ScreenCornersToWorld.cs

using UnityEngine;

/// <summary>
/// The following script converts camera viewport corners' coordinates from viewport to world space.
/// </summary>

public class ScreenCornersToWorld : MonoBehaviour
{
    public Camera Camera;
    public Vector3 BottomLeftCorner, TopLeftCorner, TopRightCorner, BottomRightCorner;

    void Start()
    {
        if (!Camera) Camera = Camera.main;

        BottomLeftCorner = Camera.ViewportToWorldPoint(new Vector3(0, 0, Camera.nearClipPlane));
        TopLeftCorner = Camera.ViewportToWorldPoint(new Vector3(0, 1, Camera.nearClipPlane));
        TopRightCorner = Camera.ViewportToWorldPoint(new Vector3(1, 1, Camera.nearClipPlane));
        BottomRightCorner = Camera.ViewportToWorldPoint(new Vector3(1, 0, Camera.nearClipPlane));
    }
}