Tjeerd M. H. Dijkstra

The effects of viewing angle, camera angle, and sign of surface curvature on the perception of three-dimensional shape from texture

Computational models for determining three-dimensional shape from texture based on local foreshortening or gradients of scaling are able to achieve accurate estimates of surface relief from an image when it is observed from the same visual angle with which it was photographed or rendered. These models produce conflicting predictions, however, when an image is viewed from a different visual angle. An experiment was performed to test these predictions, in which observers judged the apparent depth profiles of hyperbolic cylinders under a wide variety of conditions. The results reveal that the apparent patterns of relief from texture are systematically underestimated; convex surfaces appear to have greater depth than concave surfaces, large camera angles produce greater amounts of perceived depth than small camera angles, and the apparent depth-to-width ratio for a given image of a surface is greater for small viewing angles than for large viewing angles. Because these results are incompatible with all existing computational models, a new model is presented based on scaling contrast that can successfully account for all aspects of the data.

Postural responses to stationary and moving scenes as a function of distance to the scene

Abstract Amplitude of postural sway in fore-after direction was investigated as a function of suggested distance between the observer and a stationary or moving scene. These experiments were done with a set-up in which movements of the observer were fed back real-time to a graphics workstation to modify the video image projected on the screen. The results show that sway amplitude for a stationary scene increases with increasing distance. Sway amplitude in response to a scene moving along the line of sight with constant amplitude did not depend on distance. Both of these results can be explained by assuming a threshold for visual motion perception and by assuming that observers move so as to minimize retinal slip.

The effects of field of view on the perception of 3D slant from texture

Observers judged the apparent signs and magnitudes of surface slant from monocular textured images of convex or concave dihedral angles with varying fields of view between 5 degrees C and 60 degrees C. The results revealed that increasing the field of view or the regularity of the surface texture produced large increases in the magnitude of the perceptual gain (i.e., the judged slant divided by the ground truth). Additional regression analyses also revealed that observers slant judgments were highly correlated with the range of texture densities (or spatial frequencies) in each display, which accounted for 96% of the variance among the different possible dihedral angles and fields of view.

A gentle introduction to the dynamic set-point model of human postural control during perturbed stance

Three models of increasing complexity to capture three observed phenomena of postural control are described. The first-order model mimics the increase in standard deviation of sway with eyes closed, but none of the other phenomena. The damped spring model captures both the increase in standard deviation of sway with eyes closed and the change from phase-lag to phase-lead with increasing frequency of oscillation of a visual surround. The final model, the dynamic set-point model, extends the damped spring model by a degree of freedom that reflects the observed slow drift in baseline. Under the assumption of a strong coupling to positional information, the dynamic set-point model predicts a dependence between natural frequency and position coupling constant of the damped spring model. It is shown that this prediction is borne out in data obtained from haptically perturbed stance but not in data obtained from visually perturbed stance.

Stability and Variability: Indicators for Passive Stability and Active Control in a Rhythmic Task

Using a rhythmic task where human subjects bounced a ball with a handheld racket, fine-grained analyses of stability and variability extricated contributions from open-loop control, noise strength, and active error compensation. Based on stability analyses of a stochastic-deterministic model of the task--a surface contacting the ball by periodic movements--open-loop or dynamic stability was assessed by the acceleration of the racket at contact. Autocovariance analyses of model and data were further used to gauge the contributions of open-loop stability and noise strength. Variability and regression analyses estimated active error compensation. Empirical results demonstrated that experienced actors exploited open-loop stability more than novices, had lower noise strength, and applied more active error compensations. By manipulating the model parameter coefficient of restitution, task stability was varied and showed that actors graded these three components as a function of task stability. It is concluded that actors tune into task stability when stability is high but use more active compensation when stability is reduced. Implications for the neural underpinnings for passive stability and active control are discussed. Further, results showed that stability and variability are not simply the inverse of each other but contain more quantitative information when combined with model analyses.

Perception of three-dimensional shape from ego- and object-motion: comparison between small- and large-field stimuli.

We compare the performance in the detection of the shape of concave, planar and convex surfaces for small-field (8 deg) and large-field (90 deg) stimuli. Shape is perceived from head translations, object translations and object rotations. We find large differences between small-field and large-field stimulation. For small-field stimulation performance is best for object rotation, intermediate for self-motion and worst for object translation. For large-field stimulation performance is similar across conditions. Few errors on the sign of the curvature are found for self-motion for both field sizes, indicating that self-motion information disambiguates the curvature sign. For object rotation with small-field stimulation, the concave-convex ambiguity is strong with many apparent deformations. In contrast, large-field curvature signs are always accurately reported, suggesting that the weight of the rigidity hypothesis depends on field size.

3D Shape Representation: Transforming Polygons into Voxels

We developed a method that transforms a polygonal representation of an object into a voxel representation on a 3D regular grid. By applying scale-space theory we derived expressions for an anti-aliased voxel representation of points, lines, and triangles. The algorithm ‘paints’ the triangles on the voxel grid and ‘fills’ the resulting surface. The method can be applied in 3D (medical) imaging and shape analysis in 3+1 dimensional scale-space.

Extraction of three-dimensional shape from optic flow: a geometric approach

We show how a scale-invariant measure of three-dimensional shape can be derived from the velocity field generated by a rigid curved surface patch under perspective projection. We use invariance under rotation of the image plane [the Lie group SO(2)] to decompose the second-order velocity field in differential invariants. From a combination of these invariants we construct an approximation of the absolute value of Koenderink’s shape index [ Image Vis. Comput.10557 ( 1992)]. We show that the effect of these approximations on the shape index is small, especially under parallel projection. Furthermore, we provide an explanation for the psychophysical finding that elliptical shapes are more readily detected than parabolic or hyperbolic shapes. From the invariants we can also derive approximations of the principal directions, the curvedness, the slant, and the tilt.

Gene regulation in the intraerythrocytic cycle of Plasmodium falciparum

MOTIVATION To date, there is little knowledge about one of the processes fundamental to the biology of Plasmodium falciparum, gene regulation including transcriptional control. We use noisy threshold models to identify regulatory sequence elements explaining membership to a gene expression cluster where each cluster consists of genes active during the part of the developmental cycle inside a red blood cell. Our approach is both able to capture the combinatorial nature of gene regulation and to incorporate uncertainty about the functionality of putative regulatory sequence elements. RESULTS We find a characteristic pattern where the most common motifs tend to be absent upstream of genes active in the first half of the cycle and present upstream of genes active in the second half. We find no evidence that motifs score, orientation, location and multiplicity improves prediction of gene expression. Through comparative genome analysis, we find a list of potential transcription factors and their associated motifs. SUPPLEMENTARY INFORMATION Supplementary data are available at Bioinformatics online.

The beholder’s share in the perception of orientation of 2-D shapes

A considerable amount of research demonstrates that people perceive cardinal orientations (horizontal and vertical) more accurately than other orientations; this is termed theoblique effect. We investigated the interaction of this effect with the degree of elongation of the stimulus. Our stimuli were ellipses with a wide range of aspect ratios, varying from a circle (aspect ratio = 1) to a line (aspect ratio = 123.5). The task was to set a probe line in the same orientation as the long axis of the ellipse. In our first experiment, we determined that performance is degraded as the aspect ratio decreases; furthermore, the bias and response variability are linearly related to a transformation of aspect ratio (roundness). We found significant individual differences; the results show high within-subjects correlations and low between-subjects correlations. In our second experiment, we had observers judge the orientation of circles randomly mixed in with ellipses of low aspect ratio. The observers demonstrated intrinsic preferences and generated reproducible distributions of orientation settings with idiosyncratic profiles. These distributions predict the influence on the response to ellipses with an aspect ratio higher than one and can be considered as thebeholder’s share in the perception of shape orientation.