  Art Gallery is a fundamental visibility problem in Computational Geometry.
The input consists of a simple polygon P, (possibly infinite) sets G and C of
points within P, and an integer k; the task is to decide if at most k guards
can be placed on points in G so that every point in C is visible to at least
one guard. In the classic formulation of Art Gallery, G and C consist of all
the points within P. Other well-known variants restrict G and C to consist
either of all the points on the boundary of P or of all the vertices of P.
Recently, three new important discoveries were made: the above mentioned
variants of Art Gallery are all W[1]-hard with respect to k [Bonnet and
Miltzow, ESA'16], the classic variant has an O(log k)-approximation algorithm
[Bonnet and Miltzow, SoCG'17], and it may require irrational guards [Abrahamsen
et al., SoCG'17]. Building upon the third result, the classic variant and the
case where G consists only of all the points on the boundary of P were both
shown to be \exists R-complete~[Abrahamsen et al., STOC'18]. Even when both G
and C consist only of all the points on the boundary of P, the problem is not
known to be in NP.
  Given the first discovery, the following question was posed by Giannopoulos
[Lorentz Center Workshop, 2016]: Is Art Gallery FPT with respect to r, the
number of reflex vertices? We focus on the variant where G and C are all the
vertices of P, called Vertex-Vertex Art Gallery. We show that Vertex-Vertex Art
Gallery is solvable in time r^{O(r^2)}n^{O(1)}. Our approach also extends to
assert that Vertex-Boundary Art Gallery and Boundary-Vertex Art Gallery are
both FPT. We utilize structural properties of "almost convex polygons" to
present a two-stage reduction from Vertex-Vertex Art Gallery to a new
constraint satisfaction problem (whose solution is also provided in this paper)
where constraints have arity 2 and involve monotone functions.
