On the complexity of sets of free lines and line segments among balls in three dimensions

We present two new fundamental lower bounds on the worst-case combinatorial complexity of sets of free lines and sets of maximal free line segments in the presence of balls in three dimensions. We first prove that the set of maximal non-occluded line segments among n disjoint unit balls has complexity Ω(n⁴), which matches the trivial O(n⁴) upper bound. This improves the trivial Ω(n²) bound and also the Ω(n³) lower bound for the restricted setting of arbitrary-size balls [Devillers and Ramos, 2001]. This result settles, negatively, the natural conjecture that this set of line segments, or, equivalently, the visibility complex, has smaller worst-case complexity for disjoint fat objects than for skinny triangles. We also prove an Ω(n³) lower bound on the complexity of the set of non-occluded lines among n balls of arbitrary radii, improving on the trivial Ω(n²) bound. This new bound almost matches the recent O(n^3+ε) upper bound [Rubin, 2010].