unity 背景图 跑起来

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using System;

public class BackgroundController : MonoBehaviour
{
    public Text text;
    public Image back1;
    public Image back2;

    public int speed = 10;                   //背景平移的速度
    public Transform endPosition;           //到达该位置,把背景移动到开始位置
    public Transform startPosition;         //背景从开始位置移动到结束位置

    private int backIndex;

    void Start()
    {
      
    }
    float TimeNum = 0;
    void Update()
    {
        TimeNum += Time.deltaTime;
        if (TimeNum>=1)
        {
            text.text = "HP:" + tt();
            TimeNum = 0;
        }
        //两个图片的平移
        back1.transform.position = new Vector3(back1.transform.position.x - speed * Time.deltaTime, back1.transform.position.y, back1.transform.position.z);
        back2.transform.position = new Vector3(back2.transform.position.x - speed * Time.deltaTime, back2.transform.position.y, back2.transform.position.z);

        //到达结束位置,回到开始位置,切换图片
        if (back1.transform.position.x