Accuracy in discriminating rigid from nonrigid motion was investigated for orthographic projections of three-dimensional rotating objects. In 3 experiments the hypothesis that magnitudes of angular velocity are misperceived in the kinetic depth effect was tested, and in 4 other experiments the hypothesis that misperceiving angular velocities leads to misperceiving rigidity was tested. The principal findings were (a) the magnitude of perceived angular velocity is derived heurisdcally as a function of a property of the first-order optic flow called deformation and (b) perceptual performance in discriminating rigid from nonrigid motion is accurate in cases when the variability of the deformations of the individual triplets of points of the stimulus displays favors this interpretation and not accurate in other cases. The human perceptual system is capable of extracting three-dimensional (3-D) information from moving images from which every static pictorial cue to depth has been removed, a phenomenon called the kinetic depth effect (Wallach & O'Cormell, 1953). Numerous attempts to reach a theoretical understanding of this phenomenon have been

Observers viewed monocular animations of rotating dihedral angles and were required to indicate their perceived structures by adjusting the magnitude and orientation of a stereoscopic dihedral angle. The motion displays were created by directly manipulating various aspects of the image velocity field, including the mean translation, the horizontal and vertical velocity gradients, and the manner in which these gradients changed over time. The adjusted orientation of each planar facet was decomposed into components of slant and tilt. Although the tilt component was estimated with a high degree of accuracy, the judgments of slant exhibited large systematic errors. The magnitude of perceived slant was determined primarily by the magnitude of the velocity gradient scaled by its direction. The results also indicate that higher order temporal derivatives of the moving elements had little effect on observers’ judgments. A fundamental issue in the theoretical analysis of threedimensional (3-D) structure from motion concerns the number of distinct views that are required for different types of perceptual judgments. Whereas the first-order relations between pairs of views provide sufficient information

Binocular disparity and motion parallax are powerful cues to the relative depth between objects. However to recover absolute depth, either additional scaling parameters are required to calibrate the information provided by each cue, or it can be recovered through the combination of information from both cues (Richards, W. (1985). Structure from stereo and motion. Journal of the Optical Society of America, 2, 343 -- 349). However, not all tasks necessarily require a full specification of the absolute depth structure of a scene and so psychophysical performance may vary depending on the amount of information available, and the degree to which absolute depth structure is required. The experiments reported here used three different tasks that varied in the type of geometric information required in order for them to be completed successfully. These included a depth nulling task, a depth-matching task, and an absolute depth judgement (shape) task. Real world stimuli were viewed (i) monocularly with head movements, (ii) binocularly and static, or (iii) binocularly with head movements. No effect of viewing condition was found whereas there was a large effect of task. Performance was accurate on the matching and nulling tasks and much less accurate on the shape task. The fact that the same perceptual distortions were not evident in all tasks suggests that the visual system can switch strategy according to the demands of the particular task. No evidence was found to suggest that the visual system could exploit the simultaneous presence of disparity and motion parallax. 2000 Elsevier Science Ltd. All rights reserved.

Perceived orientation of axis of rotation and accuracy in discriminating fixed-axis from nordixed-axis rotations were investigated for orthographic projections of three-dimensional rotating objects. The principal findings were (a) the slant of the axis of rotation was systematically misperceived; (b) in both two-view and multiview displays, the perceived slant of the axis of rotation was well-predicted by the ratio between the deformation (a property of the first-order optic flow) and the component parallel to the image plane of the global velocity vector; (c) if this ratio was kept constant in each frame transition of the stimulus sequence (or it was varied), then the stimuli tended to be judged as fixed-axis rotations (or as nonfixed-axis rotations), regardless of whether they simulated a fixed-axis rotation or not; and (d) the tilt of the axis of rotation was perceived in two-view displays with a very small error. A changing two-dimensional (2-D) projection of an object's motion gives rise to a compelling impression of a volumetric shape moving in three-dimensional (3-D) space. This phenomenon (called the kinetic depth effect [KDE] after Wallach & O'Cormell, 1953) represents an essential

Four experiments related human perception of depth–order relations in structure-from-motion displays to current Euclidean and affine theories of depth recovery from motion. Discrimination between parallel and nonparallel lines and relative-depth judgments was observed for orthographic projections of rigidly oscillating random-dot surfaces. We found that (1) depth–order relations were perceived veridically for surfaces with the same slant magnitudes, but were systematically biased for surfaces with different slant magnitudes. (2) Parallel (virtual) lines defined by probe dots on surfaces with different slant magnitudes were judged to be nonparallel. (3) Relative-depth judgments were internally inconsistent for probe dots on surfaces with different slant magnitudes. It is argued that both veridical performance and systematic misperceptions may be accounted for by a heuristic analysis of the first-order optic flow. Appropriate 2-D motions produce phenomenal impressions of movement in depth (see, e.g., Miles, 1931; Musatti, 1924; Wallach & O’Connell, 1953). Certain types of these phenomena have been named structure from motion (SFM). The questions of how these impressions arise and what type of geometric structure is derived from these motions have led to both experimental and theoretical work on depth recovery from motion. The psychophysical research has evaluated the capabilities of the human visual system in light of the constraints and the scope of the algorithms devised to derive 3-D geometric properties from 2-D motions (for a review, see Braunstein,

In three experiments, we investigated the integration of three-dimensional information provided over time by different depth cues. In the first experiment, we found that the perceptual derivation of surface orientation from the optic flow was affected by the prior presentation of static stereo information in the same spatial location. This bias weakened as the length of the motion sequence increased, but it was still present after 800 msec. In the second experiment, conversely, we found that the perceived orientation of a stereo-specified surface was not influenced by the prior presentation of a static stereo surface. In a third experiment, we found that two surfaces defined by identical disparity fields did not elicit the same perceived depth if, previously, one of them had been specified by a conjunction of stereo and motion information. This effect was found to last for at least 400 msec. Taken together, these findings indicate that interactions exist among different sources of depth information, even when they are provided at different moments of time. Human observers obtain information about threedimensional (3-D) shape from a large number of depth cues provided by the visual environment. In recent years, many studies have been carried out to investigate the

The relationship between simulated and judged depth separations for pairs of probe dots on planar surface patches was examined in a series of 6 experiments. The simulated slant of the patches was varied without varying the simulated depth separation of the probe dots by varying the depth gradient orthogonal to the direction determined by the probe dots on the image plane. Judged depth separation varied with mean slant for constant simulated depth separations. When observers judged depth separations along a closed path, the integral of the signed depths did not sum to zero, as would be required in Euclidean geometry. These results are inconsistent with the view that the mapping between simulated and perceived 3-D structure is alfme and indicate that, in general, the perceived structure cannot be represented in either a Euclidean space or an affine space. Moreover, these results are consistent with a first-order temporal analysis of the optic flow. A pattern of moving two-dimensional (2-D) features on a flat screen can give rise to a compelling impression of three-dimensionality. This phenomenon has been called the kinetic depth effect (Wallach & O'Connell, 1953) or structure

This study presents three ¢ndings concerning the mechanisms of depth perception. First, the shape of the three-dimensional percept evoked by two-frame motion is de¢ned solely by the rotation component around an axis in the frontoparallel plane; the visual system assigns a default value to this rotation component to arrive at a unique solution. Second, when the visual axes of two eyes are almost parallel, the visual system uses a default vergence value to reconstruct stereoscopic depth. Third, the default vergence and default rotation angles are highly correlated across subjects. This correlation implies that the two modalities share a common scaling default at an internal level.

Image movement provides one of the most potent twodimensional cues for depth. From motion cues alone, the brain is capable of deriving a three-dimensional representation of distant objects. For many decades, theoretical and empirical investigations into this ability have interpreted these percepts as faithful copies of the projected 3-D structures. Here we review empirical findings showing that perceived 3-D shape from motion is not veridical and cannot be accounted for by the current models. We present a probabilistic model based on a local analysis of optic flow. Although such a model does not guarantee a correct reconstruction of 3-D shape, it is shown to be consistent with human performance. To perceive the 3-D shape of objects from two-dimensional

In three experiments, observers watched displays consisting of two or more areas that contained unidirectionally moving pixels. In half of the displays, ame area of pixels contained movement that corresponded to the projection of the front surface of a rotating cylinder. The total duration of the displays and the number of stimulus areas per displaly were varied. The subjects ’ task was to indicate whether or not a given display contained rotatiam.When the display time required to reach 75 % accuracy was determined, it was found that the number of stimuli per display had no effect; nor did it interact with other variables. One control experiment eliminated “pixel crowding ” at the edges of the rotating cylinders, with little effect on the results. Another control experiment found that the ability to discriminate rotating from linear motion declines with distance away from fixation. A fourth experiment showed that under conditions similar to the first three, subjects can make accurate shape discriminations, thereby suggesting that three-dimensional information contributed to the decisions made in the original experiments. On the basis of these results and previous data, it is suggested that in the present experiments structure was recovered from motion by the short-range process, and that this recovery engages attention to a relatively constant extent, regardless of the number of stimuli contained in a display. Shape discrimination based on structure from motion may require a more effortful form of attention. Copyright @ 1996 Elsevier Science Ltd. Motion perception Structure from motion Visual attenticm Short-range process