If the integral of the visibility
function for a set E is normalized, one arrives at the Index of convexity of E, a
measure of the relative convexity of E in terms of the average “area seen” by
a variable point of E. As the visibility function is upper semicontinuous
on a compact set in En, the Index is upper semicontinuous on the class
of all compact sets in En with an appropriate metric. We also investigate
natural generalizations of convex and starshaped sets in terms of the visibility
function.