先上圖看一下，第一張是截出來的圖，第二張，執行圖。

借鑑別人程式碼，組合而成的。。。

廢話不說了，程式碼中的註釋還算詳細

using UnityEngine;
using System.Collections;
using System.IO;

public class draw : MonoBehaviour {
	public Color rColor = Color.green;
	private Vector3 start = Vector3.zero;
	private Vector3 end = Vector3.zero;
	private Material rMat = null;
	private bool drawFlag = false;
	private Rect rect;  
	private Texture2D cutImage; 
	// Use this for initialization
	void Start () {
		rMat =  new Material( "Shader \"Lines/Colored Blended\" {" +
		                        "SubShader { Pass { " +
		                        "    Blend SrcAlpha OneMinusSrcAlpha " +
		                        "    ZWrite Off Cull Off Fog { Mode Off } " +
		                        "    BindChannels {" +
		                        "      Bind \"vertex\", vertex Bind \"color\", color }" +
		                        "} } }" );//生成畫線的材質
		rMat.hideFlags = HideFlags.HideAndDontSave;
		rMat.shader.hideFlags = HideFlags.HideAndDontSave;  
	}
	
	// Update is called once per frame
	void Update () {
		if(Input.GetMouseButtonDown(0)){
			drawFlag = true;//如果滑鼠左鍵按下 設定開始畫線標誌
			start = Input.mousePosition;//記錄按下位置
			end = Input.mousePosition;//滑鼠當前位置
		}else if (Input.GetMouseButtonUp(0)){
			drawFlag = false;//如果滑鼠左鍵放開 結束畫線

		}
		if (Input.GetKeyDown (KeyCode.P)) {
			StartCoroutine(CutImage());
		}
	}
	//繪製框選
	void OnPostRender() {//畫線這種操作推薦在OnPostRender（）裡進行 而不是直接放在Update，所以需要標誌來開啟
		if(drawFlag){
			end = Input.mousePosition;//滑鼠當前位置
		}
		GL.PushMatrix();//儲存攝像機變換矩陣
			
			if (! rMat)
				return;
			
			rMat.SetPass( 0 );
			GL.LoadPixelMatrix();//設定用螢幕座標繪圖
			GL.Begin(GL.QUADS);
			GL.Color( new Color(rColor.r,rColor.g,rColor.b,0.1f) );//設定顏色和透明度，方框內部透明
			GL.Vertex3( start.x,start.y,0);
			GL.Vertex3( end.x,start.y,0);
			GL.Vertex3( end.x,end.y,0 );
			GL.Vertex3( start.x,end.y,0 );
			GL.End();
			GL.Begin(GL.LINES);
			GL.Color(rColor);//設定方框的邊框顏色 邊框不透明
			GL.Vertex3( start.x,start.y,0);
			GL.Vertex3( end.x,start.y,0);
			GL.Vertex3( end.x,start.y,0);
			GL.Vertex3( end.x,end.y,0 );
			GL.Vertex3( end.x,end.y,0 );
			GL.Vertex3( start.x,end.y,0 );
			GL.Vertex3( start.x,end.y,0 );
			GL.Vertex3(start.x,start.y,0);
			GL.End();
			GL.PopMatrix();//恢復攝像機投影矩陣
		
	}

	IEnumerator CutImage()  
	{  
		
		string date = System.DateTime.Now.ToString("yyyyMMddHHmmss");
		//圖片大小  
		cutImage = new Texture2D((int)(end.x - start.x), (int)(start.y - end.y), TextureFormat.RGB24, true);  
		
		
		//座標左下角為0  
		rect = new Rect((int)start.x, Screen.height - (int)(Screen.height - end.y), (int)(end.x - start.x), (int)(start.y - end.y));  
		
		yield return new WaitForEndOfFrame();  
		cutImage.ReadPixels(rect, 0, 0, true);   
		
		cutImage.Apply();  
		yield return cutImage;  
		byte[] byt = cutImage.EncodeToPNG();  
		//儲存截圖  
		File.WriteAllBytes(Application.streamingAssetsPath + "/CutImage" +date + ".png", byt);  


	}  


}
 

<span style="white-space:pre">			</span>GL.LoadPixelMatrix();//設定用螢幕座標繪圖
			GL.Begin(GL.QUADS);
			GL.Color( new Color(rColor.r,rColor.g,rColor.b,0.1f) );//設定顏色和透明度，方框內部透明
			GL.Vertex3( start.x,start.y,0);
			GL.Vertex3( end.x,start.y,0);
			GL.Vertex3( end.x,end.y,0 );
			GL.Vertex3( start.x,end.y,0 );
			GL.End();
 

這段去掉，或者

GL.Color( new Color(rColor.r,rColor.g,rColor.b,0.1f) );

的值改成0全透明