Effects of Image Dynamic Range on Apparent Surface Gloss.

In this paper we present results from an experiment designed to investigate the effects of image dynamic range on apparent surface gloss. Using a high dynamic range display, we present high dynamic range (HDR) and standard dynamic range (tone mapped, SDR) renderings of glossy objects in pairs and ask subjects to choose the glossier object. We analyze the results of the experiments using Thurstonian scaling, and derive common scales of perceived gloss for the objects depicted in both the HDR and SDR images. To investigate the effects of geometric complexity, we use both simple and complex objects. To investigate the effects of environmental illumination, we use both a simple area light source and a captured, real-world illumination map. Our findings are 1) that limiting image dynamic range does change the apparent gloss of surfaces depicted in the images, and that objects shown in SDR images are perceived to have lower gloss than objects shown in HDR images; 2) that gloss differences are less discriminable in SDR images than in HDR images; and 3) that surface geometry and environmental illumination modulate these effects. Introduction One of the defining characteristics of glossy surfaces is that they reflect images of their surroundings. High gloss surfaces produce sharp detailed reflection images that clearly show all the features of the surround, while low gloss surfaces produce blurry images that only show bright “highlight” features. Due to the presence of light sources and shadows, the illumination field incident on glossy surfaces can have high luminance dynamic range. This means that the reflections from glossy surfaces can also be high dynamic range. However in conventional images of glossy objects, these high dynamic range reflections must be clipped or compressed through tone mapping so the images fit within the output range of the display medium (see Figure 1). While the utility of conventional display systems demonstrates that the general characteristics of glossy surfaces are still conveyed by these tone-mapped images, an open question is whether the tone mapping process distorts apparent gloss of the imaged surfaces. In this paper we present results from an experiment designed to investigate the effects of image dynamic range on apparent surface gloss using a high dynamic range display. In the experiments we present high dynamic range (HDR) and standard dynamic range (tone mapped, SDR) renderings of glossy objects in pairs and ask subjects to choose the glossier object. We analyze the results of the experiments using Thurstonian scaling, and derive common scales of perceived gloss for the objects depicted in both the HDR and SDR images. To investigate the effects of geometric complexity, we use both simple and complex objects. To investigate the effects of environmental illumination, we use both a simple area light source and a captured, real-world illumination map. Our findings are 1) that limiting image dynamic range does change the apparent gloss of surfaces depicted in the images, and that objects shown in SDR images are perceived to have lower gloss than objects shown in HDR images; 2) that objects differing slightly in gloss are less discriminable in SDR images than in HDR images, and 3) that surface geometry and environmental illumination modulate these effects. The following sections describe our methods and results. Figure 1. High dynamic range (HDR) and standard dynamic range (SDR) images of a bunny object. The image pair on the top looks similar in limited dynamic range prints, but would appear different on a high dynamic range display that could reproduce the full luminance range in the HDR image (see the false color image pair on the bottom). Related Work The earliest modern studies of gloss perception have been attributed to Ingersoll [1] who examined the appearance of glossy papers. In 1937, Hunter [2] observed at least six different visual attributes related to apparent gloss. He defined these as: specular gloss: perceived brightness associated with the specular reflection from a surface contrast gloss: perceived relative brightness of specularly and diffusely reflecting areas 17th Color Imaging Conference Final Program and Proceedings 193 distinctness-of-image (DOI) gloss: perceived sharpness of images reflected in a surface haze: perceived cloudiness in reflections near the specular direction sheen: perceived shininess at grazing angles in otherwise matte surfaces absence-of-texture gloss: perceived surface smoothness and uniformity In 1937, Judd [3] formalized Hunter’s observations by writing expressions that related them to the physical features of surface bidirectional reflectance distribution functions (BRDFs). Hunter and Judd’s research established a conceptual framework that has dominated work in gloss perception to the present day. In 1987, Billmeyer and O’Donnell [4] published an important paper that investigated the multidimensional nature of gloss perception. They collected ratings of the differences in apparent gloss between pairs of acrylic-painted panels with varying gloss levels viewed under a fluorescent desk lamp outfitted with a chicken-wire screen, then used multidimensional scaling techniques to discover the dimensionality of perceived gloss. For their experimental conditions, they found that gloss could be described by a single dimension. However, this work was significant because it was the first to study the multidimensional nature of gloss perception without preconceptions about how many or what the dimensions might be. In a 1986 report to the CIE, Christie [5] summarized the research findings on gloss perception up to that date. Since that time, McCamy [6,7] has published a pair of review papers on the gloss attributes of metallic surfaces and Seve [8] and Lozano [9] have outlined frameworks for describing gloss that seek to improve on Hunter’s classifications. In the Imaging Science literature, there has been considerable interest in the effects of gloss on printed image quality with efforts to characterize artifacts like differential gloss, bronzing, and gloss mottle [10,11,12,13,14,15]. One of the challenges in conducting gloss perception research is producing and controlling the stimuli used in the experiments. Generating consistent physical samples is very difficult. Therefore, the development of physically-based computer graphics techniques that can produce and present radiometrically accurate images of complex scenes has been a boon to the psychophysical study of gloss perception. One of the earliest computer graphics studies was done by Nishida and Shinya [16] who rendered bumpy glossy surfaces using direct point lighting. They found that observers made consistent errors in matching gloss properties across different surface geometries and suggested that the results of their experiments could be explained with a simple image histogram matching strategy. Pellacini et al. [17] conducted a set of experiments inspired by Billmeyer and O’Donnell’s multidimensional scaling studies, but with images of a glossy ball inside a checkerboard box with a ceiling-mounted area light source. For this stimulus set, they found that observers used two dimensions to judge gloss, “c” a measure related to the contrast of the image reflected by the surface, and “d” a measure related to the sharpness of the reflected image. Ferwerda et al. [18] extended this work to characterize multidimensional gloss differences. More recent work has examined the role of natural illumination patterns [19] and complex object geometry [20] on surface gloss perception. Although computer graphics has greatly facilitated the study of gloss perception, one of the caveats of all of these studies is that they use images of glossy surfaces as stimuli rather than the physical surfaces themselves. Because the potentially high dynamic range reflections from glossy surfaces are compressed for display, there is the potential that the gloss properties of the displayed surfaces are distorted. In our experiment, we employ an HDR display to enable more accurate presentation of physicallybased glossy stimuli. Experiments We conducted a scaling experiment to investigate the effects of image dynamic range on apparent surface gloss. The stimuli and procedure are described in the following sections.