230—Recent Progress in Color Processing

Device Independent Color—Who Wants It?Peter A. Crean

Xerox Corporation, Webster Research Center, Webster, New York 14580

Robert BuckleyXerox Corporation, Webster Research Center, Webster, New York 14580

Abstract

The technology and architecture for using device indepen-dent color in documents is presented. The expectations andneeds of the desktop users of color are examined andcontrasted with the technical realities of digital color sys-tems. Several major areas of color research are identified toaddress the problems of device independent color.

1. Introduction

The advent of desktop color management systems andPostScript level 2 printers offer the creators of color imagesand documents the opportunity to work in a device indepen-dent space. The technology to create, communicate and renderdevice independent colors is technology well established inthe industry and available in many current products.

While printing from scanned input and WYSIWYGcolor editing systems would, in most cases, benefit fromdevice independent color, gamut mapping at the printer canbe an unwelcome surprise to the creator. Today, graphicsare generally conceived and described in device colors,with a confidence in a predictable appearance after theprinting process.

This paper will discuss the values and pitfalls onoperating in a device independent space and detail some ofthe color management sophistications that may be requiredfor regular users to fully utilize device independent color.

2. Device Independent Color

Color is usually described in one of two ways:a) device dependent description which gives the for-

mula for producing the color of an object or a pixel on somedevice. Examples include the ink density on some printer,the digital value to put into the frame buffer of a workstationand the percentage of pixels to turn on in an ink jet device.The use of a device dependent color specification presup-poses accurate knowledge of the printing device, in particu-lar its performance when the image will be printed.

b) device independent description which gives thecolorimetric coordinates describing the visual stimuli nec-essary to match the intended color. Colorimetric variablessuch as CIE XYZ and L*a*b* are examples of deviceindependent color specifications. The printing or display ofdevice independent color requires an accurate transforma-tion to the parameters of the display or printing device.

Device independent color leads to an architecture forconnecting scanners, printers and workstations. In this

context, architecture means the segregation of function andthe definition of interfaces. An architecture for deviceindependent color is shown in Figure 1. In this arrangement,color devices (scanners, printers, and workstations) ex-change data in device independent format, here CIE L*a*b*.The necessary transformations between their native, devicedependent formats are performed before information istransmitted. One of the advantages of this system is thateach device is responsible for its own accuracy. Addition ofnew devices to the system do not affect other computers orprinters on the system. Color images and documents can bestored in a device independent manner and later be viewedor printed on any printer.

The architecture shown in Figure 1 is not the only onewhich implements device independent color, nor is it theonly one which allows the easy addition of new devices. Itsmajor strengths are the portability and easy reuse of archiveddocument and the use of open standards including PostScriptlevel 2.

3. Customer Expectation For Color

The concept of device independence is new to the graphicsarts and not universally accepted. In the keynote speech atthe IS&T session on Device Independent Color on Feb. 6,Professor William Schreiber expressed the belief that de-vice independent color would never suffice for offset qual-ity printing because of the fundamental differences betweencomputer monitor color and the four color ink-on-paperprocesses. Thirty years ago, media guru Professor MarshallMcLuhan came to prominence with the statement that themedia is the message.

This tight binding of information presentation to itsmedia is certainly true for the art world. A masterpiece is notavailable in oil, fresco and marble. The graphics arts havefollowed this tradition. Font design, the construction ofgraphics and the preparation of pictorial images have al-ways been done with the destination print technology andthe paper choice in mind. Although computerized compo-sition and editing aids have been used for over twenty fiveyears, they have been view as just that: devices to enhancethe productivity of the skilled operator in producing betterprint material faster.

With the advent of desktop publishing coincident withthe introduction of the Apple Macintosh in 1984, the com-puter was advertised as the document creation system. Thescreen portends the final print: what you see it what you(will) get (WYSIWYG). Granted the screen did not have thecolor of paper and fonts were 75 dpi and the character fit was

Chapter I—Color Appearance—231

crude at best, the line breaks and the layout matched thefinal print, that final print only looked better than the screenversion.

It seemed that all the compositional details could bedecided on the computer without reference to a test print.That is the big promise of WYSIWYG. With the totaldominance of the desktop world by Microsoft Windows andMacintosh and their clones and the popularity of colordisplays and color capable desktop publishing software,everybody has the opportunity to get into color documentcreation.

4. Customer Needs in Color

The customer needs in color are only subtly different fromhis expectations. The first need is no surprises color—eventhough the printed color may not precisely match thedisplay or the scanned-in image, even though the color laserprint is not the same as the offset run, the customer is neithersurprised nor disappointed.

A second clear need is for the color management to beuser friendly. In this case, that hackneyed phrase translatesto an effective hiding of the technology, to the eliminationof the adjustment knobs found on graphics arts scanners andediting workstations and their software equivalent—slidersand buttons. Just as the bank of typographic editing keysfound on the ATEX terminals have given way to a fewpulldown menus, simple color controls (or better yet nocontrols) are needed. A corollary to user friendly is supportfor progressive learning: the ability for more and/or simplercontrol as the user becomes more expert with the system andthe technology.

The final customers requirement for color systems isthat the output image or document be archival and portable.The large investment in color documents demands that thedocument print on more than the local printer and that, whenit is recalled from storage at some later date, the colors areas originally intended by the author. Device independentcolor is taken as the natural answer to this requirement.

5. Technical Reality of Color

The technical reality begins with mindset with which thedesktop world looks at and describes color. Figure 2 shownthe color gamut of the typical display. This is the usual 2Dprojection of the L*a*b* presentation of the CRT gamut.Contained within this colored egg viewed here from abovethe white point are all the colors that can be produced on theCRT. It is very instructive to look at this figure from the sideas shown in Figure 3.

The first problem with WYSIWYG color is showngraphically in Figures 4 and 5 where the CRT gamut isshown in mesh superimposed on the solid gamut of a LaserColor Copier/Printer. When viewed from above, they looksimilar, but the side view shows that many bright saturatedcolors on the CRT that cannot be printed. In other words,there are colors seen on the CRT that cannot be obtained onthe printer. And conversely, a lot of the colors produced bythe printer cannot be seen on the CRT.

All printers do not have the same range of colors tochose from in rendering images. Figures 6 and 7 show themeshed Color Laser gamut overlayed on the gamut found in

offset printers, here represented by the Kodak Approvalproofing system. The Laser has a slightly greater colorrange in all regions except darkest regions where the greaterDmax of four color offset breaks through the Laser Xero-graphic range.

An ink jet printer using special coated ink jet paper hasa very different character as shown in Figures 8 and 9.Extended green performance in the top view is seen to be inthe lighter, saturated colors. The set of colors common toboth printers is significantly less than those available toeither technology alone. The laser printer has more darkreds and blues. When the ink jet is used on plain paper, thegamut shrinks, particularly in the lower region.

On the desktop, colors can be defined in a bewilderingnumber of ways: the basic RGB, the intuitive HSV, thedevice specific printing inks CMYK, and more recently thedevice independent L*a*b*. Many applications simplyoffer a visual palette for the creator to select from. Throughthe desktop is a heterogeneous, standards averse place, it isnot necessarily inaccurate. The measured colors of theAldus Persuasion 2.1 color palette display on a self-cali-brated SuperMac monitor were compared to the PostScriptdefinitions sent by the application to printer and found tohave an average L*a*b* error of 3.1, most of that attribut-able to dark colors which cannot be displayed in a moder-ately lit room.

For many of today’ s color users, the greatest problemsdo not arise from the differences in color perception on self-luminant displays and subtractive printing processes nor inthe differences in printer gamuts. The most difficult prob-lems to solve come from the many transformations that goon between the application and the printers: the conversionfrom the application’s color space and the desktop stan-dards of Apple Quickdraw or Microsoft GDI, the variousdevice profiles used by the application and the print driverin preparing the PostScript definition of the page to beprinted, and finally the decisions made by the printer inpreparing the device dependant image representation. Thesetransformations occur at different places in the softwareinfrastructure, and are often made by modules that do nothave a consistent view of the problem.

These problems are not solved by color science ortechnology but by coherent actions by application andoperating systems vendors. Actions such as the ColorSyncstandard are directed to solving this type of problem.

The technical challenges for device independent colorcome in three areas directly related to the print process itselfwhich are often overlooked by color scientists:

l) determining the gamut mapping intent2) developing the black printer3) compensating for process shortcomings such as

dot gain and trappingAs shown in the figures, a many colors can be requested

of a printer that it cannot really print. The approximationcan favour lightness or saturation or the whole input colorspace can be squeezed to fit into the available color gamutof the printer. Present desktop applications seldom offer theuser the choice of approximations and, when specified,there is no standard way to send this information to theprinter.

Black(K) is used in addition to CMY to render color toreduce ink thickness, to avoid registration errors when

232—Recent Progress in Color Processing

printing black text, and to more consistently render nearneutral grey tones. Its use is very much an art and dependssubtly on the image content. An algorithm to produce thesuitable CMYK from a CIE color is a major challenge. Forblack text on a white background, full K with no CMY isused, while the black borders on a pie chart are printed inCMY with little K to avoid the unintended white dropshadow caused by misregistration. Each type of printing(laser xerographic, ink jet, offset) uses the K ink differentlyto solve their specific process problems.

The final device dependent problem arises at when twocolored regions abut. For offset and laser refer to thetrapping problem while for ink jet attacks inter-color bleed.In both cases, the the rendering of the pixels near theboundary are modified to minimize process-related defectsin the appearance of the final printed page.

In the these last two cases, the inks laid down are notsimply related to the color in the image but are determinedby special needs of the process. Traditionally, many of thesemodifications are inserted into the image early in the cre-

ation phase where the spatial extents of objects and thedesign intent of the artist are clearly known. This informa-tion is clearly goes beyond device independence.

6. Conclusions

Device independent color has the potential to be of greatvalue the creators and users of color documents. The suc-cessful printing of device independent color presents manyinteresting and meaningful technical challenges to imagingscientists, computer systems architects, and printer design-ers. These challenges include systems for capturing andcommunicating the color intent in a document, better per-ceptual models to enable the adjustment of the image topreserve those intentions under differing viewing condi-tions, and methods for fully utilize the gamut of eachprinting technology.

published previously in SPIE, Vol. 2171, page 267