Unity3D实现模型随机切割

public static GameObject[] Split(GameObject obj, Vector3 a, Vector3 b, Vector3 c)
 {
  if(obj == null)
  {
  return null;
  }
  MeshFilter filter = obj.GetComponent<MeshFilter>();
  if(filter == null)
  {
  return null;
  }
 
  //切割面位置调整为相对于模型的本地坐标
  a = a - obj.transform.position;
  b = b - obj.transform.position;
  c = c - obj.transform.position;
  
  List<Vector3> vertices = new List<Vector3>(filter.mesh.vertices);
  List<int> triangles = new List<int>(filter.mesh.triangles);
  List<Vector2> uvs = new List<Vector2>(filter.mesh.uv);
 
  for (int i = 0; i < filter.mesh.triangles.Length; i = i + 3)
  {
  //取三角形;
  Vector3[] p = new Vector3[3];
  for (int m = 0; m < 3; m++)
  {
   p[m] = filter.mesh.vertices[filter.mesh.triangles[i + m]];
  }
 
  //0 1 2 
  //1 2 0 ==> 切割每条边，判断是否有交点，如有交点，在交点处生成两个新的顶点:L/R
  //2 0 1
  //凡是顶点与平面相交的，一律以新顶点替换
  //判断以交点为其中一个顶点的三角形在面的哪一面
  //指定:交点到其它顶点形成的向量与平面法向量方向一致，则使用L，否则使用R
  //无交点
  //其中一个顶点在平面上
  //其中的一条边在平面上
  //整个三角形都在平面上
 
  List<Point> cross = new List<Point>();
 
  for (int m = 0; m < 3; m++)
  {
   //求线段与面的交点-无交点返回null
   Point tpoint = MathfUtils.LineCrossPlane(p[m], p[(m + 1) % 3], a, b, c);
 
   //排除线段两个端点与平面相交的情况;
   if (MathfUtils.PointAtPlane(p[m], a, b, c) || MathfUtils.PointAtPlane(p[(m + 1) % 3], a, b, c))
   {
   cross.Add(null);
   continue;
   }
 
   cross.Add(tpoint);
  }
 
  int tcount = cross.FindAll(t => t != null).Count;
 
  if (tcount == 0)
  {
   //完全没交点;
   continue;
  }
 
  
  if(tcount == 1)
  {
   //只与一条边有交点;
   //012 tidx = 0 交点x在 0-1上，则有三角形 02x 12x
   //012 tidx = 1 交点x在 1-2上，则有三角形 01x 02x
   //012 tidx = 2 交点x在 2-3上，则有三角形 01x 12x
   int tidx = cross.FindIndex(t => t != null);
   if(tidx < 0)
   {
   continue;
   }
   vertices.Add(cross[tidx].GetVector3());
   vertices.Add(cross[tidx].GetVector3());
   Vector2 tuv = (uvs[triangles[i + tidx]] + uvs[triangles[i + (tidx + 1) % 3]]) * 0.5f;
   uvs.Add(tuv);
   uvs.Add(tuv);
 
   //计算法线，保证新三角形与原来的三角形法线保持一致;
   Vector3 nor0 = Vector3.Cross((p[1] - p[0]).normalized, (p[2] - p[0]).normalized);
 
   //改一个
   triangles[i + 0] = filter.mesh.triangles[i + tidx];
   triangles[i + 1] = filter.mesh.triangles[i + (tidx + 2) % 3];
   triangles[i + 2] = vertices.Count - 2;
   Vector3 nor1 = Vector3.Cross((vertices[triangles[i + 1]] - vertices[triangles[i + 0]]).normalized,
   (vertices[triangles[i + 2]] - vertices[triangles[i + 0]]).normalized);
   if(Vector3.Dot(nor0, nor1) < 0)
   {
   //使用法线方向判断三角形顶点顺序是否与原来一致
   int tpidx = triangles[i + 1];
   triangles[i + 1] = triangles[i + 2];
   triangles[i + 2] = tpidx;
   }
 
   //新增一个
   triangles.Add(filter.mesh.triangles[i + (tidx + 1) % 3]);
   triangles.Add(filter.mesh.triangles[i + (tidx + 2) % 3]);
   triangles.Add(vertices.Count - 1);
   Vector3 nor2 = Vector3.Cross((vertices[triangles[triangles.Count - 2]] - vertices[triangles[triangles.Count - 3]]).normalized,
   (vertices[triangles[triangles.Count - 1]] - vertices[triangles[triangles.Count - 3]]).normalized);
   if (Vector3.Dot(nor0, nor2) < 0)
   {
   int tpidx = triangles[triangles.Count - 1];
   triangles[triangles.Count - 1] = triangles[triangles.Count - 2];
   triangles[triangles.Count - 2] = tpidx;
   }
 
  }
 
  if(tcount == 2)
  {
   //与两条边有交点;
   //012 tidx = 0 交点xy不在 0-1上，则有三角形 xy2 xy1 01y
   //012 tidx = 1 交点xy不在 1-2上，则有三角形 xy0 xy2 12y
   //012 tidx = 2 交点xy不在 2-3上，则有三角形 xy1 xy0 01y
   // x-y-tidx+2 是独立三角形，使用一组顶点
   int tidx = cross.FindIndex(t => t == null);
   if (tidx < 0)
   {
   continue;
   }
 
   //计算法线，保证新三角形与原来的三角形法线保持一致;
   Vector3 nor0 = Vector3.Cross((p[1] - p[0]).normalized, (p[2] - p[0]).normalized);
 
   //x
   vertices.Add(cross[(tidx + 1) % 3].GetVector3());
   vertices.Add(cross[(tidx + 1) % 3].GetVector3());
   Vector2 tuvx = (uvs[triangles[i + (tidx + 1) % 3]] + uvs[triangles[i + (tidx + 2) % 3]]) * 0.5f;
   uvs.Add(tuvx);
   uvs.Add(tuvx);
   //y
   vertices.Add(cross[(tidx + 2) % 3].GetVector3());
   vertices.Add(cross[(tidx + 2) % 3].GetVector3());
   Vector2 tuvy = (uvs[triangles[i + tidx]] + uvs[triangles[i + (tidx + 2) % 3]]) * 0.5f;
   uvs.Add(tuvy);
   uvs.Add(tuvy);
 
   //改一个
   triangles[i + 0] = filter.mesh.triangles[i + (tidx + 2) % 3];
   triangles[i + 1] = vertices.Count - 4;
   triangles[i + 2] = vertices.Count - 2;
   Vector3 nor1 = Vector3.Cross((vertices[triangles[i + 1]] - vertices[triangles[i + 0]]).normalized,
   (vertices[triangles[i + 2]] - vertices[triangles[i + 0]]).normalized);
   if (Vector3.Dot(nor0, nor1) < 0)
   {
   int tpidx = triangles[i + 1];
   triangles[i + 1] = triangles[i + 2];
   triangles[i + 2] = tpidx;
   }
 
   //新增一个
   triangles.Add(filter.mesh.triangles[i + (tidx + 1) % 3]);
   triangles.Add(vertices.Count - 3);
   triangles.Add(vertices.Count - 1);
   Vector3 nor2 = Vector3.Cross((vertices[triangles[triangles.Count - 2]] - vertices[triangles[triangles.Count - 3]]).normalized,
   (vertices[triangles[triangles.Count - 1]] - vertices[triangles[triangles.Count - 3]]).normalized);
   if (Vector3.Dot(nor0, nor2) < 0)
   {
   int tpidx = triangles[triangles.Count - 1];
   triangles[triangles.Count - 1] = triangles[triangles.Count - 2];
   triangles[triangles.Count - 2] = tpidx;
   }
 
   //新增一个
   triangles.Add(filter.mesh.triangles[i + tidx % 3]);
   triangles.Add(filter.mesh.triangles[i + (tidx + 1) % 3]);
   triangles.Add(vertices.Count - 1);
   Vector3 nor3 = Vector3.Cross((vertices[triangles[triangles.Count - 2]] - vertices[triangles[triangles.Count - 3]]).normalized,
   (vertices[triangles[triangles.Count - 1]] - vertices[triangles[triangles.Count - 3]]).normalized);
   if (Vector3.Dot(nor0, nor3) < 0)
   {
   int tpidx = triangles[triangles.Count - 1];
   triangles[triangles.Count - 1] = triangles[triangles.Count - 2];
   triangles[triangles.Count - 2] = tpidx;
   }
  }
  }
 
  //根据顶点索引数组确定mesh被分成了几份
  //经实验：不可行;因为同一个位置的点在不同的面中是不同的点，无法判断这两个三角形是否是连接起来的
  //故只能按方向将模型分成两个
  List<List<int>> ntriangles = new List<List<int>>();
  List<List<int>> temps = new List<List<int>>();
  List<List<Vector3>> nvertices = new List<List<Vector3>>();
  List<List<Vector2>> nuvs = new List<List<Vector2>>();
 
  //切割面的法向量;
  Vector3 pnormal = Vector3.Cross((c - a).normalized, (b - a).normalized);
  ntriangles.Add(new List<int>());
  ntriangles.Add(new List<int>());
  temps.Add(new List<int>());
  temps.Add(new List<int>());
  nuvs.Add(new List<Vector2>());
  nuvs.Add(new List<Vector2>());
  nvertices.Add(new List<Vector3>());
  nvertices.Add(new List<Vector3>());
  for (int i = 0; i < triangles.Count; i = i + 3)
  {
  //判断新的三角形在面的哪一侧;
  float t = 0;
  for(int j = 0; j < 3; j++)
  {
   Vector3 dir = (vertices[triangles[i + j]] - a).normalized;
   float tt = Vector3.Dot(dir, pnormal);
   t = Mathf.Abs(tt) > Mathf.Abs(t) ? tt : t;
  }
  
  int tidx = t >= 0 ? 0 : 1;
  
 
  for (int j = 0; j < 3; j++)
  {
   int idx = temps[tidx].IndexOf(triangles[i + j]);
   if (idx < 0)
   {
   ntriangles[tidx].Add(nvertices[tidx].Count);
   nvertices[tidx].Add(vertices[triangles[i + j]]);
   temps[tidx].Add(triangles[i + j]);
   nuvs[tidx].Add(uvs[triangles[i + j]]);
   continue;
   }
 
   ntriangles[tidx].Add(idx);
  }
  }
 
  if(nvertices[0].Count == 0 || nvertices[1].Count == 0)
  {
  //没有切割到物体
  return null;
  }
 
  //生成新的模型;
  List<GameObject> items = new List<GameObject>();
  MeshRenderer render = obj.GetComponent<MeshRenderer>();
  for (int i = 0; i < ntriangles.Count; i++)
  {
  GameObject tobj = new GameObject(i.ToString());
  tobj.transform.position = obj.transform.position;
  items.Add(tobj);
  MeshFilter fi = tobj.AddComponent<MeshFilter>();
  MeshRenderer mr = tobj.AddComponent<MeshRenderer>();
  
  if(render != null)
  {
   mr.material = render.material;
  }
 
  Mesh mesh = new Mesh();
  mesh.vertices = nvertices[i].ToArray();
  mesh.triangles = ntriangles[i].ToArray();
  mesh.uv = nuvs[i].ToArray();
 
  mesh.RecalculateNormals();
  mesh.RecalculateTangents();
  mesh.RecalculateBounds();
 
  fi.mesh = mesh;
  }
 
  return items.ToArray();
}