The human visual system can extract 3-D shape information of unfamiliar moving objects from their projected transformations. Computational studies of this capacity have established that 3-D shape, can be extracted correctly from a brief presentation, provided that the moving objects are rigid. The human visual system requires a longer temporal extension, but it can cope, however, with considerable deviations from rigidity. It is shown how the 3-D structure of rigid and non-rigid objects can be recovered by maintaining an internal model of the viewed object and modifying it at each instant by the minimal non-rigid change that is sufficient to account for the observed transformation. The results of applying this incremental rigidity scheme to rigid and non-rigid objects in motion are described and compared with human perceptions.

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We explore the use of the non-dominant hand to control a virtual camera while the dominant hand performs other tasks in a virtual 3D scene. Two experiments and an informal study are presented which evaluate this interaction style by comparing it to the status-quo unimanual interaction. In the first experiment, we find that for a target selection task, performance using the bimanual technique was 20% faster. Experiment 2 compared performance in a more complicated object docking task. Performance advantages are shown, however, only after practice. Free-form 3D painting was explored in the user study. In both experiments and in the user study participants strongly preferred the bimanual technique. The results also indicate that user preferences concerning bimanual interaction may be driven by factors other than simple time-motion performance advantages.

A method for structure from motion is presented. The method makes a motion assumption about the objects being viewed. The motion assumption is that all motion consists of translations and rotations about a fixed axis. Parallel projection is also assumed. This makes it possible to interpret the motion of as few as two rigidly connected points. The method works for both rigid and jointed objects. Results of a test of this method on Johansson's data are presented. 1.

In theoretical analyses of visual form perception, it is often assumed that the 3-dimensional structures of smoothly curved surfaces are perceptually represented as point-by-point mappings of metric depth and/or orientation relative to the observer. This article describes an alternative theory in which it is argued that our visual knowledge of smoothly curved surfaces can also be denned in terms of local, nonmetric order relations. A fundamental prediction of this analysis is that relative depth judgments between any two surface regions should be dramatically influenced by the monotonicity of depth change (or lack of it) along the intervening portions of the surface through which they are separated. This prediction is confirmed in a series of experiments using surfaces depicted with either shading or texture. Additional experiments are reported, moreover, that demonstrate that smooth occlusion contours are a primary source of information about the ordinal structure of a surface and that the depth extrema in between contours can be optically specified by differences in luminance at the points of occlusion. For many higher organisms, including humans, a primary source of knowledge about objects and events in the surrounding environment is provided by vision. Because of the ecological

This article reviews the more powerful 3D depth cue display techniques and then presents a formal experimental study that examines the power of cast shadows in providing depth 4

This paper describes a Computer Aided Design system for sketching free-form polygonal surfaces such as terrains and other natural objects. The user manipulates two 3D position and orientation trackers with three buttons, one for each hand. Each hand has a distinct role to play, with the dominant hand being responsible for picking and manipulation, and the less-dominant hand being responsible for context setting of various kinds. The less-dominant hand holds the workpiece, sets which re nement level that can be picked by the dominant hand, sets the constraint mode and the reshape operator, and generally acts as a counterpoint to the dominant hand. In this paper, the architecture of the system is outlined, the interaction techniques are presented, and a simple surface is shown. KEYWORDS:

Research on the perception of texture gradients has relied heavily on the subjective reports of observers engaged in free-viewing. We asked whether these findings generalized to speeded performance. Experiment 1 showed that an important aspect of subjective perception—sizeconstancy scaling with perceived distance—also predicted the speed of pop-out visual search for cylinders viewed against a texture gradient. Experiment 2 showed that this finding could not be attributed to the local contrast between search items and the background texture. Experiment 3 assessed the relative contributions of 2 separable dimensions of texture gradients—perspective (radial spreading) and compression (foreshortening)—finding them to be independent in the more rapid search conditions (long target among shorter distractors) but combined in their influence in the slower conditions (short target among longer distractors). When observers view the texture gradient shown in Figure 1A they usually report seeing a flat surface recede into the distance, despite the fact that a two-dimensional (2-D) image alone cannot specify the three-dimensional (3-D) surface that gave rise to the projection. This study asked whether the factors influencing the perceived slant of such texture gradients also influences rapid visual search for objects placed on their surface. Although a large number of previous studies have examined the perception of slant in texture gradients (e.g., Flock,

We examine a number of investigations of perceptual economy or, more specifically, of minimum tendencies and minimum principles in the visual perception of form, depth, and motion. A minimum tendency is a psychophysical finding that perception tends toward simplicity, as measured in accordance with a specified metric. A minimum principle is a theoretical construct imputed to the visual system to explain minimum tendencies. After examining a number of studies of perceptual economy, we embark on a systematic analysis of this notion. We examine the notion that simple perceptual representations must be defined within the "geometric constraints " provided by proximal stimulation. We then take up metrics of simplicity. Any study of perceptual economy must use a metric of simplicity; the choice of metric may be seen as a matter of convention, or it may have deep theoretical and empirical implications. We evaluate several answers to the question of why the visual system might favor economical representations. Finally, we examine several accounts of the process for achieving perceptual economy, concluding that those which favor massively parallel processing are the most plausible. The notions of "simplicity " and "economy" have been used in varied contexts within the sciences (see Sober, 1975). It was a commonplace of classical physics and astronomy that "nature acts by the simplest means. " Euler, Lagrange, Hamilton, and others have shown that the central equations of mechanics can be formulated isoperimetrically (in terms of maximum/minimum solutions). In a broader vein, methodologists have proposed that scientists proceed in accordance with the principle of parsimony, which holds that of two theories with equal empirical adequacy, the simpler theory should be chosen. A century ago Mach 0883/1960, 1919) referred this principle to a psychological preference of the scientific investigator for economy of thought. Finally, psychologists have found a tendency

ohject when its visible ends share a common lateral translation in space. The present work investigated the class of motion relationships that can specify object unity to infants, specifically, asking whether it includes all rigid translations. 3 experiments tested the infonnativcness ot 2 a.xes of translation not previously studied: translation in depth and vertical tianslation. These motions also allowed assessment of certain interpretations of previous results that invoke specific sensor\' consequences of lateral movement, rather than perceived motion, as underlying perceived unity\ Experiment 1 provided evidence that a small extent of translation in depth specified the unit\ ' of an object, but only to the subgroup of infants who detected the motion. Experiment 2 used a greater displacement in depth and found clear evidence for perception of object unity. Experiment 3 indicated that vertical translation, in which the 2 visible areas of the partly hidden object undergo dissimilar changes, also specifies object unity to infants. These results suggest that infants ' perception of object unity depends on perceived coherence of motion, no matter how specified, and that the class of informative motions includes all rigid translations. Kellman and Spelke (1983) reported that shape, they showed equal dishabituation to a young infants perceive the unity of partly oc- complete and a broken object (Kellman &