** Function: create mesh data which describes a plane with a hole (rectangle) inside.

TutorialFogController.cs

using UnityEngine;
using System.Collections;

/* by FVS - HaLuan */

[RequireComponent(typeof(MeshFilter))]
[RequireComponent(typeof(MeshRenderer))]
public class TutorialFogController : MonoBehaviour
{
    public void SetTutorialFog(Vector2 overallSize, Vector2 spotlightPosition, Vector2 spotlightSize)
    {
        GetComponent<MeshFilter>().mesh = GetMesh(overallSize, spotlightPosition, spotlightSize);
    }

    Mesh GetMesh(Vector2 overallSize, Vector2 spotlightPosition, Vector2 spotlightSize)
    {
        Mesh mesh = new Mesh();

        #region Size
        float haftX1 = overallSize.x / 2.0f;
        float haftY1 = overallSize.y / 2.0f;

        float haftX2 = spotlightSize.x / 2.0f;
        float haftY2 = spotlightSize.y / 2.0f;
        #endregion

        #region Vertices
        Vector3 p0 = new Vector2(-haftX1, -haftY1);
        Vector3 p1 = new Vector2(haftX1, -haftY1);
        Vector3 p2 = new Vector2(haftX1, haftY1);
        Vector3 p3 = new Vector2(-haftX1, haftY1);

        Vector3 q0 = new Vector2(-haftX1, -haftY2 + spotlightPosition.y);
        Vector3 q1 = new Vector2(haftX1, -haftY2 + spotlightPosition.y);
        Vector3 q2 = new Vector2(haftX1, haftY2 + spotlightPosition.y);
        Vector3 q3 = new Vector2(-haftX1, haftY2 + spotlightPosition.y);

        Vector3 r0 = new Vector2(-haftX2, -haftY2) + spotlightPosition;
        Vector3 r1 = new Vector2(haftX2, -haftY2) + spotlightPosition;
        Vector3 r2 = new Vector2(haftX2, haftY2) + spotlightPosition;
        Vector3 r3 = new Vector2(-haftX2, haftY2) + spotlightPosition;

        mesh.vertices = new Vector3[]
        {
            p0,p1,p2,p3,
            q0,q1,q2,q3,
            r0,r1,r2,r3,
        };

        #endregion

        #region Triangles

        mesh.triangles = new int[]
        {
            // Top
            3,2,7,
            2,6,7,

            // Bottom
            4,1,0,
            5,1,4,

            // Left
            11,8,4,
            7,11,4,

            // Right
            6,5,9,
            10,6,9
        };
        #endregion        

        ///
        return mesh;
    }
}