Abstract
Previous research has shown that human observers are quite accurate at judging the locations of local depth extrema in monocular images of smoothly curved surfaces with polka dot textures (Todd, Oomes, Koenderink & Kappers, 2004). One possible source of information on which these judgments could be based involves the relative spatial frequencies of the projected texture patterns in different local regions. For surface textures that are physically isotropic, local depth extrema (i.e. maxima or minima) in any given direction will be optically specified by local minima in the spatial frequency of the projected image texture in that direction. This correspondence between local depth extrema on a surface and local minima of spatial frequency in an image does not necessarily occur, however, for surfaces with anisotropic textures. Thus, in an effort to evaluate the extent to which observers rely on this information, the present research was designed to investigate how texture anisotropies in different orientations influence the apparent positions of local depth extrema on smoothly curved surfaces. The displays depicted randomly shaped objects with volumetric blob textures. The textures were created by carving each object from a volume of small blobs, which could be stretched in different directions to create systematic patterns of anisotropy. Observers judged the ordinal depth on each object by marking local maxima and minima along designated scan lines. The results revealed that that the perceived locations of local depth extrema were more highly correlated with the positions of local spatial frequency minima in the projected image texture, than with the locations of the actual depth extrema on the depicted surface. These findings suggest that the perception of 3D shape from random blob textures may be based on an implicit assumption that the physical texture on a surface is spatially isotropic.