我正在尝试实现以下http://repositorium.sdum.uminho.pt/bitstream/1822/6429/1/ConcaveHull_ACM_MYS.pdf
中描述的算法我正在使用以下类库。 Loyc libs来自http://core.loyc.net/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Device.Location;
using Loyc.Collections;
using Loyc.Geometry;
using Loyc;
这是基本课程
public class Hulls
{
private static List<Point<double>> KNearestNeighbors(List<Point<double>> points, Point<double> currentPoint, int k, out int kk)
{
kk = Math.Min(k, points.Count - 1);
var ret = points
.OrderBy(x => PointMath.Length(new Vector<double>(currentPoint.X - x.X, currentPoint.Y - x.Y)))
.Take(k)
.ToList();
return ret;
}
private static double Angle(Point<double> a, Point<double> b)
{
var ret = -Math.Atan2(b.Y - a.Y, b.X - a.X);
return NormaliseAngle(ret);
}
private static double NormaliseAngle(double a)
{
//while (a < b - Math.PI) a += Math.PI * 2;
//while (b < a - Math.PI) b += Math.PI * 2;
if (a < 0.0) { return a + Math.PI + Math.PI; }
return a;
}
private static List<Point<double>> SortByAngle(List<Point<double>> kNearest, Point<double> currentPoint, double angle)
{
//kNearest
// .Sort((v1, v2) => AngleDifference(angle, Angle(currentPoint, v1)).CompareTo(AngleDifference(angle, Angle(currentPoint, v2))));
//return kNearest.ToList();
kNearest = kNearest.OrderByDescending(x => NormaliseAngle(Angle(currentPoint, x) - angle)).ToList();
return kNearest;
}
private static bool CCW(Point<double> p1, Point<double> p2, Point<double> p3)
{
var cw = ((p3.Y - p1.Y) * (p2.X - p1.X)) - ((p2.Y - p1.Y) * (p3.X - p1.X));
return cw > 0 ? true : cw < 0 ? false : true; // colinear
}
private static bool _Intersect(LineSegment<double> seg1, LineSegment<double> seg2)
{
return CCW(seg1.A, seg2.A, seg2.B) != CCW(seg1.B, seg2.A, seg2.B) && CCW(seg1.A, seg1.B, seg2.A) != CCW(seg1.A, seg1.B, seg2.B);
}
private static bool Intersect(LineSegment<double> seg1, LineSegment<double> seg2)
{
if ((seg1.A.X == seg2.A.X && seg1.A.Y == seg2.A.Y)
|| (seg1.B.X == seg2.B.X && seg1.B.Y == seg2.B.Y))
{
return false;
}
if (_Intersect(seg1, seg2))
{
return true;
}
return false;
}
public IListSource<Point<double>> KNearestConcaveHull(List<Point<double>> points, int k)
{
points.Sort((a, b) => a.Y == b.Y ? (a.X > b.X ? 1 : -1) : (a.Y >= b.Y ? 1 : -1));
Console.WriteLine("Starting with size {0}", k.ToString());
DList<Point<double>> hull = new DList<Point<double>>();
var len = points.Count;
if (len < 3) { return null; }
if (len == 3) { return hull; }
var kk = Math.Min(Math.Max(k, 3), len);
var dataset = new List<Point<double>>();
dataset.AddRange(points.Distinct());
var firstPoint = dataset[0];
hull.PushFirst(firstPoint);
var currentPoint = firstPoint;
dataset.RemoveAt(0);
double previousAngle = 0;
int step = 2;
int i;
while ((currentPoint != firstPoint || step == 2) && dataset.Count > 0)
{
if (step == 5) { dataset.Add(firstPoint); }
var kNearest = KNearestNeighbors(dataset, currentPoint, k, out kk);
var cPoints = SortByAngle(kNearest, currentPoint, previousAngle);
var its = true;
i = 0;
while (its == true && i < cPoints.Count)
{
i++;
int lastPoint = 0;
if (cPoints[i - 1] == firstPoint)
{
lastPoint = 1;
}
int j = 2;
its = false;
while (its == false && j < hull.Count - lastPoint)
{
LineSegment<double> line1 = new LineSegment<double>(hull[step - 2], cPoints[i - 1]);
LineSegment<double> line2 = new LineSegment<double>(hull[step - 2 - j], hull[step - 1 - j]);
//its = LineMath.ComputeIntersection(line1, line2, out pfrac, LineType.Segment);
its = Intersect(line1, line2);
j++;
}
}
if (its == true)
{
return KNearestConcaveHull(points, kk + 1);
}
currentPoint = cPoints[i - 1];
hull.PushLast(currentPoint);
previousAngle = Angle(hull[step - 1], hull[step - 2]);
dataset.Remove(currentPoint);
step++;
}
bool allInside = true;
i = dataset.Count;
while (allInside == true && i > 0)
{
allInside = PolygonMath.IsPointInPolygon(hull, dataset[i - 1]);
i--;
}
if (allInside == false) { return KNearestConcaveHull(points, kk + 1); }
return hull;
}
}
以上假设是基于逆时针旋转点前一个边缘的最右边转弯为边界选择一条新边。代码似乎从具有最低y值的初始顶点中选择正确的第一个边缘，但是当偏移角度非零时，则不会正确地拾取下一个边缘。我认为问题是SortByAngle或Angle。 -atan2会顺时针转回来，对吗？可能我应该添加偏移角度？
编辑（解决方案）：在对问题的第一条评论中提供的Eric的有用建议后发现问题。它是SortByAngle和Angle：
private static double Angle(Point<double> a, Point<double> b)
{
var ret = Math.Atan2(b.Y - a.Y, b.X - a.X);
return NormaliseAngle(ret);
}
private static double NormaliseAngle(double a)
{
if (a < 0.0) { return a + Math.PI + Math.PI; }
return a;
}
private static List<Point<double>> SortByAngle(List<Point<double>> kNearest, Point<double> currentPoint, double angle)
{
//kNearest = kNearest.OrderByDescending(x => NormaliseAngle(Angle(currentPoint, x) - angle)).ToList();
kNearest.Sort((a, b) => NormaliseAngle(Angle(currentPoint, a) - angle) > NormaliseAngle(Angle(currentPoint, b) - angle) ? 1 : -1);
return kNearest;
}
答案 0 :(得分：1)
你有一些错误：
var kNearest = KNearestNeighbors(dataset, currentPoint, k, out kk);
将kk更改为一些var。你覆盖了＆＃34; kk＆＃34;的增量。值，然后您获得StackOverflow异常。
将您的代码更改为以下内容：
int someVal;
var kNearest = KNearestNeighbors(dataset, currentPoint, k, out someVal);