/// <summary>
        /// Remove vertices that are colinear w/ their two neighbours, within angle tolerance
        /// </summary>
        public void CollapseFlatVertices(double fMaxDeviationDeg = 5)
        {
            bool done       = false;
            int  max_passes = 200;
            int  pass_count = 0;

            while (done == false && pass_count++ < max_passes)
            {
                done = true;

                // [RMS] do modulo-indexing here to avoid pathological cases where we do things like
                // continually collapse a short edge adjacent to a long edge (which will result in crazy over-collapse)
                int       N       = Graph.MaxVertexID;
                const int nPrime  = 31337;                    // any prime will do...
                int       cur_vid = 0;
                do
                {
                    int vid = cur_vid;
                    cur_vid = (cur_vid + nPrime) % N;

                    if (!Graph.IsVertex(vid))
                    {
                        continue;
                    }

                    if (Graph.GetVtxEdgeCount(vid) != 2)
                    {
                        continue;
                    }

                    double open = Math.Abs(Graph.OpeningAngle(vid));
                    if (open < 180 - fMaxDeviationDeg)
                    {
                        continue;
                    }

                    var edges = Graph.GetVtxEdges(vid);
                    int eid   = edges.First();
                    int eid2  = edges.Last();
                    if (FixedEdgeFilterF(eid) || FixedEdgeFilterF(eid2))
                    {
                        continue;
                    }

                    Index2i ev      = Graph.GetEdgeV(eid);
                    int     other_v = (ev.a == vid) ? ev.b : ev.a;

                    DGraph2.EdgeCollapseInfo collapseInfo;
                    MeshResult result = Graph.CollapseEdge(other_v, vid, out collapseInfo);
                    if (result == MeshResult.Ok)
                    {
                        done = false;
                    }
                    else
                    {
                        throw new Exception("DGraph2Resampler.CollapseFlatVertices: failed!");
                    }
                } while (cur_vid != 0);
            }
        }