/// <summary> /// Uses a heuristic to reduce the number of points scanned to compute the hull. /// The heuristic is to find a polygon guaranteed to /// be in (or on) the hull, and eliminate all points inside it. /// A quadrilateral defined by the extremal points /// in the four orthogonal directions /// can be used, but even more inclusive is /// to use an octilateral defined by the points in the 8 cardinal directions. /// Note that even if the method used to determine the polygon vertices /// is not 100% robust, this does not affect the robustness of the convex hull. /// <para> /// To satisfy the requirements of the Graham Scan algorithm, /// the returned array has at least 3 entries. /// </para> /// </summary> /// <param name="pts">The coordinates to reduce</param> /// <returns>The reduced array of coordinates</returns> private static Coordinate[] Reduce(Coordinate[] pts) { var polyPts = ComputeOctRing(pts/*_inputPts*/); // unable to compute interior polygon for some reason if(polyPts == null) return pts; // add points defining polygon var reducedSet = new HashSet<Coordinate>(); for (int i = 0; i < polyPts.Length; i++) reducedSet.Add(polyPts[i]); /* * Add all unique points not in the interior poly. * CGAlgorithms.IsPointInRing is not defined for points actually on the ring, * but this doesn't matter since the points of the interior polygon * are forced to be in the reduced set. */ for (var i = 0; i < pts.Length; i++) if (!CGAlgorithms.IsPointInRing(pts[i], polyPts)) reducedSet.Add(pts[i]); var reducedPts = CoordinateArrays.ToCoordinateArray((ICollection<Coordinate>)reducedSet);// new Coordinate[reducedSet.Count]; Array.Sort(reducedPts); // ensure that computed array has at least 3 points (not necessarily unique) if (reducedPts.Length < 3) return PadArray3(reducedPts); return reducedPts; }
///<summary> /// Determines whether a point lies in a LinearRing, using the ring envelope to short-circuit if possible. ///</summary> /// <param name="p">The point to test</param> /// <param name="ring">A linear ring</param> /// <returns>true if the point lies inside the ring</returns> private static bool IsPointInRing(ICoordinate p, ILinearRing ring) { // short-circuit if point is not in ring envelope if (!ring.EnvelopeInternal.Intersects(p)) { return(false); } return(CGAlgorithms.IsPointInRing(p, ring.Coordinates)); }
/// <summary> /// /// </summary> /// <param name="pt"></param> /// <returns></returns> public bool IsInside(Coordinate pt) { return(CGAlgorithms.IsPointInRing(pt, pts)); }