BackfaceMesh.cs

using UnityEngine;

// Adds a backface to the mesh on Start()
//   Source: http://answers.unity3d.com/answers/280931/view.html
public class BackfaceMesh : MonoBehaviour {

	public void Start () {
		var mesh = GetComponent<MeshFilter>().mesh;
		var vertices = mesh.vertices;
		var uv = mesh.uv;
		var normals = mesh.normals;
		var szV = vertices.Length;
		var newVerts = new Vector3[szV * 2];
		var newUv = new Vector2[szV * 2];
		var newNorms = new Vector3[szV * 2];
		for (int j = 0; j < szV; j++) {
			// duplicate vertices and uvs:
			newVerts[j] = newVerts[j + szV] = vertices[j];
			newUv[j] = newUv[j + szV] = uv[j];
			// copy the original normals...
			newNorms[j] = normals[j];
			// and revert the new ones
			newNorms[j + szV] = -normals[j];
		}
		var triangles = mesh.triangles;
		var szT = triangles.Length;
		var newTris = new int[szT * 2]; // double the triangles
		for (int i = 0; i < szT; i += 3) {
			// copy the original triangle
			newTris[i] = triangles[i];
			newTris[i + 1] = triangles[i + 1];
			newTris[i + 2] = triangles[i + 2];
			// save the new reversed triangle
			int j = i + szT;
			newTris[j] = triangles[i] + szV;
			newTris[j + 2] = triangles[i + 1] + szV;
			newTris[j + 1] = triangles[i + 2] + szV;
		}
		mesh.vertices = newVerts;
		mesh.uv = newUv;
		mesh.normals = newNorms;
		mesh.triangles = newTris; // assign triangles last!
	}
  
}