Shadows and depth illusions.

The visual system uses shadows to compute perceived depth of features on an uneven surface, and in doing so makes the assumption of a single light source in most circumstances. Three illusory depth effects are described (depth reversal, accentuation, and flattening) that can be demonstrated with 3-D materials, and all of which are based on the single-source assumption. Ambiguous lighting conditions may produce moment-to-moment depth reversals and/or the subjective sensation of textural 'strangeness'. It is suggested that these are both subjective experiences which occur when the system cannot compute depth with certainty.