unity3D 从摄像机发射射线，检测画线

最近做项目的时候，有一个需要从摄像机发出一条射线，当射线检测到有碰撞物体的时候，点击鼠标左键开始画线。我是创立了一个平板drawing，当然Cube什么的也是可以的。
c#脚本代码，将这个脚本拖到摄像机里，然后拖拽材质到画线材质，拖拽碰撞体到脚本那里即可以。
代码脚本（附有注释）：
<pre><code>
using UnityEngine;
using System.Collections;

public class test : MonoBehaviour {

//-----------划线所能够用到材质  
 public Material lineMaterial  ;  
//  
  int maxPoints = 5000;  
  //全局变量的定义方法  
 public static float lineWidth = 4.0f;  
 int minPixelMove = 2;  // Must move at least this many pixels per sample for a new segment to be recorded  
 public Vector2[] linePoints;  
 private VectorLine  line ;  
 private int  lineIndex = 0;  
  private   Vector2   previousPosition;  
  private int  sqrMinPixelMove ;  
  private bool canDraw = false;  
//记录当下画板的位置，判断平板是否平移等  
  public Vector3 boardPosition;  
  public GameObject board;  
//记录当下画板的大小，判断画板是否缩放等     
 public Vector3 boardScale;   
//初始化平板的大小与位置  
void IntializeScaleAndPosition()  
{  
    boardPosition=Camera.mainCamera.WorldToScreenPoint(board.transform.position);  
    boardScale=board.transform.localScale;  
      
}  
// 初始化  
void Start () {  
    linePoints = new Vector2[maxPoints];  
    line = new VectorLine("DrawnLine", linePoints,lineMaterial, lineWidth, LineType.Continuous);  
    sqrMinPixelMove = minPixelMove * minPixelMove;  
    IntializeScaleAndPosition();  
}  
// Update is called once per frame  
void Update () {  
    Vector2 mousePos = Input.mousePosition;  
    //当画板移动时，将所有的点按照统一方向移动相同的距离
   //当前画板的位置  
   Vector3 currentBoardPositon=Camera.mainCamera.WorldToScreenPoint(board.transform.position);  
      
    //画板的移动距离  
    Vector3  moveDistance=currentBoardPositon-boardPosition;  
      
    if(moveDistance.x!=0||moveDistance.y!=0)  
    {  
        //将所有的画下的点都平移  
        for(int i=0;i<linePoints.Length;i++)  
        {  
        Vector2 dis=new Vector2(moveDistance.x,moveDistance.y);  
        linePoints[i]+=dis;  
        }  
        //销毁当前画下的线  
        Vector.DestroyLine(ref line);  
          
        //距离平移后，所有的点的位置已经重新获得，重新画下新的线  
          
         line = new VectorLine("DrawnLine", linePoints, lineMaterial, lineWidth, LineType.Continuous);  
         line.minDrawIndex = 1;  
         line.maxDrawIndex = lineIndex;  
        Vector.DrawLine(line);  
          
    }     
    boardPosition=Camera.mainCamera.WorldToScreenPoint(board.transform.position);  
    //平板的缩放操作，对平板进行缩放，同时画出的线也要能够实现缩放。  
    //检测平板是否已经开始缩放  
    float currentScale=board.transform.localScale.x;  
    float scaleFactor=currentScale/(boardScale.x);  
    //Debug.Log("scaleFactor "+scaleFactor+" currentScale "+currentScale+" boardScale "+boardScale.x);  
    boardScale=board.transform.localScale;  
    if(scaleFactor!=1||scaleFactor!=1)  
    {     
        // 求出画板中心的位置  
        Vector3 boardPostionToSceern=Camera.mainCamera.WorldToScreenPoint(board.transform.position);  
          
          //当平板缩放时，所有的点都要同比例缩放。  
        for(int i=0;i<linePoints.Length;i++)  
        {  
        Vector3 pointPositon=new Vector3(linePoints[i].x,linePoints[i].y,0);  
        Vector3 PointToCenterDistance=(pointPositon-boardPostionToSceern)*scaleFactor;  
        linePoints[i]=new Vector2((boardPostionToSceern+PointToCenterDistance).x,(boardPostionToSceern+PointToCenterDistance).y);  
        }         
        //销毁当前画下的线  
        Vector.DestroyLine(ref line);  
        //画板缩放后，所有的点的位置已经重新获得，重新画下新的线  
         line = new VectorLine("DrawnLine", linePoints, lineMaterial, lineWidth, LineType.Continuous);  
         line.minDrawIndex = 1;  
         line.maxDrawIndex = lineIndex;  
        Vector.DrawLine(line);    
    }  
    //从摄像头发出一条射线  
    Ray ray=Camera.mainCamera.ScreenPointToRay(Input.mousePosition);  
    RaycastHit hit;  
    if(Physics.Raycast(ray,out hit,1000))  
    {     
    //检测到物体时才开始画  
    if(hit.transform.gameObject.name=="drawing")  
    {         
        if (Input.GetMouseButtonDown(0))  
        {  
          //  Vector.ZeroPointsInLine(line);  
           // line.minDrawIndex = lineIndex;  
          //  Vector.DrawLine(line);  
           // linePoints[lineIndex] = mousePos;  
            previousPosition  = mousePos;  
            lineIndex++;  
            linePoints[lineIndex] = mousePos;  
            canDraw = true;  
        }  
        else if (Input.GetMouseButton(0) && (mousePos - previousPosition).sqrMagnitude > sqrMinPixelMove && canDraw)  
        {  
            previousPosition = linePoints[++lineIndex] = mousePos;  
            line.minDrawIndex = lineIndex - 1;  
            line.maxDrawIndex = lineIndex;  
            if (lineIndex >= maxPoints - 1) canDraw = false;  
            Vector.DrawLine(line);  
        }                 
    }     
}  

</code></pre>