Extrude a 2D sprite to a mesh from a Polygon Collider in Unity3D

Extruder.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

/// <summary>
/// 
/// </summary>
/// <remarks>Source: https://forum.unity3d.com/threads/trying-extrude-a-2d-polygon-to-create-a-mesh.102629/ </remarks>
[RequireComponent(typeof(MeshFilter), typeof(MeshRenderer), typeof(PolygonCollider2D))]
public class ExtrudeSprite : MonoBehaviour
{
  public Color extrudeColor = Color.white;
  public float frontDistance = -0.249f;
  public float backDistance = 0.249f;

  void Start()
  {
    PolygonCollider2D pol = GetComponent<PolygonCollider2D>();
    Mesh m = CreateMesh(pol.points, frontDistance, backDistance);

    GetComponent<MeshFilter>().sharedMesh = m;
    GetComponent<MeshRenderer>().material.color = extrudeColor;

    pol.isTrigger = true;
    pol.enabled = false;
  }

  private static Mesh CreateMesh(Vector2[] poly, float frontDistance = -10, float backDistance = 10)
  {
    frontDistance = Mathf.Min(frontDistance, 0);
    backDistance = Mathf.Max(backDistance, 0);

    // convert polygon to triangles
    Triangulator triangulator = new Triangulator(poly);
    int[] tris = triangulator.Triangulate();
    Mesh m = new Mesh();
    Vector3[] vertices = new Vector3[poly.Length * 2];

    for (int i = 0; i < poly.Length; i++)
    {
      vertices[i].x = poly[i].x;
      vertices[i].y = poly[i].y;
      vertices[i].z = frontDistance; // front vertex
      vertices[i + poly.Length].x = poly[i].x;
      vertices[i + poly.Length].y = poly[i].y;
      vertices[i + poly.Length].z = backDistance;  // back vertex    
    }
    int[] triangles = new int[tris.Length * 2 + poly.Length * 6];
    int count_tris = 0;
    for (int i = 0; i < tris.Length; i += 3)
    {
      triangles[i] = tris[i];
      triangles[i + 1] = tris[i + 1];
      triangles[i + 2] = tris[i + 2];
    } // front vertices
    count_tris += tris.Length;
    for (int i = 0; i < tris.Length; i += 3)
    {
      triangles[count_tris + i] = tris[i + 2] + poly.Length;
      triangles[count_tris + i + 1] = tris[i + 1] + poly.Length;
      triangles[count_tris + i + 2] = tris[i] + poly.Length;
    } // back vertices
    count_tris += tris.Length;
    for (int i = 0; i < poly.Length; i++)
    {
      // triangles around the perimeter of the object
      int n = (i + 1) % poly.Length;
      triangles[count_tris] = i;
      triangles[count_tris + 1] = n;
      triangles[count_tris + 2] = i + poly.Length;
      triangles[count_tris + 3] = n;
      triangles[count_tris + 4] = n + poly.Length;
      triangles[count_tris + 5] = i + poly.Length;
      count_tris += 6;
    }
    m.vertices = vertices;
    m.triangles = triangles;
    m.RecalculateNormals();
    m.RecalculateBounds();
    m.Optimize();
    return m;
  }
}

/// <summary>
/// 
/// </summary>
/// <remarks>Source: http://wiki.unity3d.com/index.php?title=Triangulator </remarks>
public class Triangulator
{
  private List<Vector2> m_points = new List<Vector2>();

  public Triangulator(Vector2[] points)
  {
    m_points = new List<Vector2>(points);
  }

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

    int n = m_points.Count;
    if (n < 3)
      return indices.ToArray();

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

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

      int u = v;
      if (nv <= u)
        u = 0;
      v = u + 1;
      if (nv <= v)
        v = 0;
      int 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()
  {
    int n = m_points.Count;
    float A = 0.0f;
    for (int p = n - 1, q = 0; q < n; p = q++)
    {
      Vector2 pval = m_points[p];
      Vector2 qval = m_points[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;
    Vector2 A = m_points[V[u]];
    Vector2 B = m_points[V[v]];
    Vector2 C = m_points[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;
      Vector2 P = m_points[V[p]];
      if (InsideTriangle(A, B, C, P))
        return false;
    }
    return true;
  }

  private 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));
  }
}