Beyond the Locus of Pure Spectral Color and the Promise of HDR Display Technology
In emerging media technologies, particularly High-Dynamic-Range (HDR) displays, much more is possible in achieving brighter and more-vibrant colors and a richer visual experience that may even transcend what we see every day. Here is how it works.
by Rodney L. Heckaman and Mark D. Fairchild
IN HIS TREATISE on the stained-glass windows at the Cathedral at Chartres, "The Radiance of Chartres," 1 James Rosser Johnson writes that "…the experience of seeing these windows…is a very complicated experience …that spans many aspects of perception." Yet, fundamentally, "…when the spectator enters the Cathedral from the bright sunlight …the visitor must step with caution until his eyes have made a partial dark adaptation… then the details of the interior will seem lighter and clearer while, at the same time, the [stained-glass] windows become richer and more intense."
Adaptation has played a powerful role in this instance depicted in Johnson's narrative. By adapting to the darkness or lower perceived diffuse white of the cathedral's interior, the colors of the windows appear exceedingly brilliant, invoking a perception that "… transcend[s] the statics of the building masses, the realities of this world…[creating] a world of illusion, shaped by and for the heavenly light of the enormous stained-glass windows." 2 While such a perceptual experience is certainly complex and affected by the many artifacts of the human visual system (HVS), the richness of it is largely and simply made possible by the broad extent of sensitivity of the HVS and its innate ability to adapt to its surround.
The HVS is capable of adapting to an incredible range of luminance extending well over eight orders of magnitude, from a starlit moonlit night in 0.0001 cd/m2 to a brightly lit summer's day in 600–10,000 cd/m2. Equally incredible, more than five orders of magnitude are available across the retinal field at any given time for the perception of complex visual fields that are typically experienced every day (Fig. 1).
Adaptation is thought to occur relative to diffuse white – an area in the scene that appears white. The perceptions of lightness and chroma are then relative to this white. The higher its brightness, the lower the brightness and chroma of similarly illuminated objects in the scene appear. The lower its brightness, the brighter and more colorful such objects appear. Figure 2 illustrates this effect. In each side of the two figures, the center stimuli are the same. Yet, in the brighter surround, the center stimulus appears darker on the left and more chromatic on the right. In other words, changing the stimulus that appears white – the surround, in this case – affects the appearance of all other stimuli in the scene.
From there, a logical question ensues: How can the powers of adaptation be used in display media to, in effect, expand the gamut of the media in the perceptual sense?
In his 1935 paper, "Maximum Visual Efficiency of Colored Materials," 3 David MacAdam stated that "one of the most compelling objectives of pigment and dye chemists has been to…produce colors of ever-greater purity without the sacrifice of brightness." In the interest of ensuring that reasonable expectations be set in this regard, MacAdam computed what are now known as the MacAdam Limits – the theoretical maximum color gamut or spectrum limit of ideal materials.
Fig. 1: The light-sensitivity range of the human visual system (HVS).
In effect, these limits bound the range of possible object colors, yet they represent only a portion of what we see every day – think of a scene where the sun is filtered through foliage or reflected off the ripples of water in a lake; or a ray of sunlight shining through a dark, cloudy October day illuminating colorful foliage; or the brilliance of a spot-lit dancer and her costume in the dark surround of a theater. It is in such instances that the greater promise of emerging display technol-ogies – particularly high-dynamic-range (HDR) displays – to take us beyond the spectrum limits of pure colors becomes apparent. This experience is well within our ability to perceive and is enabled by the perceptual gamut expansion offered by this technology.
Perceptual Gamut and a Computational Methodology
Figure 3 shows a traditional representation of the gamut of a typical digital display device with RGB primaries in a CIE Chromaticity Diagram superimposed on the MacAdam Limits for pure spectral colors. Such a representation does not give any insight into their respective appearance attributes, yet this representation is typically used in the display industry as a point of comparison. Furthermore, in traditional applications, the display is characterized and its white point set to the maximum output of the display. By definition, such a display is configured to render only within the realm of object colors, and colors outside this realm are rendered by employing various gamut compression strategies.
In this article, the powers of adaptation of the HVS are exploited to define an expanded perceptual gamut. The strategy of this methodology (Fig. 4) is simply to "push down" the white point in relative luminance and extrapolate a gamut expansion in perceived lightness, chroma, brightness, and colorfulness – an expansion that actually exceeds the spectrum locus of pure colors in the perceptual sense.
For convenience, a typical baseline color display monitor was taken from Berns,4,5 having a maximum luminance of 100 cd/m2 with a dynamic range or contrast ratio of 100:1. The color-appearance model, CIECAM02, was implemented6,7 to compute perceptual gamut in lightness, chroma, brightness, and colorfulness under various viewing conditions of interest.
For each iteration N (see Fig. 4), the maximum luminance of the baseline display and its corresponding conversion matrix MN is scaled up by a factor of 2N while maintaining diffuse white at 100 cd/m2. The gamut in CIEXYZ tri-stimulus values is then computed from the matrix of all possible combinations of scalar RGB values according to
Perceptual gamuts in lightness, chroma, brightness, and colorfulness are then computed using the CIECAM02 color-appearance model from this computed gamut in tri-stimulus values, diffuse white, and the characteristics of the surround.
Fig. 2: Lightness and chroma adaptation.
Fig. 3: The gamut of a typical digital display device in CIE chromaticities superimposed on the locus of pure spectral colors.
The number of iterations (N) was increased to the equivalent of 13 bits of luminance channel encoding (N=5) with the intent of approximating the range of a fully adapted HVS. Such a display with 13 bits of encoding is equivalent to a display with a maximum luminance of 3200 cd/m2, a contrast ratio of 3200:1, a white point of 100 cd/m2 mapped to 1/32nd of the maximum display luminance, and encoded to 8 bits to preserve object detail. Figure 5 plots the computed perceptual gamut volume in CIECAM02 lightness (J) and chroma (acbc) for each of these successive display encodings under CIECAM02 dark viewing conditions.
In the chroma (acbc) plane shown in Fig. 6, the respective chroma precepts are plotted along with those of the MacAdam Limits (shown in red) computed at 100 cd/m2 for diffuse white. Because the MacAdam Limits represent the locus of pure spectral colors, the display gamut virtually contains the locus of pure spectral colors at a maximum luminance of 800 cd/m2 or 11 bits of luminance channel encoding. Similarly, perceptual gamut volume in CIECAM02 brightness and colorfulness also contains the locus of pure spectral colors in colorfulness within 11 bits of luminance channel encoding. At 11 bits and beyond, the CIECAM02 extrapolated colors are predicted to be perceived brighter and more colorful than those within the MacAdam Limits of pure spectral colors.
Figure 7 quantifies the relative increase in perceptual gamut volume (volume ratio) in lightness and chroma as shown in Fig. 5 and further illustrates the effect of viewing condition as the display's maximum luminance or number of encoded bits in luminance is increased. Under normal viewing conditions, the relative gamut volume in lightness and chroma effectively doubles for each doubling of maximum luminance or added bit of luminance encoding. Under dim and dark viewing conditions as the surround becomes successively less bright and the display lower in perceived contrast, the effect diminishes correspondingly.
The pairs of images in Fig. 8 – (a) Grand Tetons, (b) Neon, and (c) the Stanford Memorial Church – are included only to illustrate the effect of "pushing down" the white point in relative luminance. In each of the three images, a region was chosen as diffuse white: the patch of snow on the mountain in Grand Tetons, the white paper in the foreground of Neon, and the skylight in the rotunda of the Stanford Memorial Cathedral. Each version of these images was rendered by mapping diffuse white to a white point 25% below the original 8-bit image (i.e., rendered as 6-bit images).
Fig. 7: Relative increase in perceptual gamut volume in lightness and chroma as a function of the number of bits of luminance encoding for the CIECAM02 viewing conditions of dark, dim, and normal.
Fig. 8: (a) The Grand Tetons, (b) Neon, and (c) the Stanford Memorial Church images clipped to the display's white point on the left and fully rendered on the right. The illustrations on the right allow for two additional bits of encoding beyond diffuse white (i.e., the display maximum luminance is four times that used to represent diffuse white).
On the left, those portions of the images with a luminance above the white point were clipped to 6 bits and represent the more traditional methodology of rendering the white point to the maximum luminance of the media. On the right, luminance above the white point – the sunlit trees, the neon sign, and the stained-glass-window lit portions of the cathedral – were maintained at 100% of the original for a full 8 bits.
The fact that these images appear dark as rendered for this article is due to the limitations of the printed media. Viewed in a dark surround on the Munsell Color Science Laboratory's (MCSL) HDR display, which has a dynamic range of over five orders of magnitude, the images do not appear dark, and those portions above their respective diffuse white appear strikingly brilliant – more representative of what perhaps was actually seen and demonstrating a significant perceived gamut expansion in lightness, chroma, brightness, and colorfulness. In addition, a presentation is available on the MCSL Web site (http:// mcsl.rit.edu/) that demonstrates the effect when viewed at a leisurely pace in a dark room using a digital projector.8
A methodology that shows promise for producing a fuller visual experience, particularly in the emerging HDR-display media technologies,9 was demonstrated both empirically and in a limited set of images. In this methodology, knowledge of the powers of adaptation and CIECAM02 are exploited to expand the perceptual gamut in lightness, chroma, brightness, and colorfulness beyond the MacAdam Limits of pure spectral colors by simply "pushing down" the white point of the display.
This effect of "pushing down" the white point is not unknown and continues to be common practice in photographic systems where diffuse white is encoded at a density greater than the minimum available in transparencies or less than the maximum available in negative films in order to render those "…parts of the scene having luminance greater than that of the reference white (such as specular reflections)…" 10 Similarly, the powers of adaptation are not unknown. For the spot-lit dancer referenced earlier, adaptation has been fully exploited by a theater's lighting director. And returning to the cathedral at Chartres, as far back as the 11th and 12th centuries in France, gothic architects used light and adaptation to invoke those perceptions described in the stained-glass windows at Chartres.
While the methodology presented here seems straightforward on the surface, the implications of implementing such a methodology are not. "Pushing down" the white point is only feasible if contrast resolution is maintained throughout the full dynamic range of the display. Yet, contrast resolution in an expanded perceptual gamut is only maintained if the image data are encoded to more than the 8 bits per channel. While such an expanded encoding is supported in professional graphic-arts markets today, it is not yet fully supported in today's consumer and commercial media markets with imaging standards (e.g., JPEG and MPEG), nor is it supported in the image capture, processing, storage, and display devices themselves and their respective interfaces. The promise of emerging media technologies such as HDR displays capable of displaying the full range of the visual experience should be more than compelling enough to motivate media manufacturers to develop such new standards and devices as these technologies continue to develop.
1J. R. Johnson, The Radiance of Chartres: Studies in the Early Stained Glass of the Cathedral, Columbia University Studies in Art History and Archaeology 4 (Random House, New York, 1965).
2V. Scully, Architecture (St. Martin's Press, New York, 1991), pages 123-125.
3D. MacAdam, "Maximum Visual Efficiency of Colored Materials," J. Opt. Soc. Am. 25, 36 (1935).
4R. S. Berns, Billmeyer and Saltzman's Principles of Color Technology (John Wiley & Sons, 2000), pp. 168-169.
5R. L. Heckaman and M. D. Fairchild, "Expanding Display Color Gamut Beyond the Spectrum Locus," Color Research and Application31, No. 6, 475-482 (2006).
7CIE, A Colour Appearance Model for Colour Management Systems: CIECAM02, CIE Technical Report, 2003.
8M. D. Fairchild and R. L. Heckaman, "Using HDR display technology and color appearance modeling to create display color gamut that exceed the spectrum locus," ISCC Special Topics Conference on Precision and Accuracy in the Determination of Color in Images (2005); download at www.cis.rit.edu/ fairchild/pubs.html.
9E. Reinhard, G. Ward, S. Pattanaik, and P. Debevec, High Dynamic Range Imaging: Acquisition, Display, and Image-Based Lighting(Morgan Kaufmann Publishers, San Francisco, 2005).
10R. W. G. Hunt, The Reproduction of Colour in Photography, Printing & Television, 4th ed. (Fountain Press, Tolworth, England, 1987), p. 54.
The authors acknowledge the Macbeth-Engel Fellowship in color science for making this work possible. •