The medial axis of the union of inner Voronoi balls in the plane

Consider a dense sampling S of the smooth boundary of a planar shape O. We show that the medial axis of the union of Voronoi balls centered at Voronoi vertices inside O has a particularly simple structure: it is the union of all Voronoi vertices inside O and the Voronoi edges connecting them. Therefore, the medial axis of the union of these inner balls can be computed more efficiently and robustly than for a general union of balls.