Distortions of depth-order relations and parallelism in structure from motion

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.     

Misperceptions of angular velocities influence the perception of rigidity in the kinetic depth effect

Accuracy in discriminating rigid from nonrigid motion was investigated for orthographic projections of three-dimension 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 heuristically 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.     

Postfusional latency in stereoscopic slant perception and the primitives of stereopsis.

Random dot stereograms of slanted surfaces were constructed, each representing one or two slanted surfaces in different relative arrangements and with different axes. Latency to fusion and from fusion to stereoscopic resolution was measured for each stimulus. It was found that latency to fusion was always very brief but that latency to stereoscopic resolution varied markedly, depending upon the orientation and arrangement of the stereoscopic surfaces. A gradient of discontinuities at a surface boundary produced an instant slant response for that surface, whereas a gradient of absolute disparities across the surface did not, except under conditions where vertical declination (a form of orientation disparity) was present. We conclude that stereopsis is not based on the primitives used in matching the images for fusion and that it is, at least initially, a response to disparity discontinuities which play no role in the fusion process. We also conclude that vertical declination is responded to globally as a slant around a horizontal axis but that other forms of orientation disparity are ineffective. The evidence from our experiments does not support the existence of a stereoscopic ability to respond globally to differences in magnification (or spatial frequency). It is suggested that stereoscopic perception of slant around a vertical axis is slow because it results from the integration of local processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perceived orientation of axis rotation in structure-from-motion.

Perceived orientation of axis of rotation and accuracy in discriminating fixed-axis from nonfixed-axis rotations were investigated for orthographic projections of three-dimensional rotating objects. The principal findings were (1) the slant of the axis of rotation was systematically misperceived; (2) 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; (3) 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 (4) the tilt of the axis of rotation was perceived in two-view displays with a very small error. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Speed gradients and the perception of surface slant: analysis is two-dimensional not one-dimensional

Motion parallax provides cues to the three-dimensional layout of a viewed scene and, in particular, to surface tilt and slant. For example, as a textured surface, inclined around a horizontal axis, translates horizontally relative to an observer's view point, then, in the absence of head and eye movements, the observer's retinal flow will contain a one-dimensional (1D) vertical speed gradient. The direction of this gradient indicates the direction of surface tilt, and its magnitude and sign can be used in calculating the magnitude and sign of the surface slant. Alternatively, the same retinal flow contains a 1D translating component, plus a two-dimensional (2D) component of rotation (curl), and a 2D component of deformation (def). On this view, the direction of surface tilt is related to the orientation of def and the magnitude and sign of the surface slant is related to the magnitude and sign of def. We used computer generated random dot patterns as stimuli to determine whether the human visual system employs a 1D analysis (i.e. 1D speed gradients) or a 2D analysis (i.e. deformation) of surface slant from motion parallax. Using a matching technique we found compelling impressions of slant when we vector summed a translation field with (i) vertical shear, horizontal shear or deformation (made from vertical and horizontal shear), but not rotation; and (ii) vertical compression, horizontal compression or deformation (made from vertical and horizontal compression), but much less so for expansion. In both cases, the first three conditions contain def, but the fourth does not, and the last three conditions contain 1D speed gradients orthogonal to the perceived axis of inclination, but the first one does not. Therefore, the results from the first and fourth conditions distinguish between the two processing strategies. They support the idea that surface slant is coded by combining both horizontal and vertical speed gradients in a way similar to the 2D differential invariant def and oppose the view that surface slant is encoded by a 1D analysis of motion in a direction orthogonal to the perceived axis of inclination. In a further experiment, we found essentially no effect of reducing the field size from 18 to 9 deg.     

Three theoretical views of slant perception.

3 current theories of slant perception are described. It is shown that the Gestaltist and Helmholtzian theories of slant perception depend on the shape-slant invariant and concomitantly on the familiarity of the presented shapes composing the surface (Helmholtzian) or on the goodness of the presented shapes (Gestaltist). In contrast the Gibsonian theory of slant perception depends on complex angular relations (optical texture gradients) in the optical texture (array of patterned light to the eye) without regard to the familiarity or goodness of the textural shapes composing the distal surface. (44 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The stereoscopic anisotropy: Individual differences and underlying mechanisms.

Observers are more sensitive to variations in the depth of stereoscopic surfaces in a vertical than in a horizontal direction; however, there are large individual differences in this anisotropy. The authors measured discrimination thresholds for surfaces slanted about a vertical axis or inclined about a horizontal axis for 50 observers. Orientation and spatial frequency discrimination thresholds were also measured. For most observers, thresholds were lower for inclination than for slant and lower for orientation than for spatial frequency. There was a positive correlation between the 2 anisotropies, resulting from positive correlations between (a) orientation and inclination thresholds and (b) spatial frequency and slant thresholds. These results support the notion that surface inclination and slant perception is in part limited by the sensitivity of orientation and spatial frequency mechanisms. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perceived orientation of axis of rotation in structure-from-motion

Perceived orientation of axis of rotation and accuracy in discriminating fixed-axis from nonfixed-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.     

Effect of Fiber Orientation and Ply Mix on Fiber Reinforced Polymer-Confined Concrete

This paper explores the effects of fiber orientation and ply mix on load–deformation behavior and failure modes of fiber reinforced polymer (FRP) confined concrete by testing under uniaxial compression a designed array of plain concrete cylinders wrapped with different fabric orientation. Depending on the jacket confinement stiffness, either a strain hardening or a strain softening behavior was observed beyond the kink point where there was a sharp reduction in slope in the load–deformation curve. Kinking was seen to have a definable graphical relationship with the critical concrete lateral strain while the kink stress was found to upshift with increasing jacket stiffness. It is concluded that while hoop fiber wrapped concrete leads to brittle failures, angular fiber wrapped concrete tends to fail in a ductile manner, attributed to a fiber reorientation mechanism. Ply mix sequence plays an important role in the overall deformation and failure behavior. Existing models are found to be adequate in describing load–deformation behavior of angular fiber wrapped concrete as long as equivalent FRP properties in the hoop direction are used.     

Understanding collision dynamics.

In two experiments we investigated people's ability to judge the relative mass of two objects involved in a collision. It was found that judgments of relative mass were made on the basis of two heuristics. Roughly stated, these heuristics were (a) an object that ricochets backward upon impact is less massive than the object that it hit, and (b) faster moving objects are less massive. A heuristic model of judgment is proposed that postulates that different sources of information in an event may have different levels of salience for observers and that heuristic access is controlled by the rank ordering of salience. It was found that observers ranked dissimilarity in mass on the basis of the relative salience of angle and velocity information and not proportionally to the distal mass ratio. This heuristic model was contrasted with the notion that people can veridically extract dynamic properties of motion events when the kinematic data are sufficient for their specification. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Angular velocity and head direction signals recorded from the dorsal tegmental nucleus of gudden in the rat: Implications for path integration in the head direction cell circuit.

When a rat navigates through space, head direction (HD) cells provide an ongoing signal of the rat's directional heading. It is thought that these cells rely, in part, on angular path integration of the rat's head movements. This integration requires that the HD cell system receive information about angular head movements and that this information be combined with the current directional signal, to generate the next "predicted" direction. Recent data suggest that the dorsal tegmental nucleus (DTN) may play a critical role in helping to generate the HD cell signal. To test this, recordings were made from cells in the DTN in freely moving rats. The following cell types were found: (a) "classic" HD cells, (b) angular velocity cells, and (c) cells that fired as a function of both head direction and angular velocity. Thus, DTN cells exhibit firing characteristics that are critical to the neural circuit hypothesized for generation of the HD cell signal. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Types of size disparity and the perception of surface slant

We examined (i) perceived slant of a textured surface about a vertical axis as a function of disparity magnitude for horizontal-size disparity, vertical-size disparity, and overall-size disparity; and (ii) interactions between patterns with various types and magnitudes of size disparity and superimposed or adjacent zero-disparity stimuli. Horizontal-size disparity produced slant which increased with increasing disparity, was enhanced by superimposed zero-disparity stimuli, and induced contrasting slant in superimposed or adjacent zero-disparity stimuli. Vertical-size disparity produced opposite slant (induced effect) which was reduced to near zero by a superimposed zero-disparity pattern and both patterns appeared as one surface. Adjacent vertical-size-disparity and zero-disparity patterns appeared as separate surfaces with a wide curved boundary. Overall-size disparity produced slant which was enhanced by a superimposed zero-disparity pattern and less so by a zero-disparity line, and induced more slant in a zero-disparity line than in a zero-disparity pattern. The results are discussed in terms of depth underestimation of isolated surfaces, depth enhancement, depth contrast, and the processing of deformation disparity.     

Discriminating constant from variable angular velocities in structure from motion

We investigated accuracy in discriminating between constant and variable angular velocities for orthographic projections of three-dimensional rotating objects. The reported judgments of "constant" or "variable" angular velocity were only slightly influenced by the projected angular velocities, but they were greatly affected by the variations of the deformation, a first-order component of the optic flow. When viewing either a rotating ellipsoidal volume or a planar surface that accelerated and decelerated over the course of rotation, observers' tendencies to report a variable angular velocity were increased when the temporal phase of the acceleration pattern increased the range of variation of the median deformation; the tendencies were decreased when the same acceleration pattern was used to decrease the range of variation of the median deformation. These results provide evidence contrary to the hypothesis that the visual system performs a mathematically correct analysis of the optic flow.     

Variation in surface strain on the equine hoof wall at the midstep with shoeing, gait, substrate, direction of travel, and hoof shape

Objectives were to examine the deformation of the healthy equine front hoof during locomotion, by recording strains on its outer surface, and to test whether its mechanical behaviour is significantly altered under different locomotory conditions and variation in hoof shape. Strains were recorded in vivo from 5 rosette gauges around the circumference of the right forehooves of 12 horses. The magnitudes and orientations of principal strains at the midstep were compared statistically for different conditions of shoeing (shod vs. unshod), gait (walk vs. trot), substrate (treadmill vs. ground), and direction of travel (straight, right turn, left turn). Principal strains were regressed on 4 variables describing hoof shape-toe length, toe angle, and medial and lateral wall angle--to describe their contribution to variations in strain and hoof deformation. Shoeing did not essentially change the magnitudes of the larger, compressive principal strain, but caused some strain reorientation. Shoes decreased the variation in strains indicating that they tend to stabilise the deformation of the hoof. Strain magnitudes were significantly greater at trot than walk, but there was little change in orientation indicating that the general pattern of deformation of the hoof is constant between these 2 gaits. Strain patterns showed small but significant differences between locomotion on the treadmill and on ground, with the differences being more apparent at the toe than at the sides of the hoof. When turning, the quarter on the inside of the turn experienced 40% more strain than during straightline motion, while strain was similarly reduced on the opposite quarter. Strain magnitudes increase with toe length and toe angle, but were inversely proportional to medial and lateral angles. The change with toe length correlated with the range of body size of the animals in the sample. The change with toe angle was contrary to that found in in vitro tests. The change with medial and lateral angles indicated that hooves with more upright quarters are stiffer and possibly provide less impact absorption.     

Recovering three-dimensional shape from perspective translations and orthographic rotations.

Perceived orientation in depth and 3-dimensional (3D) shape was investigated for perspective projections of translations and orthographic projections of rotations of 3D dihedral angles. The principal findings were that (1) perceived orientation in depth depends on the sign of the velocity gradient, even in the case of orthographic projections; (2) the relationship between perceived orientation and the sign of the velocity gradient is greater for shallower gradients in orthographic projections of rotations, consistent with previous findings for perspective translations; (3) the magnitudes of simulated dihedral angles were underestimated (relative depth overestimated) for orthographic projections of rotations but were overestimated for perspective projections of translations; and (4) the judged magnitude of the dihedral angle depends on the velocity ratio and on image compression; it cannot be predicted from the velocity ratio or the velocity gradient alone. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Plaid slant and inclination thresholds can be predicted from components

We investigated whether stereoscopic slant and inclination thresholds for surfaces defined by two component plaids could be predicted from the interocular differences in their individual component gratings. Thresholds were measured for binocular images defined by single sinusoidal gratings and two component plaids. In both cases thresholds showed a marked dependence on component orientation. For absolute component orientations greater than 45 deg we found that inclination thresholds were smaller than slant thresholds. However, for absolute component orientations less than 45 deg, we found a reversal: slant thresholds were smaller than inclination thresholds. We considered three models that might account for these data. One assumed that thresholds stemmed from interocular position differences of corresponding image points. The other two assumed a combination of position, orientation and/or spatial-frequency differences. The best fits were obtained from those models that explicitly represented orientation differences. From the model combining orientation and spatial-frequency differences, we estimated the relative cue sensitivity to be 1.7:1, respectively. For plaids, we found that thresholds obtained from the individual components could be used to predict thresholds for plaids, even though an additional disparity cue from the contrast beat was available.     

Effect of Post-Rolling after ECAP on Superplastic Behavior of Commercial Al-Mg Alloy

A commercial Al-Mg alloy was subjected to equal channel angular pressing of 4 passes with and without postrolling, and the effects of post-rolling on the deformation characteristics of the alloy at 723 K were examined. Post-rolling was found to influence the deformation behavior significantly. The deformation behavior of the alloy processed only by equal channel angular pressing was characterized by (a) localized deformation indicated by severe surface prominence and depression, (b) the strain rate sensitivity of 0.33, and (c) moderate high strain rate superplastic elongations. By contrast, that of the alloy processed by equal channel angular pressing and post-rolling (70 % thickness reduction) was manifested by (a) uniform deformation associated with grain boundary sliding throughout the sample, (b) a sigmoidal behavior showing the strain rate sensitivity of 0.45 at the intermediate strain rates in the logarithmic stress-strain rate curve, and (c) very large high strain rate superplastic elongations.     

Evolution of surface recrystallization during indirect extrusion of 6xxx aluminum alloys

The fundamentals of coarse grain surface recrystallized structure formation in extrusion of 6xxx aluminum alloys are not yet completely understood. The objective of this article is to understand the metallurgical origins and mechanisms of the formation of the peripheral coarse grain (PCG) structure as the first step to understanding surface behavior of extruded aluminum alloys. Small-scale indirect extrusion tests were performed in which deformation parameters of strain, strain rate, and temperature were closely controlled. The deformed material was characterized via traditional metallography and orientation imaging microscopy (OIM) in order to understand the influence of processing conditions and alloy chemistry on surface grain formation. It was found that decreasing recrystallization-inhibiting elements such as Cr as well as increasing the starting extrusion temperature, extrusion ratio, and ram speed all increased the depth of the PCG. Additionally, a mechanism for favorable coarse grain formation at the surface of the extrudate is proposed based on microstructure development during extrusion.     

Plastic Deformations of Impulsively Loaded, Rigid-Plastic Beams

Rigid body dynamics is used to determine the deformation of a fixed-end, rigid-plastic beam subjected to uniformly distributed impulsive loading. The proposed solution methodology allows calculations of deformations at plastic hinges and can be used to establish rigid-plastic fracture criteria for rigid-plastic beams. Unlike previous solutions to this problem, rotary inertia and the shear deformations at the support are considered. The solution for beam deformations is described in three phases: shear, bending, and membrane. Each phase ends when the corresponding component of the strain rate vector vanishes. The initial shear phase is completed when the transverse shear velocity at the support vanishes. The beam then undergoes only rigid body rotation and axial stretching at plastic hinges in the bending phase. The bending phase ends when the angular velocity vanishes. In the membrane phase, the beam acts like a string until the transverse velocity vanishes. It has been found that beams subjected to low impulse velocity attain permanent deformation in the bending phase, while beams subjected to high impulse velocity reach permanent deformation in the membrane phase. The predictions of the beam deflections using the proposed methodology are within 15% of the experimental results.