Measuring the sizes of concavities

A concavity in a region R is a simply-connected region S whose boundary is composed of a boundary B to R and the chord K of B. Three measures of the size of S are studied: the area of S; the maximum distance of B from K; and the maximum distance of K from B. Relationships among these measures are established. It is shown that only the last measure guarantees (when its value is high) that S contains a large 'block' of the complement of R, so that S is 'detectable' if R is coarsely digitized. Similarly, if the last measure is large relative to the length of K, S is 'locally detectable' if R is digitized to a suitable degree of coarseness. Local detectability of S at a coarse level of digitization corresponds, intuitively, to S being visually conspicuous.