The action system which can change the relations between parts of the chain. Senses find how surfaces vary, and when they vary, and thereby find where they vary. Surfaces can be flat or curved. They can face to or away from a vantage point. Virtually all objects whose shapes can be perceived are combinations of surfaces.
Surfaces form corners - concave or convex.
Convex corners can form occluding edges. Rounded surfaces can offer occlusion too. A convex corner arises with some rounded surfaces, like teeth. If pictures can show edges of surfaces, they can show much of the visible environment. Klatzky et al - blind can do route tasks including short cuts.