Zygmunt Pizlo

The uniqueness of shape as a perceptual property lies in the fact that it is both complex and structured. Shapes are perceived veridically—perceived as they really are in the physical world, regardless of the orientation from which they are viewed. The constancy of the shape percept is the sine qua non of shape perception; you are not actually studying shape if constancy cannot be achieved with the stimulus you are using. Shape is the only perceptual attribute of an object that allows unambiguous identification.