Shape Constancy

Shape constancy is the perceptual capacity to perceive an object as maintaining its three-dimensional form despite changes in viewpoint that alter the shape of its retinal projection. A door viewed from an oblique angle projects a trapezoid on the retina; it is perceived as rectangular because the visual system infers the most probable three-dimensional structure consistent with the projection and prior knowledge about object rigidity. Shape constancy is closely related to object recognition: to identify an object across viewpoints, the system must extract representations invariant under rotation, scale, and occlusion. The phenomenon demonstrates that the perceptual system does not register geometric properties but reconstructs them — that shape, like size and color, is an inferred property of distal objects, not a measured property of proximal stimuli. The failure modes of shape constancy — the apparent distortion of familiar figures in unusual spatial contexts — are not perceptual errors but correct inferences from incorrect priors.