unity3d：ugui世界坐标朝向摄像机

using UnityEngine;
using System.Collections;

public class LookAtCam : MonoBehaviour {
    public Transform m_camTrans;
    // Use this for initialization
    void Start () {

    }

    void Update()
    {
        Vector3 relativePos = transform.position - m_camTrans.position;
        Quaternion lookAtRotation = Quaternion.LookRotation(relativePos, Vector3.up);
        this.transform.rotation = Quaternion.Lerp(this.transform.rotation, lookAtRotation,  Time.deltaTime);
    }
}