It is interesting and revealing to compare the optical information for a solid wrinkled surface as shown in Figure 5.9 and the information for a liquid wavy surface, which the reader will have to visualize. Both consist of convexities and concavities, but they are motionless on the solid surface and moving on the liquid surface. In both cases the convexities are lighted on one slope and shadowed on the other. In both cases the surface is all of the same color or reflectance. The difference between the two arrays is to be found chiefly in the two forms of fluctuation of light and shade. In the terrestrial array, light and shade exchange places slowly in one direction; they do not oscillate. In the aquatic array, light and shade interchange rapidly in both directions; they oscillate. In fact, when the sun is out and the ripples act as mirrors, the reflection of the sun can be said to flicker or flash on and off. This specific form of fluctuation is characteristic of a water surface.
When ambient light at a point of observation is structured it is an ambient optic array. The point of observation may be stationary or moving, relative to the persisting environment. The point of observation may be unoccupied or occupied by an observer.
The structure of an ambient array can be described in terms of visual solid angles with a common apex at the point of observation. They are angles of intercept, that is, they are determined by the persisting environment. And they are nested, like the components of the environment itself.
The concept of the visual solid angle comes from natural perspective, which is the same as ancient optics. No two such visual angles are identical. The solid angles of an array change as the point of observation moves, that is, the perspective structure changes. Underlying the perspective structure, however, is an invariant structure that does not change. Similarly, the solid angles of an array change as the sun in the sky moves, that is, the shadow structure changes. But there are also invariants that underlie the changing shadows.
The moving observer and the moving sun are conditions under which terrestrial vision has evolved for millions of years. But the invariant principle of reversible occlusion holds for the moving observer, and a similar principle of reversible illumination holds for the moving sun. Whatever goes out of sight will come into sight, and whatever is lighted will be shaded.