In one study we investigated
the perception of local surface orientation for a simple object with regions
of elliptic (egg-shaped) and hyperbolic (saddle-shaped) points. The local surface
orientation was measured by the slant and tilt of the tangent plane at different
points of the surface under several different illumination conditions. We found
an underestimation of the perceived slant, and a larger variance for the perceived
tilt. We also found that subjects were better at estimating the surface orientation
when the shape was locally egg-shaped rather than saddle-shaped or cylindrical.
A method is described in which we find the light source direction most consistent
with an observer's settings. We also describe computer simulations for a shape-from-shading
algorithm that is optimal in a Bayesian sense under the specified constraints.
From converging evidence based on (i) the light direction most consistent with
the observer's settings, (ii) a supplementary experiment where the object is
displayed as a silhouette, and (iii) the computer simulations of the shape from
shading algorithm, we conclude that the occluding contour was the dominant source
of information used by our observers.
Mamassian P & Kersten D. (1996) Illumination, shading and the perception of local orientation. Vision Research., 36 (15), 2351-2367.
In a second study we asked: How well can observers make categorical judgements of local surface shape? We investigated this question using a smooth ``croissant", a simple object made of two connected regions of elliptic (egg-shaped) and hyperbolic (saddle-shaped) points. The object was rendered on a computer screen, using a Lambertian shading model. The object was seen from a generic viewpoint, under four different lighting conditions: the light source was either placed at the viewpoint, above, below or behind the object. Observers decided whether the surface was locally elliptic or hyperbolic at various points on the object. We found that observers' categorization judgements were accurate and invariant over illumination direction. The main departure from veridicality was a bias towards ellipticity, that is a tendency to perceive elliptic some points which are actually hyperbolic. We offer a novel description of local solid shape that separates the view-dependent from the view-independent component. Using this description, we show that observers' judgments were most accurate near the occluding and attached shadow contours.
Mamassian, P., Kersten, D. &;Knill, D.C. Categorical local shape perception. To appear in Perception.
If you would like more information, contact Pascal Mamassian who is now at the University of Glasgow. pascal@psy.gla.ac.uk