Boids/Assets/Triangulator.cs

using System.Collections.Generic;
using UnityEngine;

/// <summary>
/// This script can be used to split a 2D polygon into triangles.
/// The algorithm supports concave polygons, but not polygons with holes, 
/// or multiple polygons at once.
/// Taken from <see cref="http://wiki.unity3d.com/index.php?title=Triangulator"/>
/// </summary>
public class Triangulator
{
    private readonly List<Vector2> _mPoints;

    public Triangulator(IEnumerable<Vector2> points)
    {
        _mPoints = new List<Vector2>(points);
    }

    public int[] Triangulate()
    {
        var indices = new List<int>();

        var n = _mPoints.Count;
        if (n < 3)
            return indices.ToArray();

        var V = new int[n];
        if (Area() > 0) {
            for (var v = 0; v < n; v++)
                V[v] = v;
        } else {
            for (var v = 0; v < n; v++)
                V[v] = n - 1 - v;
        }

        var nv = n;
        var count = 2 * nv;
        for (int m = 0, v = nv - 1; nv > 2;) {
            if (count-- <= 0)
                return indices.ToArray();

            var u = v;
            if (nv <= u)
                u = 0;
            v = u + 1;
            if (nv <= v)
                v = 0;
            var w = v + 1;
            if (nv <= w)
                w = 0;

            if (Snip(u, v, w, nv, V)) {
                int a, b, c, s, t;
                a = V[u];
                b = V[v];
                c = V[w];
                indices.Add(a);
                indices.Add(b);
                indices.Add(c);
                m++;
                for (s = v, t = v + 1; t < nv; s++, t++)
                    V[s] = V[t];
                nv--;
                count = 2 * nv;
            }
        }

        indices.Reverse();
        return indices.ToArray();
    }

    private float Area()
    {
        var n = _mPoints.Count;
        var A = 0.0f;
        for (int p = n - 1, q = 0; q < n; p = q++) {
            var pval = _mPoints[p];
            var qval = _mPoints[q];
            A += pval.x * qval.y - qval.x * pval.y;
        }
        return A * 0.5f;
    }

    private bool Snip(int u, int v, int w, int n, int[] V)
    {
        int p;
        var A = _mPoints[V[u]];
        var B = _mPoints[V[v]];
        var C = _mPoints[V[w]];
        if (Mathf.Epsilon > (B.x - A.x) * (C.y - A.y) - (B.y - A.y) * (C.x - A.x))
            return false;
        for (p = 0; p < n; p++) {
            if (p == u || p == v || p == w)
                continue;
            var P = _mPoints[V[p]];
            if (InsideTriangle(A, B, C, P))
                return false;
        }
        return true;
    }

    private static bool InsideTriangle(Vector2 A, Vector2 B, Vector2 C, Vector2 P)
    {
        float ax, ay, bx, by, cx, cy, apx, apy, bpx, bpy, cpx, cpy;
        float cCROSSap, bCROSScp, aCROSSbp;

        ax = C.x - B.x;
        ay = C.y - B.y;
        bx = A.x - C.x;
        by = A.y - C.y;
        cx = B.x - A.x;
        cy = B.y - A.y;
        apx = P.x - A.x;
        apy = P.y - A.y;
        bpx = P.x - B.x;
        bpy = P.y - B.y;
        cpx = P.x - C.x;
        cpy = P.y - C.y;

        aCROSSbp = ax * bpy - ay * bpx;
        cCROSSap = cx * apy - cy * apx;
        bCROSScp = bx * cpy - by * cpx;

        return aCROSSbp >= 0.0f && bCROSScp >= 0.0f && cCROSSap >= 0.0f;
    }
}