The Windmill Principle

The four conditions of tone in relation to surroundings. At (1) is light against light, at (2) light against dark; at (3) dark against dark; and at (4) dark against light. Note too that the subordinate edges at (1) and (3) are blurred a little more.

The resulting effect marries the subject to the background in a way that both separates it and embeds it.