unity3d中用2D背景当作图片

using UnityEngine;
using System.Collections;


public class Backdrop : MonoBehaviour
{
const string SHADER_CODE = 
"Shader \"UnlitAlpha\"{" +
"Properties {" +
"_Color (\"Color Tint (A = Opacity)\", Color) = (1,1,1,1) " +
"_MainTex (\"Texture (A = Transparency)\", 2D) = \"\"" +
"} " +
"SubShader { " +
"Tags {" +
"Queue = Transparent" +
"}" +
"ZWrite Off Blend SrcAlpha OneMinusSrcAlpha Pass {" +
"SetTexture[_MainTex] {" +
"Combine texture * constant ConstantColor[_Color]" +
"}" +
"}" +
"}" +
"}";

public Texture2D image;
public float distance = 500;

GameObject backdrop;
Mesh mesh;
float prevDistance;
Vector3 prevRotation, prevPosition;
Texture2D prevImage;

void Start ()
{
//check that we are on a camera!
if(camera == null){
Debug.LogError("Backdrop must be used on a camera!");
Destroy(this);
return;
}

//create backdrop GameObject and components:
backdrop = new GameObject("Backdrop");
backdrop.AddComponent<MeshFilter>();
backdrop.AddComponent<MeshRenderer>();
backdrop.transform.parent = transform;

backdrop.renderer.material = new Material(SHADER_CODE);

mesh = backdrop.GetComponent<MeshFilter>().mesh;
mesh.vertices = CalcVerts();
mesh.uv = new Vector2[] {new Vector2(0,0), new Vector2(1,0), new Vector2(0,1), new Vector2(1,1)};
mesh.triangles = new int[] {1,0,3,3,0,2};

}

void Update(){

//reset texture if it has changed:
if( backdrop.renderer.material.mainTexture != image ){
backdrop.renderer.material.mainTexture = image;
}

//nothing needs to be recalculated if distance hasn't changed or a move/rotate was attempted on billboard moved without our permission:
if(distance == prevDistance &&
  backdrop.transform.position == prevPosition &&
  backdrop.transform.localEulerAngles == prevRotation
  ) return;

//error for attempting a backdrop placement beyond camera's far clip plane:
if(distance > camera.farClipPlane){
Debug.LogError("Backdrop's distance is further than the camera's far clip plane. Extend the camera's far clip plane or reduce the billboard's distance.");
return;
}

//error for attempting a backdrop placement before camera's near clip plane:
if(distance < camera.nearClipPlane){
Debug.LogError("Backdrop's distance is closer than the camera's near clip plane. Extend the distance or reduce the camera's near clip plane.");
return;
}


//set backdrop's placement:
backdrop.transform.position = transform.forward * distance;

//calculate mesh:
mesh.vertices = CalcVerts();
mesh.RecalculateNormals();

//readjust comparison values:
prevDistance = distance;
prevPosition = backdrop.transform.position;
prevRotation = backdrop.transform.localEulerAngles;
}

Vector3[] CalcVerts()
{
return new Vector3[] {
backdrop.transform.InverseTransformPoint(camera.ScreenToWorldPoint(new Vector3(0,0,distance))),
backdrop.transform.InverseTransformPoint(camera.ScreenToWorldPoint(new Vector3(Screen.width,0,distance))),
backdrop.transform.InverseTransformPoint(camera.ScreenToWorldPoint(new Vector3(0,Screen.height,distance))),
backdrop.transform.InverseTransformPoint(camera.ScreenToWorldPoint(new Vector3(Screen.width,Screen.height,distance)))
};
}
}