Cast shadows in wide perspective.

We investigated the apparent spatial layout of cast shadows up to very wide fields of view. We presented up to 130 degrees wide images in which two 'flat poles' were standing on a green lawn under a cloudless blue sky on a sunny day. The poles threw sharp cast shadows on the green, of which one was fixed.

What are the uncurved lines in our visual field? A fresh look at Helmholtz's checkerboard.

What are the uncurved lines in our visual field? To answer this question, Helmholtz developed a geometrical model of line-curvature perception, and demonstrated it with his famous checkerboard pattern with pin-cushion distortion. He claimed it looked perfectly regular when viewed monocularly at close range while fixating the centre. Recently, doubts have been expressed whether this demonstration actually works.

The perception of doubly curved surfaces from anisotropic textures.

Most existing computational models of the visual perception of three-dimensional shape from texture are based on assumed constraints about how texture is distributed on visible surfaces. The research described in the present article was designed to investigate how violations of these assumptions influence human perception. Observers were presented with images of smoothly curved surfaces depicted with different types of texture, whose distribution of surface markings could be both anisotropic and inhomogeneous.

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 the oblique 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.

Object pose: perceiving 3-d shape as sticks and slabs.

Estimating the pose (three-dimensional orientation) of objects is an important aspect of 3-D shape perception. We studied the ability of observers to match the pose of the principal axes of an object with the pose of a cross consisting of three perpendicular axes. For objects, we used a long and a flat spheroid and eight symmetric objects with aspect ratios of dimensions of approximately 4:2:1.

Generic and non-generic conditions for the perception of surface shape from texture.

Li and Zaidi (Vision Research 40 (2000) 217; 41 (2001) 1519) have recently argued that there are two necessary conditions for the perception of 3D shape from texture: (1) the texture pattern must have a disproportionate amount of energy along directions of principal curvature; and (2) the surface must be viewed with a noticeable amount of perspective. In the present article we present evidence that these conclusions are only valid under a limited set of non-generic viewing conditions. Other relevant factors that need to be considered in this context include the distribution of curvature on an object's surface and the set of possible viewing directions from which it can be observed.