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Color Management - Introduction, ICC architecture, ICC profile, Profile Connection Space (PCS), Module (CMM)

device viewing colorimetric colors

SABINESSTRUNK, Ph.D.
Ecole Polytechnique Fédérale de Lausanne

Introduction

Color management is a term that describes the standards, tools, and applications used in color reproduction to insure that the appearance of color image is what the user intended. As a direct result of the different physical characteristics of input and output devices and media, as well as the viewing conditions under which we capture or view color data, certain colors may not be captured or reproduced correctly by certain devices in various applications. The same document, for example, may look different when printed on different printers, viewed on different monitors, printed on a printer and viewed on a monitor, or viewed in a light booth and under office lighting. Devices, drivers, operating systems, and applications also can all interpret and reproduce colors differently. Color management takes into account these issues by providing transformations from source to destination, taking into consideration the color characteristics of both devices as well as the viewing conditions.

The International Color Consortium (ICC) is an industrial consortium that has standardized most of the color management architecture applied in color reproduction today. The ICC defines color management as the “communication of the associated data required for unambiguous interpretation of color content data, and application of color data conversions, as required, to produce the intended reproductions.” 5. Color content includes pictorial images, but can also be text, line art, and graphics. In this section, the discussion will be limited to ICC color management. Note that the ICC specifications are still evolving, and the specifications [ICC04] and the ICC website (www.color.org) are available for more information.

ICC color management architecture

It is always possible to design a color transform that maps the digital code values from one device to another, based on some color-rendering criterion and specific viewing conditions. However, such a system would not be very general: For each new device, rendering intent, and viewing condition a new transform would have to be designed. The ICC color management architecture thus provides a way to map from a device-specific color space to a standard color space, and vice versa. The transforms from device to standard color space are embedded in the ICC profile. The standard color space is called profile connection space (PCS).

ICC profile

The ICC profile contains the transforms from “device” to PCS. There are several kinds of profiles specified: input device (scanner, digital camera, etc.), output device (printers, film recorders, etc.), and display (CRTs, LCDs, projectors, etc.), but also device link (dedicated device-to-device), color image encoding (sRGB, CIE XYZ, L*a*b *, etc.), abstract (effects, PCS-to-PCS, etc.), and named color (Pantone, Truematch, etc.). The profile architecture is thus not restricted to actual physical devices, but can contain any kind of color transform from source to destination.

The color transformation is implemented as a series of lookup-tables (LUTs) and matrices, depending on the complexity of the transform, the PCS color space, and the number of colorants of the device. A profile may also contain more than one color transformation to accommodate different rendering intents. A rendering intent describes a different mapping of color values that may be useful for various imaging workflows. The colorimetric intents preserve the colorimetry of in-gamut colors at the expense of out-of-gamut colors. The mapping of the out-of-gamut colors is vendor-specific. The perceptual and saturation rendering intents modify colorimetric values to account for different devices, media, and viewing conditions.

The media-relative colorimetric intent maps the white-point of the actual medium to the white-point of the PCS illuminant and rescales the in-gamut color accordingly. The ICC-absolute colorimetric intent applies a chromatic adaptation to map the white-point of the illuminant to the PCS illuminant, but then does not change the (chromatically adapted) in-gamut colors. The media-relative colorimetric transform is useful for colors that have already been mapped to the intended reproduction colorimetry, whereas the ICC-absolute colorimetric transform is useful for spot colors and when simulating one medium on another in proofing applications. Both of these transforms are based on actual colorimetric measurements of an input or output device under specific viewing conditions. The saturation intent is used to render graphics and text. The transforms are vendor-specific, and usually involve tradeoffs for hue to maintain maximum saturation. The perceptual intent is used for general reproduction of images, particularly pictorial or photographic-type images. The goal here is to obtain best appearance on a given output, and not a colorimetric match. The PCS value thus represents the appearance of an image, viewed in the reference viewing environment by a human observer adapted to that environment.

Profile Connection Space (PCS)

The PCS color space is either CIELAB or CIE XYZ under illuminant D50, encoded as either 8 bits or 16 bits per channel for CIE Lab or 16-bit for CIE XYZ. To accommodate the perceptual intent, a reference medium and viewing conditions are also defined. The reference medium is a hypothetical print on a substrate with a white having a neutral reflectance of 89 percent and a density range of 2,4593. The viewing reference corresponds to a graphics arts and photography print comparison environment using a D50 illuminant at an illumination level of 500 lux.

Color Management Module (CMM)

The ICC profile is a file that can be embedded in a color document. The following file formats support the inclusion of an ICC profile: TIFF, EPS, PICT, JFIF, JP2, EXIF, and GIF, as well as proprietary formats. An ICC profile can also reside separately. The common extension for an ICC profile is .icc in the Apple OS and .icm in Windows OS.

The conversion of a document’s color data using ICC transforms is done in a color management module (CMM). A CMS can either be a standalone application or be embedded in an application (such as Adobe Photoshop), a device driver, or the operating system (Apple Color-Sync). According to source, destination, and rendering intent, a CMM converts the color data of a document by concatenating the transforms given by the source and destination profiles.

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