Arts Technology

Recent Developments in Color Management

Philip Galanter

As desktop computing technology has matured, artists and publishers, as well as media non-professionals, have been offered easy-to-use tools for scanning, manipulating, and printing color imagery. All too often, however, the results exhibit disappointing hue shifts, fading, darkening, or other color distortions relative to the original source.

The practice of maintaining color fidelity from scan to display to print or film output is referred to as color management. Color management on desktop computers has significantly evolved since last reported on in the November 1993 issue of Connect.

This article provides an introduction to the issues, industry solutions, and theoretical basis for color management. A second article in the next issue of Connect will present specific practical steps one can take to implement color management.

The Problem

It's natural to want to use color in a computer-mediated process. Color is an aesthetic enhancement which also adds useful information. Upon closer inspection, however, it is remarkable that real-world color can pass through a digital pipeline with any recognizable fidelity at all. There are numerous complications due to color gamuts, illuminants, colorants, ambient lighting and context.

A given range of color is referred to as a color gamut or simply a gamut. The human visual system supports a very wide color gamut. While various digital devices may surpass the resolution of human vision in terms of discriminating between small details, they typically exhibit a more limited color gamut. As shown in the diagram on page 37, photographic film and computer monitors offer only a subset of the colors available to the human visual system, and the colors offered by process printing are even more limited.

Additional complications include differences between illuminants, such as indirect light reflected off a page versus the direct light from a monitor. Various colorants such as ink, dye, wax and toner exhibit spectral differences which make an exact match impossible. The same object will appear to have a different color if there are changes in the ambient lighting created by the room lights and reflections in the surrounding environment, or in the perceptual context of the colors in nearby objects.

The Solution

Color management today provides a strong partial solution to most of these problems. The first step is to calibrate devices used in the production. Over time, a component will drift somewhat in its color response, and calibration is intended to return the device to its design specification tolerances. Next, a hardware device called a colorimeter is used to measure and capture as data the absolute color response of the individual device. This information is encapsulated as a file in a standard format called an ICC Profile. For example, two monitors of the same brand and model will be calibrated to the same specification, but each monitor should be measured with a colorimeter to generate its own unique custom profile.

Next, a software-based Color Management System (CMS) is used along with the profiles of the devices involved to match colors which are within the gamut of both devices, map colors which are out of gamut to within the gamut of later devices, convert color information from one system to another, and preview colors by using a monitor or printer to simulate the color-look of the final output device.

For example, scanners measure colors as a mix of red, green and blue light (RGB), while printers typically layer a mix of Cyan, Magenta, Yellow and blacK ink (CMYK). The color management system will use a scanner and printer profile to convert a scanned RGB image to a CMYK print in a way that is closely tailored to those two physical devices.

The CMS can use a third profile for a draft-quality proofing device to simulate what the colors will look like when they are sent to the final printer. A calibrated and profiled monitor can be controlled by the CMS to simulate the look of the printer. The CMS in this case will limit the visual range of the monitor. While this will seem to make the monitor appear degraded, this is in fact what is needed so that "what you see" on the monitor is "what you get" from the printer.

Color Management Standards and Products

Prior to 1993, every vendor of color management tools worked independently, and a number of competing application programs were available from companies such as Kodak, AGFA, EFI, Candela and others. There was little in the way of interoperability, and no clear winners or de facto standards.

In 1993, Apple introduced ColorSync 1.0, a unified operating system-level CMS. Unfortunately, the software at its core, the Color Transform Engine (CTE), was slow and produced inferior color separations. In addition, ColorSync 1.0 was not supported by any major applications. The basic concept of operating system support for color management, however, gained interest and attention.

Apple released ColorSync 2.0, a greatly improved CMS, in 1995. As its engine, ColorSync 2.0 used the LinoColor CTE, a leading, expensive, software product licensed from Linotronic and then bundled into the Mac OS. Not only did this greatly improve the quality of ColorSync transformations, it also radically improved system performance. For example, a color space conversion of a 20 MB image that took 30 minutes in ColorSync 1.0 takes only 5 seconds in ColorSync 2.0.

The second significant improvement ColorSync 2.0 introduced was the use of an industry standard profile format. Apple developed the format, and then formed the International Color Consortium (ICC) along with Adobe, AGFA, Kodak, Microsoft, Silicon Graphics, Sun and Taligent to help ensure industry-wide adoption. The ICC Profile Format is the catalyst that created the current color management industry, and allows multiple vendors to create inter- operable scanners, printers, monitors, calibration and spectral measurement devices, application software and more.

In the first quarter of 1998, Apple plans to roll out ColorSync 2.5, a major revision to the current version 2.1. ColorSync 2.5 will support multiple simultaneous color transform engines, and will ship with both the LinoColor and Kodak CTE. This allows the user to choose which CTE he prefers, and to take advantage of private data fields which can yield small improvements when the profile and the CTE are from the same vendor. In addition, ColorSync 2.5 will include tools for profile library management, built-in software calibration and monitor profiling, multi-processor support, and AppleScript support for batch conversion of multiple image files.

Microsoft color management tools for Intel platforms are not yet a practical alternative. Windows95 does include an ICC-based CMS called Image Color Management (ICM) 1.0. Unfortunately, it uses a low quality CTE based on an earlier proprietary system from Kodak, is not currently supported by available applications, and requires specially created print drivers to apply device profiles.

In mid-1998, Microsoft is expected to release ICM 2.0 as part of Windows 98 and Windows NT 5.0. This will include the same LinoColor CTE Apple adopted in 1995, and should allow the use of the same ICC standard profiles used by ColorSync. It remains to be seen, however, how broadly and quickly Windows applications will be revised to take advantage of ICM 2.0. In addition, Microsoft and Hewlett Packard have been promoting a color space definition called sRGB which is primarily oriented towards the consistent monitor display of web applications. Having both ICM and sRGB may cause confusion and slow the adoption of either by software and hardware vendors.

Looking Under the Hood -- Color Specification

A fundamental aspect of color management is color specification. Within a typical 24-bit color application, 256 levels of red, blue and green each are represented by 8 bits per primary color. While this allows for an RGB color space with "millions" of colors, there is no particular absolute real-world color defined or intended for a given set of values. The 24-bit RGB colors are simply relative values, and each will be rendered in a way which is specific to the given device. The same is true of images in a CMYK color space, and so both RGB and CMYK are referred to as "device specific."

In 1931, the CIE (Commission Internationale de L'Éclairage, or International Commission on Illumination) defined a set of device-independent color spaces. First, based on a number of experimental trials with human subjects, tristimulus values called X, Y and Z were defined based on the three types of stimuli the eye detects. This CIE XYZ space includes all visible light where X, Y and Z can range from zero to just over 100 percent. The CIE Yxy space is a simple mathematical transform of the XYZ space, and in both cases the Y component corresponds to the lightness (closeness to white) of the color. The problem with the XYZ and Yxy spaces is that they are non-linear relative to human perception. That is, XYZ and Yxy values that are close may or may not appear visually close as colors. The CIE L*a*b*, or simply CIELAB, space corrects for this nonlinearity, so that throughout the space any given distance corresponds visually to about the same color difference. It is said to be perceptually linear.

CIELAB is the interchange color space that a CMS uses internally for all color transformation calculations. Device profiles provide the CMS with the information it needs to convert between the device-independent CIELAB and the device-specific RGB or CMYK values. For example, a scan is made yielding RGB data for the image. The CMS uses the profile for that particular scanner to convert from RGB to CIELAB. It then uses the profile for the particular printer being used to convert from CIELAB to CMYK. That CMYK data will print with color fidelity on that particular printer and no other.

In the print industry there is another form of device-independent color. Proprietary spot color systems such as those from Pantone, Trumatch and AGFA use carefully controlled proportions of 14 or more inks to create color matching samples and standardized printing supplies. Because so many inks are used, the spot color gamut is much wider than the typical CMYK process color gamut. For example, typical CMYK printing can only match about 30 percent of the available Pantone colors. An extended print process called Hexachrome uses six inks and can match up to about 80 percent of the Pantone colors.

Practical Steps

The practical steps towards color management include calibrating monitors, using generic profiles, creating and using custom profiles, and using the color management system to simulate final output devices on the computer monitor or with a less expensive printer. Those working only with graphics for the Web may find that monitor calibration is sufficient. Others, such as those working in a pre-press capacity, will need to find their own combination of options based on their workflow and quality expectations. In the next issue of Connect we will explore these options in practical detail. [ C ]

Philip Galanter is Associate Director for Arts Technology at ACF.
galanter@nyu.edu

Posted January 20, 1998