Sangaku contains classes for Point, Line, Polygon, AABB and so on in 2D. It can find the 'pole of inaccessibility' for a polygon. And it can simply the boundary of a polygon to minimise points while retaining detail. It works well with Gosu.
Sangaku Eyeball uses Gosu, a 2D Game Library, to display lines, polygons and so on as defined by the Sangaku gem. It allows you to draw arbitrary polygons and then locate the Pole of Inaccessibility (usually the best place to display a label) using a fast, iterative algorithm.