new-horizons/NewHorizons/Utility/MakeMeshDoubleFaced.cs

using UnityEngine;
namespace NewHorizons.Utility
{
    public class MakeMeshDoubleFaced : MonoBehaviour
    {
        private 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 (var 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 (var 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
                var 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!
        }
    }
}