LetterboxAndScale.cs

using UnityEngine;

/// <summary>
/// Letterboxes the view to the aspect ratio given in targetSize.
/// If stretch is true, view fills camera with given aspect ratio.
/// If stretch is false, view is letterboxed to sizes that are only *1, *3, *3, etc of targetSize.
/// </summary>
[ExecuteInEditMode, RequireComponent(typeof(Camera))]
public class LetterboxAndScale : MonoBehaviour
{
    public Vector2 targetSize = new Vector2(400, 240);

    private new Camera camera;

    public bool stretch = true;

    private void Awake()
    {
        camera = GetComponent<Camera>();
    }

    private void Start()
    {
        AddLetterbox();
    }

    private void Update()
    {
        AddLetterbox();
    }

    private void AddLetterbox()
    {
        Rect cameraRect = camera.rect;

        if (stretch)
        {
            float targetAspectRatio = targetSize.x / targetSize.y;

            float windowAspectRatio = Screen.width / (float)Screen.height;

            float scaleHeight = windowAspectRatio / targetAspectRatio;

            if (scaleHeight < 1f)
            {
                cameraRect.width = 1f;
                cameraRect.height = scaleHeight;
                cameraRect.x = 0;
                cameraRect.y = (1f - scaleHeight) / 2f;
            }
            else
            {
                float scaleWidth = 1f / scaleHeight;

                cameraRect.width = scaleWidth;
                cameraRect.height = 1f;
                cameraRect.x = (1f - scaleWidth) / 2f;
                cameraRect.y = 0;
            }

            camera.rect = cameraRect;
        }
        else
        {
            int nearestWidth  = Mathf.FloorToInt(Screen.width / targetSize.x) * (int)targetSize.x;
            int nearestHeight = Mathf.FloorToInt(Screen.height / targetSize.y) * (int)targetSize.y;
            int xScaleFactor  = Mathf.FloorToInt(nearestWidth / targetSize.x);
            int yScaleFactor  = Mathf.FloorToInt(nearestHeight / targetSize.y);

            int scaleFactor = yScaleFactor < xScaleFactor ? yScaleFactor : xScaleFactor;

            Vector2 scaled = targetSize * scaleFactor;

            cameraRect.width = scaled.x / Screen.width;
            cameraRect.height = scaled.y / Screen.height;
            cameraRect.x = (1f - cameraRect.width) / 2f;
            cameraRect.y = (1f - cameraRect.height) / 2f;

            camera.rect = cameraRect;
        }
    }
}