三角頂点を扱う＆トライアングルから、平均的に何かを出す！スクリプト

SpawnFromTriangle.cs

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

public class SpawnFromTriangle : MonoBehaviour
{
	public Transform[] hanas;
	public Triangle[] triangles;
	public int numSpawn = 10;
	public float emission = 10f;
	public Transform notSpawnArea;
	int count = 0;

	// Use this for initialization
	void Start ()
	{
		triangles = Triangle.GetTriangles (GetComponent<MeshFilter> ().mesh);
		notSpawnArea.parent = transform;
	}
		
	//	Update is called once per frame
	void Update ()
	{
		for (int i = 0; i < numSpawn; i++) {
			Triangle tr = triangles [Random.Range (0, triangles.Length)];

			if (emission * tr.area < Random.value) 
				continue;

			Vector3 pos = tr.RandomPoint ();
			if ((notSpawnArea.localPosition - pos).magnitude < notSpawnArea.localScale.x)
				continue;

			Quaternion rot = Quaternion.FromToRotation (Vector3.forward, tr.normal);
			Transform t = (Transform)Instantiate (hanas [count = count % hanas.Length], pos, rot);
			t.parent = transform;
			t.localPosition = pos;
			t.localRotation = rot;
			t.localScale *= Random.Range (0.7f, 1f);
			count ++;

		}
	}
}

Triangle.cs

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

public class Triangle {
	public static Triangle[] GetTriangles(Mesh m){
		Vector3[] vertices = m.vertices;
		int[] triangles = m.triangles;
		
		List<Triangle> tList = new List<Triangle>();
		
		for (int i = 0; i < triangles.Length/3; i++)
			tList.Add(new Triangle(vertices[triangles[i*3]], vertices[triangles[i*3+1]], vertices[triangles[i*3+2]]));
		return tList.ToArray();
	}

	Vector3 point1, point2, point3;
	public float area;
	public Vector3 center,normal;

	public Triangle (Vector3 p1, Vector3 p2, Vector3 p3){
		point1 = p1; point2 = p2; point3 = p3;

		Vector3 cross = Vector3.Cross(point2-point1, point3-point1);
		area = cross.magnitude/2f;

		center = (point1+point2+point3)/3f;
		normal = cross.normalized;

	}

	public Vector3 RandomPoint(){
		Vector3
			p1 = Vector3.Lerp (point1, point2, Mathf.Sqrt(Random.value)),
			p2 = Vector3.Lerp (point1, point3, Mathf.Sqrt(Random.value));
		return Vector3.Lerp (p1, p2, Random.value);
	}
}