Specific alignments of illumination and anisotropic surface geometry predict failures of shape constancy.

Images of an object can vary considerably due to changes in illumination direction and material. Despite this, the human visual system often recovers stable estimates of shape, suggesting the presence of invariant image structure. Prior research shows that perceived shape can be affected by the direction of illumination, but the general conditions that lead to failures of shape constancy remain unknown. Here, we show that certain shape-illumination configurations can generically cause failures of shape constancy. Aligning the illuminant with anisotropic shape features' axis of elongation (e.g., a cylindrical ridge) causes local deletion of shading gradients, resulting in substantial reduction of its perceived relief. This can affect the perception of adjacent surface geometry demonstrated by stimuli containing a pair of smoothly intersecting ridges where perceived flattening of one of the ridges results in illusory segmentation of the other. We also present an example of qualitative differences in perceived shape as a function of illumination direction. We discuss these results in the context of recent theories of shape from shading.