Color management
Encyclopedia
In digital imaging systems, color management is the controlled conversion between the color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

 representations of various devices, such as image scanner
Image scanner
In computing, an image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...

s, digital camera
Digital camera
A digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor. It is the main device used in the field of digital photography...

s, monitors, TV screens, film printers, computer printer
Computer printer
In computing, a printer is a peripheral which produces a text or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable or, in most new printers, a...

s, offset presses, and corresponding media.

The primary goal of color management is to obtain a good match across color devices; for example, the colors of one frame of a video should appear the same on a computer LCD monitor, on a plasma TV screen, and as a printed poster. Color management helps to achieve the same appearance on all of these devices, provided the devices are capable of delivering the needed color intensities.

Parts of this technology are implemented in the operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 (OS), helper libraries, the application, and devices. A cross-platform view of color management is the use of an ICC-compatible color management system. The International Color Consortium
International Color Consortium
The International Color Consortium was formed in 1993 by eight industry vendors in order to create a universal color management system that would function transparently across all operating systems and software packages....

 (ICC) is an industry consortium which has defined:
  • an open standard for a Color Matching Module (CMM) at the OS level.
  • color profiles
    ICC profile
    In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium...

     for:
    • devices; this includes devicelink-profiles representing a complete color transformation from source device to target device.
    • working spaces, the color spaces in which color data is meant to be manipulated.


There are other approaches to color management besides using ICC profile
ICC profile
In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium...

s. This is partly due to history and partly because of other needs than the ICC standard covers. The film and broadcasting industries make use of many of the same concepts, but they more frequently rely on boutique solutions. The film industry, for instance, often uses 3D LUT
3D LUT
In the film industry, 3D LUTs are used to calculate preview colors for a monitor or digital projector of how an image will be reproduced on the final film print. A 3D LUT is a 3D lattice of output color values. Each axis is one of the 3 input color components and the input color thus defines a...

s (lookup table
Lookup table
In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than...

) to represent a complete color transformation. At the consumer level, color management currently applies more to still images than video, in which color management is still in its infancy.

Characterization

In order to describe the behavior of the various output devices, they must be compared (measured) in relation to a standard color space
Color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...

. Often a step called linearization is performed first, in order to undo the effect of gamma correction
Gamma correction
Gamma correction, gamma nonlinearity, gamma encoding, or often simply gamma, is the name of a nonlinear operation used to code and decode luminance or tristimulus values in video or still image systems...

 that was done to get the most out of limited 8-bit color
8-bit color
8-bit color graphics is a method of storing image information in a computer's memory or in an image file, such that each pixel is represented by one 8-bit byte. The maximum number of colors that can be displayed at any one time is 256....

 paths. Instruments used for measuring device colors include colorimeter
Colorimeter
For articles on Colorimeter see:* Colorimeter * Tristimulus colorimeter...

s and spectrophotometers. As an intermediate result, the device gamut
Gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...

 is described in the form of scattered measurement data. The transformation of the scattered measurement data into a more regular form, usable by the application, is called profiling. Profiling is a complex process involving mathematics, intense computation, judgment, testing, and iteration. After the profiling is finished, an idealized color description of the device is created. This description is called a profile.

Calibration

Calibration is like characterization, except that it can include the adjustment of the device, as opposed to just the measurement of the device. Color management is sometimes sidestepped by calibrating devices to a common standard color space such as sRGB; when such calibration is done well enough, no color translations are needed to get all devices to handle colors consistently. This avoidance of the complexity of color management was one of the goals in the development of sRGB.

Embedding

Image formats themselves (such as TIFF, JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

, PNG, EPS
Encapsulated PostScript
Encapsulated PostScript, or EPS, is a DSC-conforming PostScript document with additional restrictions which is intended to be usable as a graphics file format...

, PDF
Portable Document Format
Portable Document Format is an open standard for document exchange. This file format, created by Adobe Systems in 1993, is used for representing documents in a manner independent of application software, hardware, and operating systems....

, and SVG
Scalable Vector Graphics
Scalable Vector Graphics is a family of specifications of an XML-based file format for describing two-dimensional vector graphics, both static and dynamic . The SVG specification is an open standard that has been under development by the World Wide Web Consortium since 1999.SVG images and their...

) may contain embedded color profiles
ICC profile
In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium...

 but are not required to do so by the image format. The International Color Consortium
International Color Consortium
The International Color Consortium was formed in 1993 by eight industry vendors in order to create a universal color management system that would function transparently across all operating systems and software packages....

 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
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...

. This allows the embedding of color profiles into images as well as storing them in a database or a profile directory.

Working spaces

Working spaces, such as sRGB, Adobe RGB
Adobe RGB color space
The Adobe RGB color space is an RGB color space developed by Adobe Systems in 1998. It was designed to encompass most of the colors achievable on CMYK color printers, but by using RGB primary colors on a device such as the computer display...

 or ProPhoto are color spaces that facilitate good results while editing. For instance, pixels with equal values of R,G,B should appear neutral. Using a large (gamut) working space will lead to posterization
Posterization
Posterization of an image entails conversion of a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another. This was originally done with photographic processes to create posters...

, while using a small working space will lead to clipping
Clipping (photography)
In digital photography and digital video, clipping is a result of capturing or processing an image where the intensity in a certain area falls outside the minimum and maximum intensity which can be represented. It is an instance of signal clipping in the image domain...

. This trade-off is a consideration for the critical image editor.

Color transformation

Color transformation, or color space conversion, is the transformation of the representation of a color from one color space
Color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...

 to another. This calculation is required whenever data is exchanged inside a color-managed chain. 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. Color transformations can be represented by two profiles (source profile and target profile) or by a devicelink profile.

Profile connection space

In the terminology of the International Color Consortium
International Color Consortium
The International Color Consortium was formed in 1993 by eight industry vendors in order to create a universal color management system that would function transparently across all operating systems and software packages....

, a translation between two color spaces can go through a profile connection space (PCS): Color Space 1 → PCS (CIELAB or CIEXYZ) → Color space 2; conversions into and out of the PCS are each specified by a profile.

Gamut mapping

Since different devices don't have the same gamut
Gamut
In color reproduction, including computer graphics and photography, the gamut, or color gamut , is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a...

, 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 the dark highly saturated purplish-blue color of a typical computer monitor’s “blue” primary is impossible to print on paper with a typical CMYK printer. The nearest approximation within the printer’s gamut will be much less saturated. Conversely, an inkjet printer’s “cyan” primary, a saturated mid-brightness greenish-blue, is outside the gamut of a typical computer monitor. The color management system can utilize various methods to achieve desired results and give experienced users control of the gamut mapping behavior.

Rendering intent

When the gamut of source color space exceeds that of the destination, saturated colors are liable to become clipped
Clipping (photography)
In digital photography and digital video, clipping is a result of capturing or processing an image where the intensity in a certain area falls outside the minimum and maximum intensity which can be represented. It is an instance of signal clipping in the image domain...

 (inaccurately represented), or more formally burned
Burned (image)
An image is said to be burned when its original gamut considerably exceeds the target gamut, or when the result of processing considerably exceeds the image's gamut, resulting in clipping...

. The color management module can deal with this problem in several ways. The ICC specification includes four different rendering intents: absolute colorimetric, relative colorimetric, perceptual, and saturation.

Absolute colorimetric :
Absolute colorimetry and relative colorimetry actually use the same table
but differ in the adjustment for the white point media. If the output device
has a much larger gamut than the source profile, i.e., all the colors in the
source can be represented in the output, using the absolute colorimetry rendering intent would
"ideally" (ignoring noise, precision, etc.) give an exact output of the specified
CIELAB values. Perceptually, the colors may appear incorrect, but instrument measurements of the resulting output would match the source. Colors outside of the proof print system's possible color are mapped to the boundary of the color gamut.
Absolute colorimetry is useful to get an exact specified color (e.g., IBM blue), or to quantify the accuracy of mapping methods.

Relative colorimetric :
The goal in relative colorimetry is to be truthful to the specified color,
with only a correction for the media. Relative colorimetry is useful in
proofing applications, since you are using it to get an idea of how a print
on one device will appear on a different device. Media differences are the
only thing you really would like to adjust for. Obviously there has to be
some gamut mapping going on also. Usually this is done in a way where hue
and lightness are maintained at the cost of reduced saturation. Relative colorimetric is the default rendering intent on most systems.

Perceptual and Saturation :
The perceptual and saturation intents are where the results really depend
upon the profile maker. This is even how some of the competitors in this
market differentiate themselves. These intents should be created by the
profile maker so that pleasing images occur with the perceptual intent while
eye-catching business graphics occur with the saturation intent. This is
achieved through the use of different perceptual remaps of the data as well
as different gamut mapping methods. Perceptual rendering is recommended for color separation.

In practice, photographers almost always use relative or perceptual intent, as for natural images, absolute causes color cast, while saturation produces unnatural colors. Relative intent handles out-of-gamut by clipping (burning) these colors to the edge of the gamut, leaving in-gamut colors unchanged, while perceptual intent smoothly moves out-of-gamut colors into gamut, preserving gradations, but distorts in-gamut colors in the process. If an entire image is in-gamut, relative is perfect, but when there are out of gamut colors, which is more preferable depends on a case-by-case basis.

Saturation intent is most useful in charts and diagrams, where there is a discrete palette of colors which one wishes to have saturated (to "pop"), but where the specific hue is less important.

Color management module

Color matching module (also -method or -system) 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.

Some well known CMMs are ColorSync
ColorSync
ColorSync is Apple Inc's color management API for the Mac OS and Mac OS X.-Version history:Apple developed the original 1.0 version of ColorSync as a Mac-only architecture, which made it into an operating system release in 1993. In the same year, Apple co-founded the International Color Consortium...

, Adobe CMM, LittleCMS
LittleCMS
LittleCMS or LCMS is an open source color management system, released as a software library for use in other programs which will allow the use of International Color Consortium profiles. It is licenced under the MIT License Agreement....

, and ArgyllCMS.

Operating system level

Apple's Mac operating systems
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface...

 have provided OS-level color management since 1993, through ColorSync
ColorSync
ColorSync is Apple Inc's color management API for the Mac OS and Mac OS X.-Version history:Apple developed the original 1.0 version of ColorSync as a Mac-only architecture, which made it into an operating system release in 1993. In the same year, Apple co-founded the International Color Consortium...

.

Since 1997 color management in Windows is handled at the OS level through an ICC color management system. Beginning with Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

, Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 introduced a new color architecture known as Windows Color System
Windows Color System
Windows Color System is a platform for color management first included with Windows Vista that aims to achieve color consistency across various software and hardware, including cameras, monitors, printers and scanners. Different devices interpret the same colors differently, according to their...

. WCS supplements the Image Color Management (ICM) system in Windows 2000 and Windows XP, originally written by Heidelberg
Heidelberger Druckmaschinen
Heidelberger Druckmaschinen AG is a German precision mechanical engineering company with head offices in Heidelberg . It is a manufacturer of offset printing presses sold globally. The company has a worldwide market share of more than 47% in this area and is the largest global manufacturer of...

.

Operating systems which use the X Window System
X Window System
The X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...

 for graphics use ICC profile
ICC profile
In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium...

s, and support for color management on Linux
Linux color management
Linux color management has the same goal as the color management systems for other operating systems: to achieve the best possible color reproduction throughout an imaging workflow from its source , through imaging software , and finally onto an output medium...

, still less mature than on other platforms, is coordinated through OpenICC at freedesktop.org
Freedesktop.org
freedesktop.org is a project to work on interoperability and shared base technology for free software desktop environments for the X Window System on Linux and other Unix-like operating systems. It was founded by Havoc Pennington from Red Hat in March 2000.The organisation focuses on the user....

 and makes use of LittleCMS
LittleCMS
LittleCMS or LCMS is an open source color management system, released as a software library for use in other programs which will allow the use of International Color Consortium profiles. It is licenced under the MIT License Agreement....

.

Application level

As of 2005, most web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...

s ignored color profiles. Notable exceptions were Safari
Safari (web browser)
Safari is a web browser developed by Apple Inc. and included with the Mac OS X and iOS operating systems. First released as a public beta on January 7, 2003 on the company's Mac OS X operating system, it became Apple's default browser beginning with Mac OS X v10.3 "Panther". Safari is also the...

, starting with version 2.0, and Firefox
Mozilla Firefox
Mozilla Firefox is a free and open source web browser descended from the Mozilla Application Suite and managed by Mozilla Corporation. , Firefox is the second most widely used browser, with approximately 25% of worldwide usage share of web browsers...

 starting with version 3. Although disabled by default in Firefox 3.0, users can enable ICC v2 and ICC v4 color management by using an add-on or setting a configuration option. Starting from Firefox 3.5 color management is enabled by default only for tagged images, although support is limited to ICC v2 profiles owing to a change in color management systems from 3.0. Firefox 8.0 has partial ICC v4 profiles support. Internet Explorer 9 is the first Microsoft browser to partly support ICC profiles, but it does not render images correctly according to the Windows ICC settings (it only converts non-sRGB images to the sRGB profile) and therefore provides no real color management at all. As of 2011, Google Chrome
Google Chrome
Google Chrome is a web browser developed by Google that uses the WebKit layout engine. It was first released as a beta version for Microsoft Windows on September 2, 2008, and the public stable release was on December 11, 2008. The name is derived from the graphical user interface frame, or...

 does not support color management by default.

See also

  • Color chart
    Color chart
    In color-related fields, a color chart is a flat, physical object colored with an arrangement of standardized color samples, used for color comparisons and measurements such as checking the color reproduction of an imaging system...

  • Digital printing
    Digital printing
    Digital printing refers to methods of printing from a digital based image directly to a variety of media. It usually refers to professional printing where small run jobs from desktop publishing and other digital sources are printed using large format and/or high volume laser or inkjet printers...

  • International Color Consortium
    International Color Consortium
    The International Color Consortium was formed in 1993 by eight industry vendors in order to create a universal color management system that would function transparently across all operating systems and software packages....

  • IT8
    IT8
    IT8 is a set of American National Standards Institute standards for color communications and control specifications. Formerly governed by the IT8 Committee, IT8 activities were merged with those of the Committee for Graphics Arts Technologies Standards in 1994.-Standards List:The following is a...

  • Linux color management
    Linux color management
    Linux color management has the same goal as the color management systems for other operating systems: to achieve the best possible color reproduction throughout an imaging workflow from its source , through imaging software , and finally onto an output medium...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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