In digital imaging systems,
color management is the controlled conversion between the
colorColor or colour is the visual perceptual property corresponding in humans to the categories called red, yellow, 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 scannerIn computing, a 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 window for scanning...
s,
digital cameraA digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor....
s, monitors, TV screens, film printers,
computer printerIn computing, a printer is a peripheral which produces a hard copy 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 newer printers, a USB...
s, offset presses, and corresponding media.
The primary goal of color management is to obtain a good match across color devices; for example, a video which should appear the same color 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 devices are capable of delivering the needed color intensities.
Parts of this technology are implemented in the
operating systemAn operating system is an interface between hardware and user which is responsible for the management and coordination of activities and the sharing of the resources of the computer that acts as a host for computing applications run on the machine. As a host, one of the purposes of an operating...
(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 ConsortiumThe 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, and
color profilesIn 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 the devices and
working space (color space the user edits in).
There are other approaches to color management besides using
ICC profileIn 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, uses
3D LUTIn 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 tableIn 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 characterize color. At the consumer level, color management currently applies more to 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 spaceA 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 correctionGamma 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 color8-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
colorimeterFor articles on Colorimeter see:* Colorimeter * Tristimulus colorimeter...
s and spectrophotometers. As an intermediate result, the device
gamutIn 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,
JPEGIn computing, JPEG is a commonly used method of compression for photographic images. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality...
, PNG,
EPSEncapsulated PostScript, or EPS, is a DSC-conforming PostScript document with additional restrictions intended to make EPS files usable as a graphics file format...
,
PDFPortable Document Format is a file format created by Adobe Systems in 1993 for document exchange. PDF is used for representing two-dimensional documents in a manner independent of the application software, hardware, and operating system....
, and
SVGScalable Vector Graphics is a family of specifications of an XML-based file format for describing two-dimensional vector graphics, both static and dynamic Scalable Vector Graphics (SVG) is a family of specifications of an XML-based file format for describing two-dimensional vector graphics, both...
) may contain embedded
color profilesIn 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 ConsortiumThe 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
metadataMetadata is "data about data", of any sort in any media. Metadata is text, voice, or image that describes what the audience wants or needs to see or experience. The audience could be a person, group, or software program...
. 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 RGBThe 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
posterizationPosterization 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
clippingIn 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 translation
Color translation, or color space conversion, is the translation of the representation of a color from one
color spaceA 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.
Profile Connection Space
In the terminology of the
International Color ConsortiumThe 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
gamutIn 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 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 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
clippedIn 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). 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
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
ColorSyncColorSync 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,
LittleCMSLittleCMS or LCMS is an open source colour management system, released as a software library for use in other programs. It is licenced under the MIT License Agreement....
, and
ArgyllCMS.
Operating system level
Beginning with
Windows VistaWindows Vista is a line of operating systems developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
, color management in Windows is handled at the OS level through an ICC V4-compatible color management standard and API known as
Windows Color SystemWindows 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
HeidelbergHeidelberger 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...
.
Apple's
Mac operating systemsMac OS is the trademarked name for 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
ColorSyncColorSync 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...
.
Operating systems which use the
X Window SystemThe X Window System is a computer software system and network protocol that provides a graphical user interface for networked computers, and was initially developed as part of Project Athena...
for graphics use
ICC profileIn 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, but support for
color management on LinuxLinux 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 , onto imaging software and finally its output...
is still quite nascent, with only a handful of applications supporting, some through
LittleCMSLittleCMS or LCMS is an open source colour management system, released as a software library for use in other programs. It is licenced under the MIT License Agreement....
.
Application level
Most
web browserA 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 ignore color profiles. Notable exceptions are
SafariSafari is a web browser developed by Apple Inc. First released as a public beta on 7 January 2003 on the company's Mac OS X operating system, it became Apple's default browser beginning with Mac OS X v10.3 "Panther". Apple has also made Safari the native browser for the iPhone OS...
, starting with version 2.0, and
FirefoxMozilla Firefox is a free and open source web browser descended from the Mozilla Application Suite and managed by Mozilla Corporation. Firefox has 23.75% of the recorded usage share of web browsers , making it the second most popular browser in terms of current use worldwide after Microsoft's...
starting with version 3.0. Although disabled by default in Firefox 3.0, users can enable color management by using an add-on or setting the value "gfx.color_management.enabled" to "true" in Firefox 3's "
about:configabout: is an internal URI scheme in various web browsers to display certain built-in functions...
" file.. The current Beta version of Firefox 3.1 (Minefield 3.1b1pre) has color management enabled by default only for tagged images.
FastPictureViewerFastPictureViewer is a commercial 32 and 64-bit image viewer for Windows 7, Vista and XP. Its aim is to facilitate quick review and rating of large quantities of digital images in the early steps of the digital workflow, with an emphasis on simplicity and speed.As of v1.0, a basic version is...
, a commercial image viewer for Windows, features full color management support (monitor profile and image profiles).
External links