Abstract
This study examined perceptual differentiation of specular from diffuse shading for the recovery of surface color and gloss. In Experiment 1, we parametrically varied the mesoscale relief height of globally planar surfaces, specular sharpness and the orientation of the surface relative to the light source. We obtained psychophysical matches for perceived color saturation and value (HSV), but also considered whether the main effects could be influenced by color space used when transforming data to perceptually-uniform CIE LCH space. Results revealed strong interactions between perceived color attributes and the lighting conditions, the structure of specular reflections, and surface relief. Declines in saturation were observed with increasing specular roughness (using an HSV color representation), but no similar decline was observed in chroma (using a CIE LCH color representation). Experiment 2 found strong negative correlations between perceived gloss and specular roughness. Perceived gloss also depended on mesoscopic relief height and orientation of the surface relative to the light source. Declines in perceived gloss moderately accounted for the variability in color saturation and value matches obtained in Experiment 1. We found information about perceived specular coverage could further improve the model’s accountability of perceived color saturation and lightness (Experiment 3). These findings together suggest that perceived color saturation and color value depends on the visual system’s ability to distinguish the underlying diffuse shading from specular highlights in images.
Highlights
Surfaces vary in shape, color, gloss and a host of other properties
We focused on HSV because the matching task was configured to perform the task in this way as it was the simplest color space to use when instructing participants on making different dimensional settings for their color matches
In Experiment 2, we found that perceived gloss could account for much of the variability in perceived color saturation and value imposed by specular roughness alone, but not the variability introduced by changes in physical relief height
Summary
Color, gloss and a host of other properties (e.g., texture and opacity). The formation of natural images depends on complex interactions between the structure of prevailing illumination, 3D surface shape and reflectance, as well as the viewing direction. Much of this structure in images can be modeled using an idealized bi-directional reflectance distribution function (BRDF) (Nicodemus, 1965). Separate diffuse and specular components within this model respectively characterize different reflectance properties of surfaces. The diffuse component is determined by Lambertian reflectance, which is viewpoint-independent shading generated by the orientation of surface normals relative to the light source. The specular component describes the viewpoint-dependent shading generated by the orientation of surface normals relative to both the viewing direction and the prevailing light source(s). Specular shading is informative of surface gloss, and the 3D shape of surfaces (Fleming et al, 2003, 2004)
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