Figure-Ground Relationship
The perceptual separation of a subject from its background, which determines whether it reads clearly.
In depth
Separation is achieved through contrast in tone, colour, sharpness or motion — a dark subject against a light ground, a sharp subject against a blurred one. When the relationship is ambiguous, the eye cannot decide what is subject and what is setting, which is why a dark suit against dark panelling produces a picture that reads as a shapeless mass.
Examples in practice
Moving one step so a subject's head falls against a bright window rather than a dark bookcase resolves the separation instantly.