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A color management module (CMM) is a component that implements color-matching, color conversion, and gamut-checking services. A CMM uses profiles to convert and match a color in a given color space on a given device to or from another color space or device.
Each profile header includes a field that specifies a CMM to use for performing color matching involving that profile. If two profiles in a color-matching session specify different CMMs, or if a specified CMM is unavailable or unable to perform a requested function, the ColorSync Manager follows an algorithm, described in How the ColorSync Manager Selects a CMM , to determine which CMM to use.
ColorSync ships with a robust CMM that is installed as part of the ColorSync extension. This CMM supports all the required and optional functions defined by the ColorSync Manager, so it can always be used as a default CMM when another CMM is unavailable or unable to perform an operation.
The ColorSync Manager includes color conversion functions that allow your application or driver to convert colors between color spaces belonging to the same base families without the use of CMMs; CMMs themselves can also call these color conversion functions. However, color conversion and color matching across color spaces belonging to different base families always entail the use of a CMM.
When colors from one device's gamut are displayed on a device with a different gamut, as shown in Figure 2-7 , ColorSync can minimize the perceived differences in the displayed colors between the two devices by mapping the out-of-gamut colors into the range of colors that can by produced by the destination device.
A CMM uses lookup tables and algorithms for color matching, previewing color reproduction capabilities of one device on another, and checking for out-of-gamut colors (colors that cannot be reproduced). Although ColorSync provides a default CMM, device manufacturers and peripheral developers can create their own CMMs, tailored to the specific requirements of their device. For information on creating CMMs, see Developing Color Management Modules and ColorSync Reference for Color Management Modules . ColorSync also provides the Kodak CMM as a "custom install" feature, for users who wish to work with that CMM.
Starting with version 2.5, the ColorSync control panel, shown in Figure 1 , lets you choose a preferred CMM from any CMMs that are present (that is, registered with the Component Manager--see Creating a Component Resource for a CMM for a description of how to create a CMM the ColorSync Manager can use).
If you choose a preferred CMM with the ColorSync control panel, and if that CMM is available, ColorSync will attempt to use that CMM for all color conversion and matching operations. If you specify "Automatic" instead, or if the specified CMM is no longer present or cannot provide the required matching service, or for versions prior to version 2.5, ColorSync follows an algorithm described in How the ColorSync Manager Selects a CMM to determine which available CMM to use for matching.
Note that if you want color conversion and matching operations to use the same CMM-selection algorithm they did in versions prior to ColorSync 2.5, specify "Automatic" in the ColorSync control panel.
Starting with ColorSync 2.5, your application can determine the preferred CMM by calling the function
CMGetPreferredCMM
.
Rendering intent refers to the approach taken when a CMM maps or translates the colors of an image to the color gamut of a destination device--that is, a rendering intent specifies a gamut-matching strategy. The ICC specification defines a profile tag for each of four rendering intents: perceptual matching , relative colorimetric matching , saturation matching , and absolute colorimetric matching . These rendering intents are described in Table 2-1 .
perceptual matching | All the colors of one gamut are scaled to fit within another gamut. Colors maintain their relative positions. Usually produces better results than colorimetric matching for realistic images such as scanned photographs. The eye can compensate for gamuts differences, such as in Figure 2-7 , and when printed on a CMYK device, the image may look similar to the original on an RGB device. A side effect is that most of the colors of the original space may be altered to fit in the new space. | photographic |
relative colorimetric matching | Colors that fall within the overlapping gamuts of both devices are left unchanged. For example, to match an image from the RGB gamut onto the CMYK printer gamut in Figure 2-7 , only the colors in the RGB gamut that fall outside the printer gamut are altered. Allows some colors in both images to be exactly the same, which is useful when colors must match quantitatively. A disadvantage is that many colors may map to a single color, resulting in tone compression. All colors outside the printer gamut, for example, would be converted to colors at the edge of its gamut, reducing the number of colors in the image and possibly altering its appearance. Colors outside the gamut are usually converted to colors with the same lightness, but different saturation, at the edge of the gamut. The final may be lighter or darker overall than the original image, but the blank areas will coincide. | spot colors |
saturation matching | The relative saturation of colors is maintained as well as can be achieved from gamut to gamut. Colors outside the gamut of the "to" space are usually converted to colors with the same saturation of the "from" space, but with different lightness, at the edge of the gamut. Can be useful for some graphic images, such as bar graphs and pie charts, when the actual color displayed is less important than its vividness. | business graphics |
absolute colorimetric matching | Preserves native device white point of source image instead of mapping to D50 relative. Most often using in simulation or proofing operations where a device is trying to simulate the behavior of another device and media. For example, simulating newsprint on a monitor with absolute colorimetric intent would allow white space to be displayed onscreen as yellowish background because of the differences in white points between the two devices. | no typical content (most often used in proofing) |
Color professionals and technically-sophisticated users are likely to be familiar with the ICC terms for rendering intent and the gamut-matching strategies they represent. If your application is aimed at novice users, however, you may prefer to add a simplified terminology based on the typical image content associated with a rendering intent (for example, perceptual matching is commonly used for photographic images). Table 2-1 lists the ICC-specified rendering intents and the corresponding image content term, where applicable. Note that there is no simplified terminology for describing absolute colorimetric matching. However, novice users are not likely to need this rendering intent.
For information about the actual rendering intent tags, you can obtain the latest ICC profile specification by visiting either the ICC Web site at http://www.color.org or the ColorSync Web site at http://colorsync.apple.com .