General lighting can overcome accidental viewing.

When seeing an object in a scene, the presumption of seeing that object from a general viewpoint (as opposed to an accidental viewpoint) is a useful heuristic to decide which of many interpretations of this object is correct. Similar heuristic assumptions on illumination quality might also be used for scene interpretation. Here we tested that assumption and asked if illumination information helps determine object properties when seen from an accidental viewpoint. Test objects were placed on a flat surface and illumination was varied while keeping the objects' images constant. Observers judged the shape or rigidity of static or moving simple objects presented in accidental view. They also chose which of two seemingly very similar faces was familiar. We found: (1) Objects might appear flat without shadow information but were perceived to be volumetric objects or non-planar in the presence of cast shadows. (2) Apparently non-rigid objects became rigid with shadow information. (3) Shading and shadows helped to infer which of two face was the familiar one. Previous results had shown that cast shadows help determine spatial layout of objects. Our study shows that other properties of objects like rigidity or 3D-shape can be disambiguated by shadow information.