Unity3D画线

using UnityEngine;
using System.Collections;
public class Line : MonoBehaviour
{ 
    public Material mat;
    public Color color = Color.red;
    public Vector3 pos1;
    public Vector3 pos2;
    public bool isReady = false;

    void Start()
    {
        mat.color = color;
    }

    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            pos1 = Input.mousePosition;
        }
        if (Input.GetMouseButtonUp(0))
        {
            pos2 = Input.mousePosition;
            isReady = true;
        }
    }

    void OnPostRender()
    {
        if (isReady)
        {
            GL.PushMatrix();
            mat.SetPass(0);
            GL.LoadOrtho();
            GL.Begin(GL.LINES);
            GL.Color(color);
            GL.Vertex3(pos1.x/Screen.width, pos1.y/Screen.height, pos1.z);
            GL.Vertex3(pos2.x / Screen.width, pos2.y / Screen.height, pos2.z);
            GL.End();
            GL.PopMatrix();
        }
    }
}