  A hidden guard set $ G $ is a set of point guards in polygon $ P $ that all
points of the polygon are visible from some guards in $ G $ under the
constraint that no two guards may see each other. In this paper, we consider
the problem for finding minimum hidden guard sets in histogram polygons under
orthogonal visibility. Two points $ p $ and $ q $ are orthogonally visible if
the orthogonal bounding rectangle for $ p $ and $ q $ lies within $ P $. It is
known that the problem is NP-hard for simple polygon with general visibility
and it is true for simple orthogonal polygon. We proposed a linear time exact
algorithm for finding minimum hidden guard set in histogram polygons under
orthogonal visibility. In our algorithm, it is allowed that guards place
everywhere in the polygon.
