise than the perceptuel mode. Most important here is the precision of the colors. It can however breaks in the ranges because of the losses which it generates. Particularly, this mode functions well on flat tint vectorials.

          Absolute colorimetric mode:

          The difference with the three preceding cases is that it takes color from the white. The white of destination is then parameterized differently from that of reference and one can note a drift proportional of the restitution of the colors according to the difference between these two white points.

Color Management and operating system

          Role of Colorsync

          Colorsync, used on the Macintosh platform, is an extension to the software system of APPLE which is used for color management. Colorsync is an A.P.I (Application Programming Interface). It can be able to manage colorimetric adjustments such as the calibration of the monitor, the management of the profiles (selection, suppression, posting of information of the profile, modification etc.). These actions are usable in the form of scripts, in the tableux one of ColorSync edge or in ColorSync Plug-in of Photoshop.
APPLE Colorsync was introduced into the Macintosh system in 1993 under its version 1.0. Version 2.0 of Colorsync was introduced in 1995 to control the color management at the level of the operating system. It is supported by a great number of software such as Adobe Photoshop, InDesign, Illustrator, XPress Quark, Macromedia Freehand etc. For Platform PC, Microsoft introduced an equivalent module, the ICM.
Colorsync uses Color Management Module or C.M.M by default, a module developed by Linotype-Hell and APPLE. Intermediary conversions and modifications are made by itself.

          Colorsync currently became inevitable in the field of the color management (v.3.3), as well as standard developed by ICC in 1993. Colorsync is in conformity with this standard and can, moreover, functions with other C.M.M. that provided by default. This compatibility with others CMM and other applications is an advantage with respect to its competitor ICM.
For example, when one creates a tiff file via an application using Colorsync, a Colorsync profile creates automatically. It has information relating to the colors of the file, properties of display, seizure and colorimetric reproduction.
To ensure the colorimetric correspondence with different applications and between different peripherals, is a difficult task, that the Colorsync module contributes to facilitate.

          Image Color Matching ICM

          This module was created by the Eastman Kodak company, a format dedicated to the digital photographic equipment.. ICM 2.0 is also an A.P.I (Application Programming Interface). It is available on the platform of Windows 98, but is not present on the current version of Windows NT. It has the same function as Colorsync, since it is supposed to establish a permanent dialogue between the peripherals and to manage the constancy of the colors between them. It is used very less in the field of Art-Graphs, the PC does not have the same control in terms of color management in comparison to the Colorsync system set up on the Macintosh platform. This A.P.I. is integrated in a system of color management, support shaping ICC. It plays the part of interface between the various applications using this type of profiles and conversions by mathematical calculations are carried out by Color Management modulates or C.M.M. and it is used in Windows by default.

What is an inserted profile?

          An inserted profile (or Embedded profiles), as its name indicates, it is a profile associated with the job file. This profile posesses some characteristics and options which differs according to the type of file to which it is overlapping. It is necessary that the application with which this file associated with its profile, support this type of configuration. The overlapping profile thus shows the limits of the colorimetric space of the file which it is associated, and it is thus possible for another application to use this file by carrying out the modifications of colorimetric space if necesary.

         The numeric files are generated by several applications such as page-setting software, final improvement of an image, vectorial drawing and are used on multiple peripherals (printers, systems of épreuvage, presses, etc.). Use of inserted or imbricated profiles, allows an essential traceability of the colorimetric properties of the original file. The transfer of file from one application to peripheral of another application is currently facilitated by the inserted profile. The Silverfast software, pilot of the scanners Lynx, Pro 42 and Pro 48 of Qubyx, have the property to bind a ICC profile to the digitized file. The overlap of profile effectuate files whose formats are as follows: PICT, EPS, Tiff, JFIF and GIF.
The original file can thus, via an inserted profile, keep its initial properties, and re-use later on in its rough form.
Here an example of overlapping ICC Profile via the Silverfast software, pilot of the Qubyx scanners.



Figure 10: Example of insertion of ICC Profile under Silverfast AI
- PART I - CHAPTER VI - ICC PROFILES -
 
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