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this article and Chick Hebert and Andrew Beall for their help in constructing the apparatus. Correspondence concerning this article should be addressed to John W. Philbeck, Department of Psychology, University of Cal- ifornia, Santa Barbara, California 93106-9660. Electronic mail may be sent via Internet to philbck@psych.ucsb.edu

We are surrounded by surfaces that we perceive by visual means. Understanding the basic principles behind this perceptual process is a central theme in visual psychology, psychophysics and computational vision. In many of the computational models employed in the past, it has been assumed that a metric representation of physical space can be derived by visual means. Psychophysical experiments, as well as computational considerations, can convince us that the perception of space and shape has a much more complicated nature, and that only a distorted version of actual, physical space can be computed. This paper develops a computational geometric model that explains why such distortion might take place. The basic idea is that, both in stereo and motion, we perceive the world from multiple views. Given the rigid transformation between the views and the properties of the image correspondence, the depth of the scene can be obtained. Even a slight error in the rigid transformation parameters c...

Two experiments explored four factors that may influence people's judgments of exocentric (interobject) distances in virtual environments. Participants freely navigated in a simple virtual environment and repeatedly made magnitude estimations of exocentric distances. Distances were generally overestimated. An exponential model (Steven's power law) fit the data and exponent estimates were generally less than unity. Geometric field of view and the presence of feedback were found to have the strongest effect on accuracy. Display type (head-mounted vs. desktop) and the presence of additional perspective cues were less influential. Distance perception in VE's 3 . 1. Introduction In the last few years, major claims have been made about the potential efficacy of virtual environments (VE's) for training knowledge and skills (see Seidel & Chatelier, 1997; Winn, 1998). These applications generally require trainees to learn the spatial characteristics of a computer-generated environment and the...

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In this investigation of monocular perception of egocentric distance, the authors advocate the necessity of a perception-action approach because calibration is intrinsic to definite distance perception. A helmet-mounted camera and display were used to isolate optic flow generated by participants ' head movements toward a target, and participants ' reaches to place a stylus either in a target hole (Experiments 1, 2, and 4) or aligned under a target surface (Experiment 3) were analyzed. Conclusions are that binocular distance perception is accurate, monocular distance perception yields compression that is not eliminated by feedback, but feedback is used to eliminate underestimation generated by restriction of the size of the visual field. The study of definite distance perception requires a perception-action approach. As we argue, the reason is twofold. First, definite distance perception entails calibration and, therefore, a task-specific action that provides both feedback and a standard of accuracy. Calibration is complete once measurements are within a task-specific tolerance. The tolerance is determined by error variability and task requirements,

How the eye measures reality and virtual reality We, as a species, seem to have been fascinated with pictures throughout our history. The paintings at Niaux, Altamira, and Lascaux (Clottes, 1995; Ruspoli, 1986), for example, are known to be about 14,000 years old, but with the recently discovered paintings in the Grotte Chauvet, the origin of representational art appears to have been pushed back even further (Chauvet, Brunel Deschamps, & Hillaire, 1995; Clottes, 1996), to 20,000 years ago if not longer. 1 Thus, these paintings date from about the time at which homo sapiens sapiens first appeared in Europe (Nougier, 1969). We should remember these paintings in the context of virtual reality; our fascination with pictures is by no means recent. My intent is threefold: first, to discuss our perception of the cluttered layout, or space, that we normally find around us; second, to discuss the development of representational art up to our current appreciation of it; and third, to apply this knowledge to virtual reality systems. The first discussion focuses on the use of multiple sources of information specifying ordinal depth relations, within the theoretical framework that I have called directed perception

Introduction Virtual environments offer a safe and cost effective way to expose people to situations that are inaccessible, dangerous, or simply too costly to otherwise expose them to. As such, their use is highly desirable for many types of training operations. The utility of virtual environment for training is restricted, however, by poor transfer of spatial knowledge from the virtual environment to the real world (Witmer, Bailey, Knerr, and Parsons, 1996; Bliss, Tidwell and Guest, 1997; Waller, Hunt and Knapp, 1998; Darken and Banker, 1998). If virtual environments are to be useful for high risk training scenarios, this problem must be overcome. One possible cause of this training transfer problem may be the poor distance perception that typically accompanies immersion in a virtual environment. Numerous studies have found observers significantly underestimate egocentric distance judgements while immersed in a virtual environment (Witmer and Kline, 1998; Witmer and Sadowski,

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The allocation of perceived size and perceived motion or displacement in depth resulting from retinal size changes (changes in the visual angle of the stimulus) was investigated in situations in which all other cues of perceived changes in distance were absent. The allocation process was represented by the size–distance invariance hypothesis (SDIH), in which, for a given change in visual angle, the perceived depth was determined only by the amount of size constancy available. The changes in perceived size and perceived distance (perceived depth) were measured by kinesthetic observer (open-loop) adjustments in five situations. These situations consisted of optical expansions or contractions presented successively or simultaneously or as a mixture of successive and simultaneous presentations. The amounts of perceived motion or perceived displacement in depth obtained by kinesthetic measures were compared with those obtained from size constancy measures as applied to the SDIH. This latter measure accounted for more of the perceived depth obtained from simultaneous and mixed situations than it did for the perceived depth from the successive situations and more for the perceived depth obtained from the expansion than from the contraction situations, whether these were simultaneous or mixed. Perceived rigidity of the stimulus (perfect size constancy) clearly was not obtained in any of the situations. Significant partial size constancy and some predictive ability of the perceived sagittal motion