axiosengine/axios/Common/ConvexHull/ChainHull.cs

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2012-03-19 23:57:59 +00:00
using System.Collections.Generic;
using Microsoft.Xna.Framework;
namespace FarseerPhysics.Common.ConvexHull
{
public static class ChainHull
{
//Andrew's monotone chain 2D convex hull algorithm.
//Copyright 2001, softSurfer (www.softsurfer.com)
/// <summary>
/// Gets the convex hull.
/// </summary>
/// <remarks>
/// http://www.softsurfer.com/Archive/algorithm_0109/algorithm_0109.htm
/// </remarks>
/// <returns></returns>
public static Vertices GetConvexHull(Vertices P)
{
P.Sort(new PointComparer());
Vector2[] H = new Vector2[P.Count];
Vertices res = new Vertices();
int n = P.Count;
int bot, top = -1; // indices for bottom and top of the stack
int i; // array scan index
// Get the indices of points with min x-coord and min|max y-coord
int minmin = 0, minmax;
float xmin = P[0].X;
for (i = 1; i < n; i++)
if (P[i].X != xmin) break;
minmax = i - 1;
if (minmax == n - 1)
{
// degenerate case: all x-coords == xmin
H[++top] = P[minmin];
if (P[minmax].Y != P[minmin].Y) // a nontrivial segment
H[++top] = P[minmax];
H[++top] = P[minmin]; // add polygon endpoint
for (int j = 0; j < top + 1; j++)
{
res.Add(H[j]);
}
return res;
}
top = res.Count - 1;
// Get the indices of points with max x-coord and min|max y-coord
int maxmin, maxmax = n - 1;
float xmax = P[n - 1].X;
for (i = n - 2; i >= 0; i--)
if (P[i].X != xmax) break;
maxmin = i + 1;
// Compute the lower hull on the stack H
H[++top] = P[minmin]; // push minmin point onto stack
i = minmax;
while (++i <= maxmin)
{
// the lower line joins P[minmin] with P[maxmin]
if (MathUtils.Area(P[minmin], P[maxmin], P[i]) >= 0 && i < maxmin)
continue; // ignore P[i] above or on the lower line
while (top > 0) // there are at least 2 points on the stack
{
// test if P[i] is left of the line at the stack top
if (MathUtils.Area(H[top - 1], H[top], P[i]) > 0)
break; // P[i] is a new hull vertex
else
top--; // pop top point off stack
}
H[++top] = P[i]; // push P[i] onto stack
}
// Next, compute the upper hull on the stack H above the bottom hull
if (maxmax != maxmin) // if distinct xmax points
H[++top] = P[maxmax]; // push maxmax point onto stack
bot = top; // the bottom point of the upper hull stack
i = maxmin;
while (--i >= minmax)
{
// the upper line joins P[maxmax] with P[minmax]
if (MathUtils.Area(P[maxmax], P[minmax], P[i]) >= 0 && i > minmax)
continue; // ignore P[i] below or on the upper line
while (top > bot) // at least 2 points on the upper stack
{
// test if P[i] is left of the line at the stack top
if (MathUtils.Area(H[top - 1], H[top], P[i]) > 0)
break; // P[i] is a new hull vertex
else
top--; // pop top point off stack
}
H[++top] = P[i]; // push P[i] onto stack
}
if (minmax != minmin)
H[++top] = P[minmin]; // push joining endpoint onto stack
for (int j = 0; j < top + 1; j++)
{
res.Add(H[j]);
}
return res;
}
#region Nested type: PointComparer
public class PointComparer : Comparer<Vector2>
{
public override int Compare(Vector2 a, Vector2 b)
{
int f = a.X.CompareTo(b.X);
return f != 0 ? f : a.Y.CompareTo(b.Y);
}
}
#endregion
}
}