The effect of viewpoint on perceived visual roughness.

In previous work, we examined how the apparent roughness of a textured surface changed with direction of illumination. We found that observers exhibited systematic failures of roughness constancy across illumination conditions for triangular-faceted surfaces where physical roughness was defined as the variance of facet heights. These failures could be due, in part, to cues in the scene that confound changes in surface roughness with changes in illumination. These cues include the following: (1) the proportion of the surface in shadow, (2) mean luminance of the nonshadowed portion, (3) the standard deviation of the luminance of the nonshadowed portion, and (4) texture contrast. If the visual system relied on such "pseudocues" to roughness, then it would systematically misestimate surface roughness with changes in illumination much as our observers did despite the availability of depth cues such as binocular disparity. Here, we investigate observers' judgments of roughness when illumination direction and surface orientation are fixed and the observers' viewpoint with respect to the surface changes. We find a similar pattern of results. Observers exhibited patterned failures of roughness constancy with change in viewpoint, and an appreciable part of their failures could be accounted for by the same pseudocues. While the human visual system exhibits some degree of roughness constancy, our results lead to the conclusion that it does not always select the correct cues for a given visual task.