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Maximize Color and Contrast in Multimedia Images

Photonics Spectra
May 2003
Color management, a field of study that has come of age in the past decade, is yielding practical solutions and interesting products.

Dr. Christine Connolly, Contributing Editor

After scanning a color photograph into a computer, one assumes it will reproduce well on the screen and on those to which it has been sent via the Internet. The ease with which this can now be done, however, conceals the complexity of the underlying technology and the challenges involved in getting various input and output devices to reproduce colors well. The reason is that one monitor may use red, green and blue (RGB) primaries with additive mixing, while the printer has cyan, magenta, yellow and black (CMYK) primaries with subtractive mixing.

According to Adobe scientists, because humans can't distinguish separate cyan, magenta, yellow and black dots in an image at normal viewing size, they instead perceive colors. These are simply an additive combination of varying amounts of CMYK inks in any section of the image.

In the past 20 years, a field of study called color management has emerged to address the problem of color rendering by multiple devices. Perhaps one of the earliest applications involved prepress systems for publishing applications. All components were single-source and used proprietary hardware and software. Accuracy of color reproduction depended on the manufacturers’ knowledge of the input and output devices and the development of the appropriate transforms to map colors from an analyze scanner to the workstation screen and from the screen to an expose scanner.

Another early application involved textile design. Research at the University of Manchester Institute of Science and Technology in the UK, for example, led to the development of the Shademaster, a system that allows a designer to create a color on-screen and to output its numeric color definition so that dyers can calculate its formulation. Researchers also calibrated the required color printers so that the color would reproduce well on paper.

Around the same time, the Colour and Imaging Institute of the University of Derby in the UK was working independently on calibrating monitors to display color order systems such as Munsell (an established communication tool in dye houses) on-screen and to display the CIE tristimulus X-Y-Z values of a user-selected sample. The university also went on to calibrate color cameras so that colors could be scanned in from them.

Toward the end of the 1980s, manufacturers began developing desktop publishing systems for standard computers. At first, each company pursued its own path, and every color-management system had a different architecture. As Seattle-based Microsoft explains on its Web site, when each application uses a different proprietary system, it is impossible to get consistent color interchange among the different applications. Apple of Cupertino, Calif., addressed the problem with a software system called ColorSync that provides a framework within the operating system for exchanging color information between applications and between input and output devices.

Then in 1990, Eastman Kodak Co. in Rochester, N.Y., announced the development of its Photo CD system and a proposed worldwide standard for defining color in the digital environment of computers and computer peripherals. In 1993, the firm banded together with Microsoft, with Apple, with Agfa in Mortsel, Belgium, and with Sun Microsystems, Adobe and Silicon Graphics, all in California, to form the International Color Consortium (ICC).

To build an International Color Consortium profile of a CMYK printer, a spectro colorimeter measures 125 color patches, and the data is fed to color-management software for computation. Courtesy of DataColor.

Attacking the problem

One area to benefit from subsequent standards work of the consortium was prepress. While transforms for high-end proprietary prepress systems may work well when mapping colors between a pair of devices, they are not suitable in environments using a variety of scanning devices and cameras, and many different output printers, all connected to the computer via plug-and-play drivers. This led the consortium to adopt device-independent transformations.

These transforms map each device to one standard color space. Called the profile connection space, it provides a common language for device communication. A new device introduced to an application thus requires just one transform to the standard space. Its ICC profile contains the transform to the connection space in the form of a look-up table with either 8- or 16-bit data per channel.

The profile has a closely specified format, so that many users can access it readily and reliably. Input profiles translate the native metrics of an input device, such as a scanner or camera, into the profile-connection-space values. Output profiles apply to output devices, such as printers, and they translate profile correction space data into signals that determine the quantity of each color of ink applied to the paper.

The process of building an ICC profile is called device characterization. It relates the color characteristics of a device, such as a computer monitor, to the color space. But the profile will be used on many of these monitors in many places, so it is essential that they all behave the same. Walk through an electronics shop with various television sets on the same channel, and it becomes obvious that the color and contrast can differ dramatically from screen to screen. Each may receive the same image but may render it differently because of differences in brightness, color balance and contrast settings. Getting consistency among monitors requires that they be calibrated to display the same colors.

The characterization process relates the native color space of the device to the profile connection space. For example, scanner characterization may use a printed target consisting of patches of uniform color. The scanner produces an R, G and B signal from each of its pixels. The process involves calculating the average R, G and B value for each patch and relating the data to the Commission Internationale de l’Eclairage (CIE) X-Y-Z values of the same patch as measured by a spectrophotometer. The ICC profile includes a header that contains information such as the medium white point, data type and size, and information that is used within a specific vendor’s applications.

The whole gamut?

A major problem when transferring colors from device to device is gamut mismatch. The gamut is the range of colors producible by mixing primaries. It is impossible to produce colors that are more pure than the primary ones, so each device has a limited gamut, and the gamuts of different devices vary.

It may be possible to scan some particularly bright colors from a photographic film, yet be impossible to print those colors with CMYK primaries. Only the in-gamut colors transform accurately into the color space data. So the question becomes what to do about the out-of-gamut colors.

The ICC profile contains a collection of rendering intents that enable different solutions, depending on circumstances. In the example above, where the printer has a smaller gamut than the scanner and the aim is to produce a pleasing reproduction rather than an exact color match, the user would choose the perceptual rendering intent. This expands or compresses the full gamut of the image to fill the gamut of the destination device. It keeps the gray balance correct and ensures that the overall scene appearance is acceptable. The colors bear a similar relationship to each other, as they did in the original, although they do not actually match color for color, even within their common gamut.

For proofing, for example where a monitor simulates the appearance of a printed design or where an ink-jet printer simulates the appearance produced by a printing press, the solution would involve the use of colorimetric intent to ensure that individual colors within the common gamut match exactly. Saturation intent finds use in business graphics, where bright, eye-catching colors are important. This strategy preserves the saturation of the pixels in the image, even though there may be a change in hue and lightness.

Software aids

Once the ICC profile has been produced for a particular device, one can use it as part of a color-management system. It requires a color-management software module that interpolates between the look-up table entries of the profiles and allows one device to communicate color to another. Photoshop includes such software, as do the Macintosh and Windows operating systems.

The color-management system in Windows works without user intervention, loading the ICC profile of a printer or scanner as part of its installation and automatically using it whenever the device is operated. An override, however, allows one to specify which profile to use with a particular device.

As equipment ages, it changes its behavior, and thus requires a calibration procedure. Proper calibration ensures that colors will be acceptable in all the reproduction media. Color-management software does not solve the problems completely, but it does give a generally acceptable level of control over what was previously a very hit-and-miss affair.

The ICC profiling approach to color management makes the user responsible for maintaining equipment calibration and for using the appropriate profile for each device. In parallel with this, vendors supplying the consumer market started using a standard-default color-management approach called sRGB. It has gained the status of an international standard and is implemented in virtually every computer input and output device and on the Internet.

Based on the color space of traditional cathode-ray-tube monitors, the standard takes into account the colorimetric properties of their phosphors and their nonlinear display characteristics. It is a device-dependent color space, but this device has long been subject to the international standards of television broadcasting.

This inexpensive means of color-management works without intervention by the user and usually without the user realizing it is there. Input device manufacturers build transformations from native color space to sRGB, and printer manufacturers build the sRGB to CMYK transforms.

Products and tools

The main outcome of a decade of work by the ICC is a set of standards that are accepted and used by the membership. Information about color-management theory and practice is available on the consortium’s Web site ( In addition, the member companies have produced a collection of products that make the practice of color management readily accessible to professionals who work in markets where color use is routine.

Some software tools allow end users to build their own ICC profiles. For example, Eastman Kodak offers an ICC Input Profile Builder and ColorFlow tools that enables digital imaging and photographic professionals to develop and maintain custom ICC profiles for devices. For input devices, these scan the colors in a standard target, compute a best-fit function to relate them to X-Y-Z values, and produce a look-up table. The mathematical procedure for finding the best-fit function is proprietary to the software manufacturer. For an output device, the software causes the device to print a set of uniformly colored patches. The end user must measure the colors produced with a color-measuring instrument, and the software will use this data to relate the CMYK recipe to the actual colors printed.

There are calibration aids, such as targets produced by Gretag Macbeth, and color-measuring instruments from Gretag in Zurich, Switzerland; X-Rite Inc. in Grandville, Mich.; Datacolor in Lawrenceville, N.J.; and Color Savvy Systems Ltd. in Springsboro, Ohio. To calibrate a monitor, there are visual procedures such as Adobe Gamma, a utility built into Photoshop that generates an ICC profile for a particular monitor. A more accurate calibration involves the use of a special colorimeter to measure the colors on-screen.

To characterize the colorimetric properties of a display, end users can attach a special colorimeter such as this LCD Spyder to the screen while it displays a test pattern of colors.

Other color-management tools use the profiles to make input and output devices work together. For example, Photoshop allows artists to create images from scratch and to retouch and modify existing pictures. QuarkXPress enables the control of layout and design, text processing and color imaging when preparing print publications and Web pages.

The current ICC Profile Specification 3.4 supports Apple, Microsoft, Silicon Graphics Inc., Java and Sun operating systems. For example, Apple’s ColorSync, which is built into the Mac operating system, stores ICC profiles as part of the image file to specify how the image was captured and in what color space. Included is a monitor calibration system that corrects any irregularities in the monitor’s age, phosphor set, ambient light, white point and monitor type. End users also can calibrate monitors to ensure that colors of a displayed image correspond to those within the original photograph and can view how an image would appear in hard copy using different paper types and printers.

The tools find use in high-end and professional graphic arts and photographic devices. By creating custom profiles for their equipment, color laboratories can achieve better-quality color reproduction, especially when transferring images to a wide range of media, such as murals, exhibition stands and brochures. Kodak’s Web site describes how one of its ColorFlow users produced a 40-foot-long color print on transparent film that was applied to the sides of a white van to advertise a local zoo.

Apple’s ColorSync has been used recently to produce a series of reproductions of oil paintings on vitreous enamel panels displayed along a 450-foot passage in the Tower Hill underground station in London. This was commissioned by the Historic Royal Palaces to make tourists aware of the museum of expressionist art at the Tower of London. It is the vibrancy of color in the original paintings that makes them so attractive, so it was vital to retain accurate color rendering in this vandal-proof medium.

Color management tools continue to evolve. Datacolor now sells Shademaster technology under the name ImageMaster. Retailers use it, as do their suppliers in the apparel industry. Fashion designers develop colors for the coming season on-screen, and the system outputs the CIE X-Y-Z specifications required by dye houses to reproduce the colors on fabrics. An integral quality control system assesses dyers’ samples and produces a pass/fail verdict against the original color. As another example, Derby’s ColourTalk system has been developed further in collaboration with Coats Viyella and Fuji Film Electronic Imaging to communicate precise color specifications among remote locations.

CMYKcolor photographConsumerFeature ArticlesFeaturesinput and output devicesRGBsubtractive mixing

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