CameraRectScaler.cs

using UnityEngine;

[ExecuteAlways]
[RequireComponent(typeof(Camera))]
public class CameraRectScaler : MonoBehaviour
{
    [SerializeField] private float targetAspect = 16f / 9f;

    private int lastWidth = 0;
    private int lastHeight = 0;
    private Camera mainCamera;

    private void Start()
    {
        TryGetComponent(out mainCamera);
        ApplyRectScale();
    }

    private void Update()
    {
        // 画面サイズが変わった瞬間だけ再計算
        if (Screen.width != lastWidth || Screen.height != lastHeight)
        {
            ApplyRectScale();
        }
    }

    private void ApplyRectScale()
    {
        lastWidth = Screen.width;
        lastHeight = Screen.height;

        float windowAspect = (float)lastWidth / lastHeight;
        float scale = windowAspect / targetAspect;

        // 横長 → 左右に黒帯（ピラーボックス）
        if (scale > 1f)
        {
            float newWidth = 1f / scale;
            float xOffset = (1f - newWidth) / 2f;
            mainCamera.rect = new Rect(xOffset, 0f, newWidth, 1f);
        }
        // 縦長 → 上下に黒帯（レターボックス）
        else
        {
            float newHeight = scale;
            float yOffset = (1f - newHeight) / 2f;
            mainCamera.rect = new Rect(0f, yOffset, 1f, newHeight);
        }
    }
}