The Role Played by Extremal Edges in Figure-ground Organization

Identifying and understanding the visual cues that determine relative depth across image contours (i.e., figure-ground organization) are central problems of vision science. In this monograph we report the figural cue of extremal edges (EEs), which arise when an opaque con-vex surface smoothly curves to occlude part of itself. We also compare classical cues to figure-ground organization with the recently disco-vered cue of EE. Our results show that EEs are surprisingly powerful pictorial cues to relative depth across a contour, almost entirely dominating the well-known figure-ground cues of relative size, con-vexity, surroundedness, and shape familiarity. These results demon-strate that natural shading and texture gradients in an image provide important additional information about figure-ground organization that has been overlooked in the past 75 years in research on figure-ground organization.