Planeのアスペクト比を固定したままスケーリングする

FixPlaneAspectRatio.cs

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class FixPlaneAspectRatio : MonoBehaviour
{
    enum HEIGHT_PARAM
    {
        Y,
        Z,
    }

    // Planeに適用しているテクスチャのサイズ
    [SerializeField]
    int originalWidth;
    [SerializeField]
    int originalHeight;
    // 高さを反映する向き
    [SerializeField]
    HEIGHT_PARAM toHeightParam = HEIGHT_PARAM.Z;
    // Updateで適用する
    [SerializeField]
    bool fixOnUpdate;

    Vector3 lastScale = Vector3.zero;
    Vector3 newScale = Vector3.zero;
    // Transformキャッシュ
    Transform myTransform;

    new Transform transform {
        get {
            return myTransform != null ? myTransform : (myTransform = GetComponent<Transform> ());
        }
    }

    void Awake ()
    {
        lastScale = transform.localScale;
    }

    void Update ()
    {
        #if UNITY_EDITOR
        if (Application.isPlaying == false) {
            FixAspectRatio ();
            return;
        }
        #endif

        if (fixOnUpdate) {
            FixAspectRatio ();
        }
    }

    void FixAspectRatio ()
    {
        if (originalWidth <= 0 || originalHeight <= 0) {
            return;
        }

        bool changeX;
        if (lastScale.x != transform.localScale.x) {
            changeX = true;
        } else if (lastScale.z != transform.localScale.z) {
            changeX = false;
        } else {
            return;
        }

        if (changeX) {
            float ratio = transform.localScale.x / (float)originalWidth;
            if (toHeightParam == HEIGHT_PARAM.Y) {
                newScale.Set (transform.localScale.x, (float)originalHeight * ratio, transform.localScale.z);
            } else {
                newScale.Set (transform.localScale.x, transform.localScale.y, (float)originalHeight * ratio);
            }

        } else {
            float ratio = toHeightParam == HEIGHT_PARAM.Y ? transform.localScale.y / (float)originalHeight : transform.localScale.z / (float)originalHeight;
            newScale.Set ((float)originalWidth * ratio, transform.localScale.y, transform.localScale.z);
        }
        transform.localScale = newScale;

        lastScale = transform.localScale;
    }
}