[转]多边形点集排序--针对凸多边形，按逆时针方向进行排序

原文地址：http://www.cnblogs.com/dwdxdy/p/3230156.html

原文是C++下的，稍微的改了为C#的，呵呵

主要方法：

 1         public static void ClockwiseSortPoints(List<Point3D> vPoints)
 2         {
 3             //计算重心
 4             Point3D center = new Point3D();
 5             double X = 0, Y = 0;
 6             for (int i = 0; i < vPoints.Count; i++) {
 7                 X += vPoints[i].X;
 8                 Y += vPoints[i].Y;
 9             }
10             center.X = (int)X / vPoints.Count;
11             center.Y = (int)Y / vPoints.Count;
12 
13             //冒泡排序
14             for (int i = 0; i < vPoints.Count - 1; i++) {
15                 for (int j = 0; j < vPoints.Count - i - 1; j++) {
16                     if (PointCmp(vPoints[j], vPoints[j + 1], center)) {
17                         Point3D tmp = vPoints[j];
18                         vPoints[j] = vPoints[j + 1];
19                         vPoints[j + 1] = tmp;
20                     }
21                 }
22             }
23         }

辅助方法：

 1         //若点a大于点b,即点a在点b顺时针方向,返回true,否则返回false
 2         static bool PointCmp(Point3D a, Point3D b, Point3D center)
 3         {
 4             if (a.X >= 0 && b.X < 0)
 5                 return true;
 6             if (a.X == 0 && b.X == 0)
 7                 return a.Y > b.Y;
 8             //向量OA和向量OB的叉积
 9             int det = Convert.ToInt32((a.X - center.X) * (b.Y - center.Y) - (b.X - center.X) * (a.Y - center.Y));
10             if (det < 0)
11                 return true;
12             if (det > 0)
13                 return false;
14             //向量OA和向量OB共线，以距离判断大小
15             double d1 = (a.X - center.X) * (a.X - center.X) + (a.Y - center.Y) * (a.Y - center.Y);
16             double d2 = (b.X - center.X) * (b.X - center.Y) + (b.Y - center.Y) * (b.Y - center.Y);
17             return d1 > d2;
18         }

每多学一点知识，就少写一行代码