Ingo Fruend (York University, Toronto) and I demonstrate that only a small fraction of biologically relevant shapes can be represented by Radial Frequency (RF) pattern-based shapes and that this small fraction is perceptually distinct from the general class of all possible planar shapes. In this paper we derive a general method to compute the distance of a given shape's outline from the set of RF patterns, allowing us to scan large numbers of object outlines automatically. This analysis shows that only 1 to 6% of naturally smooth outlines can be exactly represented by RF patterns. In addition, we present results from visual search experiments, which revealed that searching RF patterns among non-RF patterns is efficient, whereas searching an RF pattern among other RF patterns is inefficient (and vice versa).
Our results suggest that RF patterns represent only a small and restricted subset of possible planar shapes and that results obtained with this special class of stimuli can not simply be expected to generalise to any arbitrary planar shape and shape representation in general.