Color Management Fundamentals

There are a few basic concepts to understand when working with color and color management systems. These will help you understand why color discrepancies exist and how color management works.

Device Gamut

Every color device is capable of reproducing a range of colors, better known as its gamut. Each device, such as a display or printer, has a unique gamut that is dictated by the characteristics of that device the types of inks it uses to print, the type of technology a scanner uses to capture images, and so on. When the gamuts of two devices don't overlap, the colors that do not overlap can't be reproduced consistently across the devices. It's as if the devices speak different languages. Even devices of the same type (say, two color printers) may have different gamuts or language dialects. The colors seen on a display are typically much brighter and more saturated than the color that comes out of a printer. The reason for this is that the color on a display is in gamut for the display, but it is not in gamut for the printer. The printer simply cannot reproduce all of the colors contained in the image. In this case, the color is said to be device dependent whether the desired color is produced depends on the capabilities of a particular device.

Note

If you haven't copied the chapter files from this book's Web site onto your hard drive, including the sample profile D65 Monitor Profile.icc, do so now before continuing. For instructions, see "Getting Started" at the beginning of the book.

To see this, let's use ColorThink to compare the gamuts of a display and color printer.

1.	Launch Chromix ColorThink and, from the main menu, choose Graph > Open 3D Graph. Tip The next series of exercises use Chromix ColorThink, an excellent color management utility, to illustrate color management concepts. A trial version of ColorThink is available from www.chromix.com. Figure 1.8. The Grapher component of ColorThink appears. Figure 1.9.
2.	In the Plot Items window on the right, click the drop-down arrow beside Add to access the Add menu; then select Open. Figure 1.10.
3.	In the Open dialog box, navigate to the C:\windows\system32\spool\drivers\color directory, where Windows XP stores color profiles for your devices, and select the profile D65 Monitor Profile.icc from the list. Tip If you have a profile for your display, select it instead of D65 Monitor Profile.icc for this exercise. Color profiles contain information about your devices, which enables color management systems to understand their capabilities. Figure 1.11. \up8 ColorThink's Grapher displays a 3D visual representation of the selected profile's gamut. The colors of the spectrum are mapped to the Lab color space to illustrate the range of colors that can be reproduced. Figure 1.12.
4.	Drag the 3D color model to view this profile's color space from multiple angles and then release the mouse. You can also zoom in or out by pressing and holding the Alt key while dragging the mouse up and down.
5.	In the Plot Items window, click the drop-down arrow beside the Add button again, and select the profile for your printer. In Figure 1.13, I selected JW E2200 PremLstr 1_27_04.icc, which is the custom profile for my Epson Stylus 2200 with Premium Luster paper. Figure 1.13.
6.	Select the printer profile by single clicking it in the Plot List. On the Color tab, change the option from True Color to Single Color, and select Red from the drop-down list. Figure 1.14. This procedure will display the gamut of the printer in red, making it easier to distinguish from the monitor gamut.
7.	Drag the graph to see the different gamuts of the devices. The following images show two different views of the graph. Figure 1.15. Figure 1.16. Colors contained in the overlapping areas can be matched between the printer and the monitor. Colors that fall outside of the overlapping areas will not be matched.
8.	To get an idea of the actual colors that are in gamut for one device but not the other, on the Color tab of the Plot Item window, select the True Color radio button. This will change the representation of the printer's gamut from solid red to the actual colors. Figure 1.17. This exercise illustrates why color matching is generally not possible, and why the colors you see on your display often do not match the colors you see on your printer.

Color Spaces

Color scientists use various mathematical models to represent color, and these models can be visualized, as demonstrated in the previous exercise. For the purposes of color management, models that represent the visible spectrum are used, as they easily contain all of the colors that an imaging device can capture or reproduce. The gamuts of desktop devices, such as displays and printers, are relatively small when compared with the visible spectrum of light.

These are color spaces where the definition of a color is not dependent on any particular device they are said to be device independent. In 1931, the CIE (Commission Internationale de l'Eclairage) established standards for a series of color spaces that represented the visible spectrum 60 years before the arrival of desktop color! The CIE color spaces form the foundation of device-independent color for color management. Many of these spaces, such as CIE XYZ and CIE Lab, are widely used in color management systems today.

The following illustration shows the gamut of the Epson Stylus Photo 2200 with Premium Luster paper compared with the CIE color space.

Figure 1.18.

In ColorThink, you can include the color space by selecting the Gamut Projection check box at the bottom of the Plot Items window.

Figure 1.19.

Color spaces are used by color management systems in the process of transforming data from one device to another. Color from one device is mapped from the device-specific value to a device-independent value in a color space. Once in an independent space, the color can be mapped to another device-specific space.

Device Profiles

Profiles are files similar to dictionaries that contain data on a specific device's color information, including its gamut, color space, colorants, and modes of operation. In other words, profiles contain all the unique color characteristics of a device and are essential to making the whole system work. The process of creating profiles is known as device characterization. Device characterization is typically performed with highly sensitive color measurement devices. You'll learn how to create profiles in Chapters 3 and 4.

Early color management systems used proprietary profile formats, which weren't compatible with each other. This made them very difficult to use in an open environment, such as on a PC running Adobe Photoshop. Fortunately, the International Color Consortium was founded to establish color management standards, the first of which was the ICC profile. ICC profiles are based on a well-defined, open standard and are now supported by virtually all vendors of color imaging hardware and software, as well as platform vendors such as Apple and Microsoft.

ICC profiles come in different flavors, or classes:

• Input profiles support scanners and cameras.

• Display profiles support both CRTs and LCDs, as well as digital projectors.

• Output profiles support RGB and CMYK printers and printing processes.

• Color-space profiles support device-independent color spaces, such as CIE Lab.

The ICC profile specification supports other profile classes, albeit uncommon ones, such as the following:

• Device-link profiles link two devices together directly, bypassing the device-independent color space during transformation. These profiles are essentially two device profiles in one file.

• Abstract profiles essentially manipulate the color of images. For example, the abstract profile Sepia Tone will convert an image from one device space into a sepia-toned image. Few profile-creation tools support abstract profiles.

• Name profiles are for vendor-specific inks, such as PANTONE. The name profile contains transformations from device-independent spaces into the specific values for the vendor's inks.

ColorThink provides a catalog of profiles installed on your system and can display them by class. In the following exercise, we'll use ColorThink to view the ICC profiles that are installed on your system.

1.	In ColorThink, choose System > Open Profile Manager. Figure 1.20. The Profile Manager appears, which enables you to view the profiles installed on your system.
2.	To sort profiles by name, type, color space, and profile creator, click the desired sort field name. You can filter the list by clicking a profile type, such as monitor or print, in the Types list. Figure 1.21.
3.	Double-click the D65 Monitor Profile.icc profile to open the Profile Inspector. Figure 1.22. The Profile Inspector provides more detailed information on the profile, including overview and detailed information contained within the profile.
4.	Click the Header Fields tab. Figure 1.23. This tab displays detailed information on the profile, such as which software was used to create it, the date and time it was created, and the preferred CMM and rendering intent to be used.

Color Matching Method

The CMM color matching method or color management module is the color engine that performs the transformations of color data between different profiles. The color management system in Windows XP, known as Image Color Management, or ICM, includes a high-quality CMM. You'll also find that some applications come with their own CMMs. For example, all Adobe applications ship with the Adobe Color Engine (ACE), but they let you choose between ACE and Microsoft's ICM.

Gamut Checks

Device profiles and the color matching method are used to transform color data between devices to ensure accurate reproduction. Specifically, the CMM translates data from one device's colors to another's via a device-independent color space. The CMM receives the necessary information from the relevant profiles so that it can accurately transform a color from one device to another, producing color that is predictable from device to device. Remember that it is not possible to achieve perfect color matches between all devices due to inherent differences in each device's color gamut.

But what happens when a color is out of gamut if, for example, you want to print a photograph that contains colors your printer can't produce? Without color management, you'd have no way of predicting or controlling what happens to those colors, or even of knowing that they were out of gamut.

One of the key benefits of color management is that it can tell you that a color is out of gamut. The hard way to discover this is to print an image and wonder why that bright blue color on your monitor prints like muddy blue from your printer.

The easier way, which the next exercise illustrates, is to use a neat feature of color management called the gamut check, or as Adobe calls it, the Gamut Warning. This feature applies a device profile to an image and tells you if certain colors can't be reproduced.

Note

If you haven't copied the chapter files from this book's Web site onto your hard drive, do so now before continuing. For instructions, see "Getting Started" at the beginning of the book.

1.	Launch Adobe Photoshop and choose File > Open.
2.	Navigate to the sample file Tulips.tif on your hard drive and click Open. Figure 1.24.
3.	If Photoshop displays the Missing Profile alert, select "Use the embedded profile (instead of the working space)" and then click OK in the Embedded Profile Mismatch dialog box. This tells Photoshop not to apply any color changes to the image. Figure 1.25.
4.	Choose View > Gamut Warning. Figure 1.26. Photoshop displays the image with the gamut warning, indicating which colors are out of gamut. Figure 1.27. The areas shaded in gray are colors that the output device cannot reproduce. In the case of the Tulips.tif image, a great deal of the colors are out of gamut. Without the gamut warning, we would print the image and be surprised at the resulting differences in color between the monitor and printer.
5.	Toggle the Gamut Warning option on and off to see the affected colors. You can check for colors that will be out of gamut for your printer by telling Photoshop to use the profile for your printer. We'll customize these settings in a later chapter.

When a color is in gamut for one device but not another, the CMM must select the next-closest reproducible color. This is called gamut mapping.

Because the use of color data varies, the intended use of a color must be specified to produce the best possible results. The ICC profile specification supports four gamut-mapping options, called rendering intents. Fortunately, each profile includes a default rendering intent, so if you're not sure which one to pick, ICM will use the default set in the profile.

Color management can also be used to simulate, or proof, the appearance of the output, including the effect of gamut mapping on out-of-gamut colors.

Rendering Intents

The ICC profile specification supports four rendering intents for mapping out-of-gamut colors:

• Perceptual rendering works to preserve the visual relationship between colors so that they are perceived as natural to the human eye, even if the colors themselves actually change. This rendering intent is suitable for photographic images.

• Saturation rendering tries to produce vivid colors and sometimes sacrifices color accuracy to do so. As a result, this rendering intent is suitable for business graphs or charts in which bright, saturated colors are more important than the exact relationship between colors.

• Relative colorimetric rendering compares the highlight of the source color space to that of the destination color space and shifts out-of-gamut colors to the closest reproducible colors in the destination color space. Relative colorimetric rendering maps whites to the target output space and preserves more of the original colors in an image than the perceptual rendering intent.

• Absolute colorimetric rendering leaves colors that fall inside the destination gamut unchanged, and it clips colors that fall out of that gamut. Out-of-gamut colors are not mapped to in-gamut colors. This rendering is based on the source's white point and is suitable for proofing to simulate the output of a particular device.

Color Management System Interfaces

Color management systems such as ICM include an interface that allows software developers to use and manage the color management capabilities of the system. These interfaces, known as application programming interfaces, or APIs, contain code that can be used as part of another application.