How direction of illumination affects visually perceived surface roughness.

We examined visual estimation of surface roughness using random, computer-generated, three-dimensional (3D) surfaces rendered under a mixture of diffuse lighting and a punctate source. The angle between the tangent to the plane containing the surface texture and the direction to the punctate source was varied from 50 to 70 deg across lighting conditions. Observers were presented with pairs of surfaces under different lighting conditions and indicated which 3D surface appeared rougher. Surfaces were viewed either in isolation or in scenes with added objects whose shading, cast shadows, and specular highlights provided information about the spatial distribution of illumination. All observers perceived surfaces to be markedly rougher with decreasing illuminant angle. Performance in scenes with added objects was no closer to constant than that in scenes without added objects. We identified four novel cues that are valid cues to roughness under any single lighting condition but that are not invariant under changes in lighting condition. We modeled observers' deviations from roughness constancy as a weighted linear combination of these "pseudocues" and found that they account for a substantial amount of observers' systematic deviations from roughness constancy with changes in lighting condition.