Fig. 1.

Object images used in the present study (top) and their configurations in the parameter space (bottom). The shapes of the objects were defined by 57 parameters, but the nine objects used were aligned on a two-dimensional hyper-plane. Three objects were located at the apices of a triangle on the hyper-plane and the other six were on the sides of the triangle, one-quarter of the distance away from an apex. Thus, objects fell into three groups composed of three similar objects.