Color management

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Color management is a term used in computer environments which describes a controlled conversion between the colors of various color devices, such as scanners, digital cameras, monitors, TV screens, film printers, printers, offset presses, and corresponding media.

The primary goal of color management is to obtain a good match across color devices. For instance, imagine a video which should look the same on a computer LCD monitor, a plasma TV screen, and on a printed frame of video. Color management helps to achieve the same appearance on all of these devices, provided the device is capable of delivering the needed color intensities.

Contents

[edit] Concepts

Various components formed by different concepts are part of a color management system:

[edit] Characterization

In order to describe the behavior of the various output devices they must be compared (calibrated) in relation to a standard color space. Often a step called linearization is performed first, in order to get most out of limited 8-bit color paths. Instruments used for measuring device colors include colorimeters and spectrophotometers. As an intermediate result the device gamut is described in the scattered measurement data. The measurement data (CGATS) are often not usable immediately. Such data needs to be prepared for high speed conversions of the actual image data. The transformation of the scattered measurement data into a more regular form, usable by the application, is called profiling. Profiling is a complex mathematical process. After the profiling is finished, an idealized color description of the device is created. This description is called a profile.

[edit] Flexibility

Transforming profiled color information to different output devices is achieved by referencing the profile data into a standard color space. It is easy to convert colors from one device to a selected standard, and from that color space to the colors of another device. By ensuring that the reference color space covers the many possible colors that humans can see, this concept allows one to exchange colors between many different color output devices.

[edit] Communicating diversity

Image formats themselves (such as JPEG and TIFF) may contain embedded color profiles, but are not required to do so by the image format. The International Color Consortium standard was created to bring various developers and manufacturers together. The ICC standard permits the exchange of output device characteristics and color spaces in the form of metadata. This allows the embedding of color profiles into images as well as storing them in a database or a profile directory.

[edit] Color space consistency

Editing spaces (better known as working spaces) are a valuable concept that facilitates good results while compositing and manipulating images. They behave differently from most output device color spaces. They must be consistent regarding gray editing and color arrangement. For instance after brightening an image a gray should not become greenish. Most often such editing color spaces are described in simple mathematical formulas, such as sRGB or Adobe RGB.

[edit] Gamut mapping

Since different devices don't have the same gamut, they need some rearrangement near the borders of the gamut. Some colors need to be shifted to the inside of the gamut as they otherwise cannot be represented on the output device and would simply be clipped. For instance to print a mostly saturated blue from a monitor to paper with a typical CMYK printer will surely fail. The paper blue will not be that saturated. Conversely, the bright cyan of an inkjet printer cannot be easily presented on an average computer monitor. The color management system can utilize various methods to achieve pleasant results and give experienced users control of the gamut mapping behavior.

[edit] Implementations

Parts of this technology are implemented in the operating system (OS), helper libraries, the application, and devices. The concept for color management in Microsoft Windows and Microsoft Office products is that every driver for an input device makes a color transformation from the color space of the device to sRGB. For the output device or the monitor, the driver has to make a color transformation from sRGB to the color space of the output device. This kind of implementation of color management is very user-friendly, because there is no need for any configuration. However, the quality of the results depends entirely on the quality of the color transformations, which are part of the drivers.

A more open concept, platform independent view of color management is the use of an ICC-compatible color management system. The International Color Consortium (ICC) is an industry consortium which has defined an open standard for a Color Matching Module (CMM) at the OS level, and color profiles (ICC profiles) for the devices and working space. Another ICC concept is to make color profiles a part of file formats like TIFF, JPEG, PNG, EPS, PDF, and SVG.

Besides ICC and WWW style color management, a lot of other color management approaches exist, partly due to history and partly because of other needs than the ICC standard covers. Particularly in the printing industry, in broadcasting and in film studios there are diverging demands and solutions.

Color matching method is a software algorithm that adjusts the numerical values that get sent to, or received from, different devices so that the perceived color they produce remains consistent. The key issue here is how to deal with a color that cannot be reproduced on a certain device in order to show it through a different device as if it were visually the same color, just as when the reproducible color range between color transparencies and printed matters are different. There is no common method for this process, and the performance depends on the capability of each color matching method.

[edit] Criticism

Newer output devices can display more saturated colors that sRGB cannot represent.

A clear implementation of color management in the user interface of the OS would have the selection of profiles for monitors and for the working space in system configuration, and the selection of device profiles in the scanner or printer driver. Analyzing the actual implementation of color management at the level of the OS, the drivers, and the applications shows that there is substantial confusion about color management amongst both the software architects for operating systems and the software developers of drivers and applications.

[edit] Problem list

  • The software architects of the operating system often have no clear concept of how to implement color management at the OS level, and commonly enough there are no guidelines for the developers of applications and drivers on how to use the color management possibilities of the OS.
    • There is no option to choose either an editing (working) space or a monitor profile in the graphics architecture and at operating-system level. On Mac OS this is possible, but most applications ignore it.
    • The OS architecture for printer drivers doesn't allow the configuration of profiles for an editing (working) space and a device space.
  • Some important older file formats like EPS are of limited use for ICC color management on the host computer. For color matching of EPS files, a PostScript RIP is necessary.
  • Color management for the printer is sometimes implemented at several levels at the same time, such as in the application, at the OS driver level, and in the PostScript RIP. This can cause double or triple color transformations, with incorrect results.
  • ICC profiles are mostly fixed entities, with only a few changeable parameters.

[edit] References (books)

[edit] External links

More information about color management for software developers can be found here:

[edit] Standards

[edit] Operating systems / implementations

[edit] Science

[edit] External links

  • monitorsetup.com - Free website for checking the monitor calibration and the color management capabilities of web browsers

[edit] Companies

[edit] Blog

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