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P Mamassian

Publications and source records attributed to P Mamassian.

7 recordsLinked to original sources
PubMed1998

Observer biases in the 3D interpretation of line drawings.

Line drawings produced by contours traced on a surface can produce a vivid impression of the surface shape. The stability of this perception is notable considering that the information provided by the surface contours is quite ambiguous. We have studied the stability of line drawing perception from psychophysical and computational standpoints. For a given family of simple line drawings, human observers could perceive the drawings as depicting either an elliptic (egg-shaped) or hyperbolic (saddle-shaped) smooth surface patch. Rotation of the image along the line of sight and change in aspect ratio of the line drawing could bias the observer toward either interpretation. The results were modeled by a simple Bayesian observer that computes the probability to choose either interpretation given the information in the image and prior preferences. The model's decision rule is noncommitting: for a given input image its responses are still probabilistic, reflecting variability in the modeled observers' judgements. A good fit to the data was obtained when three observer assumptions were introduced: a preference for convex surfaces, a preference for surface contours aligned with the principal lines of curvature, and a preference for a surface orientation consistent with an object viewed from above. We discuss how these assumptions might reflect regularities of the visual world.

Adult↗
PubMed1997

Prehension of objects oriented in three-dimensional space.

When reaching for an object, the proximity of the object, its orientation, and shape should all be correctly estimated well before the hand arrives in contact with it. We were interested in the effects of the object's orientation on manual prehension. Subjects were asked to reach for an object at one of several possible orientations. We found that the trajectory of the hand and its rotation and opening were significantly affected by the object's orientation within the first half of the movement. We also detected a slight delay of the wrist relative to the forearm and a small bias of the orientation of the fingers' tips toward the orientation of the table on which the object lay. Finally, the aperture of the hand was proportional to the physical size of the object, which shows that size constancy was achieved from the variation of the object's orientation. Taken together, these results indicate that the three components of the movement - the transport, rotation, and opening of the hand - have access to a common visual representation of the object's orientation.

Adult↗
PubMed1997

Moving cast shadows induce apparent motion in depth.

Phenomenally strong visual illusions are described in which the motion of an object's cast shadow determines the perceived 3-D trajectory of the object. Simply adjusting the motion of a shadow is sufficient to induce dramatically different apparent trajectories of the object casting the shadow. Psychophysical results obtained with the use of 3-D graphics are reported which show that: (i) the information provided by the motion of an object's shadow overrides other strong sources of information and perceptual biases, such as the assumption of constant object size and a general viewpoint; (ii) the natural constraint of shadow darkness plays a role in the interpretation of a moving image patch as a shadow, but under some conditions even unnatural light shadows can induce apparent motion in depth of an object; (iii) when shadow motion is caused by a moving light source, the visual system incorrectly interprets the shadow motion as consistent with a moving object, rather than a moving light source. The results support the hypothesis that the human visual system incorporates a stationary light-source constraint in the perceptual processing of spatial layout of scenes.

Depth Perception↗
PubMed1997

Geometry of shadows.

Shadows provide a strong source of information about the shapes of surfaces. We analyze the local geometric structure of shadow contours on piecewise smooth surfaces. Particular attention is paid to intrinsic shadows on a surface: that is, shadows created on a surface by the surface's own shape and placement relative to a light source. Intrinsic shadow contours provide useful information about the direction of the light source and the qualitative shape of the underlying surface. We analyze the invariants relating surface shape and light-source direction to the shapes and singularities of intrinsic shadow contours. The results suggest that intrinsic shadows can be used to directly infer illuminant tilt, qualitative global surface structure, and, at intersections with surface creases, the concavity/convexity of a surface. We show that the results obtained for point sources of light generalize in a straightforward way to extended light sources, under the assumption that light sources are convex.

Depth Perception↗
PubMed1996

Illumination, shading and the perception of local orientation.

We investigated the perception of local surface orientation on a simple smooth object, under several different illumination conditions. The perceived local orientation was determined for several points on the surface and quantified as slant and tilt of the local tangent plane. We found an underestimation of the perceived slant and a larger variance for the perceived tilt than for the perceived slant. We found also that subjects were less biased at estimating the surface orientation when the shape was locally egg-shaped rather than saddle-shaped or cylindrical. In order to investigate the relationship between perceived shape and light source direction, we developed a method to compute the light source direction most consistent with an observer's settings. Also we compared human errors with those of an "ideal observer" which makes explicit assumptions about the illuminations, shapes and materials in its world. 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 ideal observer, we conclude that the observers used the occluding contour of the object rather than shading to estimate the local surface orientation.

Algorithms↗
PubMed1996

Categorical local-shape perception.

How well do observers perceive the local shape of an object from its shaded image? This problem was addressed by first deriving a potential representation of local solid shape. The descriptor of local shape, called shape characteristic, provides a viewpoint-independent continuum between hyperbolic (saddle-shaped) and elliptic (egg-shaped) points. The ability of human observers to make categorical judgments of local solid shape was then studied. This question was investigated by using a smooth 'croissant', a simple object made of two connected regions of elliptic and hyperbolic points. Observers decided whether the surface was locally elliptic or hyperbolic at various points on the object. The task was natural, and the observers could reliably partition the shaded image of the object into two regions, one elliptic and one hyperbolic. The ability of observers to perform this partition shows that they can, at least implicitly, localize the parabolic curves on a surface. This ability to locate the parabolic curve could in turn be exploited for other purposes, for instance to segment an object into its parts.

Humans↗