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Unityでオブジェクトを指定の方向に向ける

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オブジェクトを指定の方向に向けたい

lookat1.jpg
図のようにCubeをSphereの方向に向かせたい場合
transform.forwardに方向ベクトルを突っ込む方法とtransform.LookAt関数を使う方法がある

検証コード1

LookAtTest.cs
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class LookAtTest : MonoBehaviour 
{
    public Transform _target;

    readonly int COUNT = 100000;

    void Update()
    {
        if(Input.GetKeyDown(KeyCode.A))
        {
            float startTime = Time.realtimeSinceStartup;
            for (int i = 0; i < COUNT; i++) {
                Vector3 direction = (_target.position - this.transform.position).normalized;
                this.transform.forward = direction;
            }
            float end1 = Time.realtimeSinceStartup - startTime;

            startTime = Time.realtimeSinceStartup;
            for (int i = 0; i < COUNT; i++) {
                Vector3 direction = (_target.position - this.transform.position);
                this.transform.forward = direction;
            }
            float end2 = Time.realtimeSinceStartup - startTime;

            startTime = Time.realtimeSinceStartup;
            for (int i = 0; i < COUNT; i++) {
                this.transform.LookAt (_target.position);
            }
            float end3 = Time.realtimeSinceStartup - startTime;

            Debug.Log (end1);    //0.04388928sec
            Debug.Log (end2);    //0.03444195sec
            Debug.Log (end3);    //0.02334642sec
        }
    }
}

手法 実行時間
forwardに方向ベクトル normalizeあり 0.04388928sec
forwardに方向ベクトル normalizeなし 0.03444195sec
LookAt関数 0.02334642sec

何も考えずにLookAt関数を使うのが一番早い
forwardに突っ込む場合もわざわざnormalize(正規化)しなくても大丈夫

オブジェクトを軸固定で指定の方向に向けたい

lookat2.jpg
図のようにCubeをSphereの方向にY軸だけ回転させて向かせたい場合
外積を用いる方法、transform.LookAt関数のtargetの座標Yを一致させる方法
targetの座標Yを一致させ、Quaternion.LookRotationを使う方法がある

検証コード2

LookAtTest2.cs
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class LookAtTest2 : MonoBehaviour 
{
    public Transform _target;

    Vector3 _RotAxis = Vector3.up;

    readonly int COUNT = 100000;

    void Update()
    {
        if (Input.GetKeyDown (KeyCode.S)) 
        {
            float startTime = Time.realtimeSinceStartup;
            for (int i = 0; i < COUNT; i++) {
                Vector3 direction = (_target.position - this.transform.position).normalized;
                Vector3 xAxis = Vector3.Cross (_RotAxis, direction).normalized;
                Vector3 zAxis = Vector3.Cross (xAxis, _RotAxis).normalized;
                this.transform.rotation = Quaternion.LookRotation(zAxis,_RotAxis);
            }
            float end1 = Time.realtimeSinceStartup - startTime;

            startTime = Time.realtimeSinceStartup;
            for (int i = 0; i < COUNT; i++) {
                Vector3 direction = (_target.position - this.transform.position);
                Vector3 xAxis = Vector3.Cross (_RotAxis, direction);
                Vector3 zAxis = Vector3.Cross (xAxis, _RotAxis);
                this.transform.rotation = Quaternion.LookRotation(zAxis,_RotAxis);
            }
            float end2 = Time.realtimeSinceStartup - startTime;

            startTime = Time.realtimeSinceStartup;
            for (int i = 0; i < COUNT; i++) {
                Vector3 target = _target.position;
                target.y = this.transform.position.y;
                this.transform.LookAt (target);
            }
            float end3 = Time.realtimeSinceStartup - startTime;

                        startTime = Time.realtimeSinceStartup;
            for (int i = 0; i < COUNT; i++) {
                Vector3 target = _target.position;
                target.y = this.transform.position.y;
                this.transform.rotation = Quaternion.LookRotation (target , _RotAxis);
            }
            var end4 = Time.realtimeSinceStartup - startTime;

            Debug.Log (end1);    //0.06525421sec
            Debug.Log (end2);    //0.03794098sec
            Debug.Log (end3);    //0.03161621sec
                        Debug.Log (end4);    //0.03028011sec
        }
    }
}

手法 実行時間
外積 normalizeあり 0.06525421sec
外積 normalizeなし 0.03794098sec
LookAt関数 0.03161621sec
LookRotation関数 0.03028011sec

LookRotation関数の方がLookAt関数よりわずかに早い

参考リンク

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