Planar motion permits perception of metric structure in stereopsis

Abstract

A fundamental problem in the study of spatial perception concerns whether and how vision might acquire information about the metric structure of surfaces in three-dimensional space from motion and from stereopsis. Theoretical analyses have indicated that stereoscopic perceptions of metric relations in depth require additional information about egocentric viewing distance; and recent experiments by James Todd and his colleagues have indicated that vision acquires only affine but not metric structure from motion-that is, spatial relations ambiguous with regard to scale in depth. The purpose of the present study was to determine whether the metric shape of planar stereoscopic forms might be perceived from congruence under planar rotation. In Experiment 1, observers discriminated between similar planar shapes (ellipses) rotating in a plane with varying slant from the frontal-parallel plane. Experimental conditions varied the presence versus absence of binocular disparities, magnification of the disparity scale, and moving versus stationary patterns. Shape discriminations were accurate in all conditions with moving patterns and were near chance in conditions with stationary patterns; neither the presence nor the magnification of binocular disparities had any reliable effect. In Experiment 2, accuracy decreased as the range of rotation decreased from 80°to 10°. In Experiment 3, small deviations from planarity of the motion produced large decrements in accuracy. In contrast with the critical role of motion in shape discrimination, motion hindered discriminations of the binocular disparity scale in Experiment 4. In general, planar motion provides an intrinsic metric scale that is independent of slant in depth and of the scale of binocular disparities. Vision is sensitive to this intrinsic optical metric.Everyday human experience and performance in perceiving constant object shapes from changing perspectives in varying contexts indicate that vision acquires information about the spatial structure of environmental objects with impressive speed, reliability, precision, and generality. Despite the familiarity and functional importance of these visual achievements, we lack a theoretical understanding of the optical information and visual mechanisms on which they are based.Indeed, the specific geometric relations that might be used by vision to perceive shape have seldom been directly evaluated by psychophysical techniques. Sperling, Landy, Dosher, and Perkins (1989) recently criticized the literature on perceiving three-dimensional (3-D) structure from motion for having failed to provide such evidence. The apparent 3-D shape constancy of perceived environmental objects, however, suggests that vision usually provides information about the metric structure of environmental This research was supported in part by NIH Grants EY05926 and P30EY08126. Experiments 1-3 were conducted by S. R. Love as part of the honors program in psychology at Vanderbilt. We are grateful to Farley Nonnan and John Kuster for their help in conducting Ex...

Compression of visual space in natural scenes and in their photographic counterparts

“…This is not to say that subjective space is necessarily nonmetric. In particular, we have not investigated any 2-D cues that could have helped or interfered with performance (Lappin & Love, 1992;Pizlo & SalachGolyska, 1994). However, our results agree better with contentions that visual space cannot be modeled by any standard geometry (Pizlo, Rosenfeld, & Weiss, 1997) than they agree with models of metric distortion.…”

Environmental context influences visually perceived distance

“…A fundamentally different prediction about visual scaling of space from optic flow is also plausible, based on the congruence of objects in different positions at different times: If A and B are two stationary objects at image positions a and b in the sagittal plane, and if the image of A moves from a to b at the same time that B moves from b to c during an observer's translation, the distance from a to b equals that from b to c. Thus, vision might possibly achieve veridical scaling of space from locomotion. Lappin and Love (1992) and Lappin and Ahlström (1994) showed both theoretically and experimentally that under certain restricted conditions, this measurement procedure can yield accurate perception of three-dimensional metric structure from motion.…”

The perceptual analysis of structure from motion for rotating objects undergoing affine stretching transformations

“…Furthermore, it has been mathematically proven (Bennett, Hoffman, Nicola, & Prakash, 1989;Huang & Lee, 1989;Koenderink & van Doom, 1991;Todd & Bressan, 1990) that a unique 3-D structure cannot be obtained from apparent motion sequences containing only two orthographic views of a moving 3-D object, except in certain specialized cases-as, for example, when a configuration of moving points is confined to a fixed plane (see Hoffman & Flinchbaugh, 1982;Lappin & Love, 1992). On theoretical grounds, it would therefore appear that three distinct orthographic views are both necessary and sufficient for determining 3-D structure from motion.…”