When an object casts its shadow on a background surface, the shadow can be informative about the shape of the object, the shape of the background surface and the spatial arrangement of the object relative to the background. Among all these roles, we found that cast shadows were perceptually most relevant for the recovery of spatial arrangement, especially when the shadow is in motion. This finding is intriguing when one considers, the ambiguities in the possible ways that shadow motion can be interpreted. We reasoned that the visual system must use a priori constraints to disambiguate the cast shadow motion. One of these constraints is that the light source is stationary. Though simple, the stationary-light-source constraint supports rich, reliable inferences about the qualitative motions of objects in three dimensions.
Bibliographical noteFunding Information:
Some of the ideas presented in this paper were presented at the annual meeting of the Association for Research in Vision and Ophthalmology in 1991 and 1992, and at the European Conference on Visual Perception in 1992. We thank Jim Enns and the other reviewer for their acute comments. Funding was provided by a grant from the National Science Foundation (NSF) SBR-9631682.