# Colloquium by Tillmann Miltzow (Utrecht University): A practical algorithm with performance guarantees for the art-gallery problem

Given a closed simple polygon P, we say two points p,q see each other if the segment pq is fully contained in P. The art gallery problem seeks a minimum size set G⊂P of guards that sees P completely. The only known algorithm to solve the art gallery problem exactly is attributed to Sharir and uses algebraic methods. As the art gallery problem is ∃R-complete, it seems impossible to avoid algebraic methods without additional assumptions.

To circumvent this problem, we introduce the notion of vision stability.

In order to describe vision stability, consider an enhanced guard that can see "around the corner" by an angle of δ or a diminished guard whose vision is "blocked" by an angle δ by reflex vertices. A polygon P has vision stability δ if the optimal number of enhanced guards to guard P is the same as the optimal number of diminished guards to guard P. We will argue that most relevant polygons are vision-stable. We describe a one-shot algorithm that computes an optimal guard set for vision-stable polygons using polynomial time besides solving one integer program.

We implemented an iterative version of the vision-stable algorithm. Its practical performance is slower, but comparable to other

state-of-the-art algorithms. Our iterative algorithm is a variation of the one-shot algorithm. It delays some steps and only computes them only when deemed necessary. Given a chord c of a polygon, we denote by n(c) the number of vertices visible from c. The chord-width of a polygon is the maximum n(c) over all possible chords c. The set of vision-stable polygons admits an FPT algorithm when parametrized by the chord-width. Furthermore, the one-shot algorithm runs in FPT time when parameterized by the number of reflex vertices.

Joint work by Simon Hengeveld & Tillmann Miltzow.

### Time & Location

Oct 19, 2020 | 04:00 PM s.t.

online