OBSERVER BIASES IN THE 3D INTERPRETATION OF LINE-DRAWINGS

Line drawings produced by contours traced on a surface can produce a v ivid impression of the surface shape. The stability of this perception is notable considering that the information provided by the surface c ontours is quite ambiguous. We have studied the stability of line draw ing 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 hyperbol ic (saddle-shaped) smooth surface patch. Rotation of the image along t he line of sight and change in aspect ratio of the line drawing could bias the observer toward either interpretation. The results were model ed by a simple Bayesian observer that computes the probability to choo se either interpretation given the information in the image and prior preferences. The model's decision rule is noncommitting: for a given i nput image its responses are still probabilistic, reflecting variabili ty in the modeled observers' judgements. A good fit to the data was ob tained when three observer assumptions were introduced: a preference f or convex surfaces, a preference for surface contours aligned with the principal lines of curvature, and a preference for a surface orientat ion consistent with an object viewed from above. We discuss how these assumptions night reflect regularities of the visual world. (C) 1998 E lsevier Science Ltd. All rights reserved.