Part and Whole in Pictorial Relief

What are “natural parts” of pictorial reliefs? Intuitively, and suggested by common lore from the visual arts, they are the bulges that stick out toward the observer. Each such bulge contains a (locally) nearest point and is bounded by one or (usually) more curvilinear ruts. The latter meet in “passes” or saddle points. This divides the relief into “natural districts”. From a formal analysis one knows that reliefs can be divided into “hill districts” or “dale districts”, these two “natural” parcellations being fully distinct. We report empirical results that strongly suggest that visual awareness is based on a partition in bulges, which are mutually only weakly connected. Such a notion immediately explains why inverted reliefs or surfaces illuminated from below appear so different as to be mutually not recognizable.