The effects of visual angle on the perception of 3D curvature from texture

Abstract

At the 2002 VSS meeting there was conflicting evidence presented about whether it is possible to perceive the direction of surface slant or the sign of surface curvature from perspective gradients of isotropic textures. In the present paper a theoretical analysis will be presented that quantifies the available information from texture gradients for a wide range of geometric conditions. An important implication of this analysis is that the discrimination of concave and convex surface regions may require integration over relatively large visual angles. A series of psychophysical experiments will also be reported that examined the effects of visual angle on observers' judgments of surface curvature from texture. Images of singly curved surfaces with polka dot or plaid textures were projected onto a translucent display screen that could be viewed at a comfortable distance with a large visual angle. Observers indicated the perceived 3D shape of each surface by adjusting a profile of its cross-section in depth on a separate monitor. The results revealed that observers were significantly less accurate at judging the sign of curvature when the visual angle of the depicted surface was reduced over a range of values from 60–20 degrees. Reductions in visual angle also lowered the perceived magnitude of curvature. These results provide strong evidence that that the perception of shape from texture can involve pooling of information over large regions of visual space, and that other reported results obtained with small visual angles may have led to erroneous conclusions about observers' perceptual capabilities.