Estimates of shape by eye, or, The little invariant that could

Five experiments were conducted to test the hypothesis that observers appre hend specific constancies under change in perspective. The constancies were projective properties of ellipses pictured to slant and tilt in depth. Obs ervers were asked to reproduce the static upright view of a moving pair of ellipses, using a computer graphics display and interface. Projective invar iants for pairs of conics were computed on the observers' productions. A fe w experimental conditions revealed near-perfect performance. When pairs of coplanar ellipses were viewed under dynamic transformation in perspective, then invariants calculated on the observers' productions were a match - in value on average - to the invariants of the transforming ellipse pairs. It is proposed that measures of projective properties afford a family of techn iques that can be applied to gauge acuity for complex shapes in the study o f visual form perception. (C) 1999 Elsevier Science B.V. All rights reserve d.