Unity 截图选择框，中间全透明，边缘半透明

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;

public class jietu : MonoBehaviour, IDragHandler, IEndDragHandler,IBeginDragHandler
{
    void Start()
    {
        //DrawTriangle(30, 0, 100, 250, 200, 100, mat);//三角形的三个定点坐标
    }

    public void OnBeginDrag(PointerEventData eventData)
    {
        dragObj = eventData.rawPointerPress;
    }

    public void OnDrag(PointerEventData eventData)
    {
        if (dragObj!=null)
        {
            dragObj.transform.position = Input.mousePosition;

            leftBottomPos   = leftBottom.transform.position;
            leftTopPos      = leftTop.transform.position;
            rightTopPos     = rightTop.transform.position;
            rightBottomPos = rightBottom.transform.position;

            if (dragObj.Equals(leftBottom))
            {
                leftTopPos.x = dragObj.transform.position.x;
                rightBottomPos.y= dragObj.transform.position.y;
                leftTop.transform.position = leftTopPos;
                rightBottom.transform.position = rightBottomPos;
            }
            else if (dragObj.Equals(leftTop))
            {
                leftBottomPos.x = dragObj.transform.position.x;
                rightTopPos.y = dragObj.transform.position.y;
                leftBottom.transform.position = leftBottomPos;
                rightTop.transform.position = rightTopPos;
            }
            else if (dragObj.Equals(rightTop))
            {
                rightBottomPos.x = dragObj.transform.position.x;
                leftTopPos.y = dragObj.transform.position.y;
                rightBottom.transform.position = rightBottomPos;
                leftTop.transform.position = leftTopPos;
            }
            else if (dragObj.Equals(rightBottom))
            {
                rightTopPos.x = dragObj.transform.position.x;
                leftBottomPos.y = dragObj.transform.position.y;
                rightTop.transform.position = rightTopPos;
                leftBottom.transform.position = leftBottomPos;
            }
        }

    }

    public void OnEndDrag(PointerEventData eventData)
    {
        dragObj = null;
    }

    GameObject dragObj;

    public GameObject leftBottom;
    public GameObject leftTop;
    public GameObject rightTop;
    public GameObject rightBottom;

    Vector2 leftBottomPos   = Vector2.zero;
    Vector2 leftTopPos      = Vector2.zero;
    Vector2 rightTopPos     = Vector2.zero;
    Vector2 rightBottomPos  = Vector2.zero;

    public Material mat;
    void OnRenderObject()
    {
        DrawTriangle();//三角形的三个定点坐标
    }
    void DrawTriangle()
    {
        GL.PushMatrix();
        mat.SetPass(0);
        GL.LoadOrtho();
        GL.Begin(GL.QUADS);//顺时针方向

        //顶部
        GL.Vertex3(leftTop.transform.position.x / Screen.width, leftTop.transform.position.y / Screen.height, 0);
        GL.Vertex3(0, 1, 0);
        GL.Vertex3(1, 1, 0);
        GL.Vertex3(rightTop.transform.position.x / Screen.width, rightTop.transform.position.y / Screen.height, 0);

        //右侧
        GL.Vertex3(rightBottom.transform.position.x / Screen.width, rightBottom.transform.position.y / Screen.height, 0);
        GL.Vertex3(rightTop.transform.position.x / Screen.width, rightTop.transform.position.y / Screen.height, 0);
        GL.Vertex3(1, 1, 0);
        GL.Vertex3(1, 0, 0);

        //底部
        GL.Vertex3(0, 0, 0);//屏幕左下角
        GL.Vertex3(leftBottom.transform.position.x / Screen.width, leftBottom.transform.position.y / Screen.height, 0);//屏幕左上角
        GL.Vertex3(rightBottom.transform.position.x / Screen.width, rightBottom.transform.position.y / Screen.height, 0);//屏幕右上角
        GL.Vertex3(1, 0, 0);//屏幕右下角

        //左侧
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(0, 1, 0);
        GL.Vertex3(leftTop.transform.position.x / Screen.width, leftTop.transform.position.y / Screen.height, 0);
        GL.Vertex3(leftBottom.transform.position.x / Screen.width, leftBottom.transform.position.y / Screen.height, 0);

        GL.End();
        GL.PopMatrix();
    }
}