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Suparna Kalghatgi, James Ferwerda; Effects of microscale and mesoscale structure on surface appearance. Journal of Vision 2010;10(7):449. doi: 10.1167/10.7.449.
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Real-world surfaces typically have geometric features at a range of spatial scales. At the microscale, opaque surfaces are often characterized by bidirectional reflectance distribution functions (BRDFs), which describe how a surface scatters incident light. At the mesoscale surfaces often exhibit visible texture – stochastic or patterned arrangements of geometric features that provide visual information about tactile surface properties such as roughness, smoothness, softness, etc.. These textures also affect how light is scattered by the surface, but the effects are at a different spatial scale than those captured by the BRDF. Normally both microscale and mesoscale surface properties contribute to overall surface appearance, however under particular illumination and viewing conditions, one or the other may dominate. In this project we investigated how microscale and mesoscale surface properties interact to determine perceived surface lightness. We measured the BRDFs and textures of flat surfaces covered with matte latex wall paints applied by spray or roller, then created computer graphics models of these surfaces and rendered center/surround targets with identical BRDFs but different textures. Observation of the images under directional lighting shows that as the viewing angle changes from normal to grazing, the lightness contrast of the center and surround regions change non-monotonically with the rougher textured surface first appearing lighter than the smoother one, then darker as the specular angle is approached, then potentially lighter again near grazing. This complex behavior is due to both the surface physics and simultaneous contrast effects, and is the cause of the well-known “touch-up problem” in the paint industry. We have conducted psychophysical studies that characterize how the perceived lightness differences of surfaces vary with BRDF and texture properties, and are developing models that can predict lightness differences for various lighting and viewing conditions, and provide prescriptions for minimizing the effect.
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