/// <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.
* PointLocation.IsInRing 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 (int i = 0; i < pts.Length; i++)
{
if (!PointLocation.IsInRing(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);
}
public static bool IsPointInRing(Coordinate p, ICoordinateSequence ring)
{
return(PointLocation.IsInRing(p, ring));
}
public static bool IsPointInRing(Coordinate p, Coordinate[] ring)
{
return(PointLocation.IsInRing(p, ring));
}