Form constancy

Form constancy denotes the appearance of the invariant shape of objects in the environment, notwithstanding the variability due to changes of the viewpoint, caused by subjects movement, to objects displacement, to illumination, lightness, color, and texture whose variation can affect the internal and external boundaries of the surfaces that fall in the simultaneous or successive views of solid objects. In early psychology of perception, shape invariance was called form constancy to highlight the perceptual ability to derive something constant from the changing forms of the multiple views of objects. Shape is an attribute of 2D surfaces and 3D objects, which are invariant across the transformations in the coordinate system of the viewer. The perception of shape can be decomposed in viewer-centered and object-centered factors.