A Review of RGAB RColoreview Spaces of RGB Color Spaces … from xyY to R’G’B’ Danny Pascale Danny Pascale Title: A Review of RGB Color Spaces …from xyY to R’G’B’ ©2002-2003 Danny Pascale The BabelColor Company 5700 Hector Desloges Montreal (Quebec) Canada H1T 3Z6 E-mail: [email protected] Front cover: The xyY representation of the sRGB color space and its corresponding R’G’B’ cube. Adobe®, Apple®, ColorChecker®, ColorMatch®, ColorSync®, Digital Origin™, GretagMacbeth™, IBM®, iMac™, Intel®, International Color Consortium®, LG™, Mac®, Macintosh®, Microsoft®, Munsell®, Pantone®, Photoshop®, PressView™, Radius®, Silicon Graphics®, SGI™, Sony®, Trinitron®, VGA™ and Windows® are Trademarks or Registered Trademarks of their respective owners. Document revised 2003-10-06. A Review of RGB Color Spaces … from xyY to R’G’B’ Danny Pascale Why another document about RGB? While there are many sources of information describing Red-Green-Blue spaces, their use, and why you should or should not use some of them, there are few self-contained sources of information on how to get there. You may find books, standards, and articles with equations on how to transform colorimetric data into a specific RGB space, or even how to translate data between some RGB spaces, but only a few spaces may be covered, or there is not enough information on how these formulas were derived and how to recalculate for different conditions, or they are not up to date with the most recent standards, or there are inaccuracies and mistakes, especially in non peer-reviewed freely accessible Web-based documents, or all of these “or”s. Why bother? Accurate colorimetric data is no longer the realm of top tier professionals for which this is the only way to survive and which are willing, and capable, to invest heavily both time and money. Accurate colorimetric data formerly required proprietary – read expensive –, often esoteric, color management equipment and software. With the availability of low cost high quality devices such as scanners, printers, monitors, and calibration equipment, and with the significant increase in the per dollar computing power, accurate color capabilities are now available to small shops as well as consumers. The incentive to bother is that most users now expect a higher quality end product. Who should read this document? This text is targeted to professionals who wish to acquire a basic understanding of colorimetry applied to computer and TV display systems and who want to see how theory translates into practice. A programmer involved in developing color transformation routines will find the flow chart and detailed conversion process of Section 4 helpful. Someone simply interested in checking the calibration of a camera or scanner using the GretagMacbeth ColorChecker card will find useful the RGB values of the card’s color patches presented, in Section 5, for many common spaces. What is not covered? Although referred to in the text, ICC profile generation and gamut mapping are not developed. Nonetheless, this text is a good introduction to these more advanced topics. 4 • A Review of RGB Color Spaces… TABLE OF CONTENT 1 Introduction.................................................................................................................................................................................................. 5 2 Color spaces.................................................................................................................................................................................................. 6 2.1 Parameters of color spaces................................................................................................................................................................ 7 2.1.1 The human eye........................................................................................................................................................................... 7 2.1.2 The “abc” of XYZ .................................................................................................................................................................... 8 2.1.3 Limitations of the CIE 1931 chromaticity diagram........................................................................................................... 14 2.1.4 Illuminants ................................................................................................................................................................................ 15 2.1.5 The Bradford Matrix............................................................................................................................................................... 16 2.1.6 Gamma...................................................................................................................................................................................... 17 2.2 The various RGB color spaces....................................................................................................................................................... 20 3 Television and multimedia systems......................................................................................................................................................... 23 4 From xyY to R’G’B’ .................................................................................................................................................................................. 26 4.1 From xyY to XYZ............................................................................................................................................................................ 26 4.2 From L*a*b* to XYZ ...................................................................................................................................................................... 27 4.3 From XYZ (Source illuminant) to XYZ (Destination illuminant) – Bradford Matrix......................................................... 27 4.4 From XYZ to RGB, and vice-versa.............................................................................................................................................. 27 4.5 From RGB to R’G’B’....................................................................................................................................................................... 29 4.6 Conversion accuracy vs. requirements.......................................................................................................................................... 29 5 A practical example: the GretagMacbeth ColorChecker .................................................................................................................... 33 A Review of RGB Color Spaces… • 5 1 Introduction Not so long ago, in fact just before the personal computers era, color displays used to be either color film (slides, prints and movies), the good old TV, or an image printed with ink on paper. These media were based on different color processes and standards, and the interchange between them, while done scientifically, involved complex dedicated machines and a good dose of black magic (some call it experience). The advent of the personal computer mixed the cards. Using an Apple Macintosh, anyone could soon do its own page layout. At first, the results were bland more often than not, and it proved once again that having a hammer does not make someone a carpenter. With time, the software evolved so that some graphic designer know-how is now integrated with the layout tools, in the form of Wizards – on screen step-by-step instructions – for example, and more time can be spent on the content than on the container. The same situation is happening with color management. In the personal computer world, you used to have only one choice, the default Apple RGB space, whatever that was, since it was not really easy to find its specification. You could use it to generate images that would be compatible at least with future generations of Macintoshes. However, an image generated on a Macintosh did not look the same on a Windows based machine simply because, among other things, they do not have the same transfer function in the graphics card (one of the elements that control the display brightness in relation to graphic data), not to mention the limited amount of colors available on first generation Intel compatible computers. Later, proprietary color management schemes were imported to or created for the personal computer, and standards slowly emerged from them. Very recently, the cross-platform acceptance of the International Color Consortium (ICC) color profiling method helped bring uniformity to the picture. It enables the input, output and display devices vendors to transparently, at least to the general user, exchange color data that conform to well characterized color spaces. The color management workflow tools are the latest trend in this development. In particular, the set of tools first offered by Adobe in their Photoshop version 5.0.2 program started a new era in the color environment controls available to the laymen. Not that the process is simple. There is a cumbersome heritage to support, to which we have to add all the proposed standards emerging from the work done on High-Definition TV (HDTV) and for the coming-real-soon-now computer-TV-multimedia convergence. The shear number of combinations resulting from the various alternatives is frightening to the novice, annoying to the expert, and some of both for anyone in between. The fact that very good color