Fig. 1.

A graphical depiction of the types of curved surfaces used in the experiment. The surfaces are arranged by shape index, which ranges from −1.0 (concave hemisphere) at the far left to +1.0 (convex hemisphere) at the far right. Concave and convex cylinders possess shape index values of negative and positive 0.5, respectively, while a symmetric "saddle" is represented by a shape index value of zero. Shape index values with a magnitude between 0.5 and 1.0 represent various ellipsoids, while values with a magnitude between zero and 0.5 represent asymmetric "saddles." For more information about shape index, see Koenderink (1990) or Koenderink and van Doorn (1992)