Given a per-triangle integer ("group"), FGroupTopology extracts a group-level topological graph from an input Mesh. The graph consists of three elements: Corners: there is a corner at each vertex where 3 or more groups meet (ie 3 or more groups in one-ring) Edges: a group edge is a list of one or more connected edges that have the same pair of different groups on either side Group: a group is a set of connected faces with the same GroupID