Unity3D Shader实现动态屏幕遮罩

Shader "Peter/DarkEffect"
{
 Properties
 {
  _MainTex ("Texture", 2D) = "white" {}
 }

 SubShader
 {
  // No culling or depth
  Cull Off ZWrite Off ZTest Always

  Pass
  {
   CGPROGRAM
   #pragma vertex vert
   #pragma fragment frag

   #include "UnityCG.cginc"

   //追踪物体最多个数
   #define ItemSize 9

   struct appdata
   {
    float4 vertex : POSITION;
    float2 uv : TEXCOORD0;
   };

   struct v2f
   {
    float2 uv : TEXCOORD0;
    float4 vertex : SV_POSITION;
   };

   sampler2D _MainTex;

   fixed4 _DarkColor;
   float _SmoothLength;
   fixed _ItemCnt;
   float4 _Item[ItemSize];

   v2f vert (appdata v)
   {
    v2f o;
    o.vertex = UnityObjectToClipPos(v.vertex);
    o.uv = v.uv;
    return o;
   }

   fixed CalcAlpha(float4 vt, float4 pt)
   {
    if(pt.z < 0)
    {
     return 1;
    }

    float distPow2 = pow(vt.x - pt.x, 2) + pow(vt.y - pt.y, 2);
    float dist = (distPow2 > 0) ? sqrt(distPow2) : 0;

    float smoothLength = _SmoothLength;
    if(smoothLength < 0)
    {
     smoothLength = 0;
    }

    float maxValue = pt.z;
    float minValue = pt.z - smoothLength;
    if(minValue < 0)
    {
     minValue = 0;
     smoothLength = pt.z;
    }

    if(dist <= minValue)
    {
     return 0;
    }
    else if (dist > maxValue)
    {
     return 1;
    }

    fixed retVal = (dist - minValue) / smoothLength;

    return retVal;
   }

   fixed4 frag (v2f i) : SV_Target
   {
    fixed alphaVal = 1;
    fixed tmpVal = 1;

    for(fixed index = 0; index < _ItemCnt; ++index)
    {
     tmpVal = CalcAlpha(i.vertex, _Item[index]);
     if(tmpVal < alphaVal)
     {
      alphaVal = tmpVal;
     }
    }

    alphaVal *= _DarkColor.a;

    return tex2D(_MainTex, i.uv) * ( 1 - alphaVal) + _DarkColor * alphaVal;
   }

   ENDCG
  }
 }
}

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

[ExecuteInEditMode]
[RequireComponent(typeof(Camera))]
public class DarkEffect : MonoBehaviour
{
 [System.Serializable]
 public class Item
 {
  [SerializeField]
  public Transform target;

  [SerializeField]
  public int radius;

  public Vector3 GetScreenPosition(Camera cam)
  {
   return cam.WorldToScreenPoint(target.position);
  }
 }

 //渐变像素数量
 public int _smoothLength = 20;
 //遮罩混合颜色
 public Color _darkColor = Color.black;
 //目标物体
 public List<Item> _items = new List<Item>();

 protected Material _mainMaterial;
 protected Camera _mainCamera;

 Vector4[] _itemDatas;
 Item _tmpItem;
 Vector4 _tmpVt;
 Vector3 _tmpPos;
 int _tmpScreenHeight;

 private void OnEnable()
 {
  _mainMaterial = new Material(Shader.Find("Peter/DarkEffect"));
  _mainCamera = GetComponent<Camera>();
 }

 private void OnRenderImage(RenderTexture source, RenderTexture destination)
 {

  if (_itemDatas == null || _itemDatas.Length != _items.Count)
  {
   _itemDatas = new Vector4[_items.Count];
  }

  _tmpScreenHeight = Screen.height;

  for (int i = 0; i < _items.Count; i++)
  {
   _tmpItem = _items[i];
   _tmpPos = _tmpItem.GetScreenPosition(_mainCamera);

   _tmpVt.x = _tmpPos.x;
   _tmpVt.y = _tmpScreenHeight - _tmpPos.y;
   _tmpVt.z = _tmpItem.radius;
   _tmpVt.w = 0;

   _itemDatas[i] = _tmpVt;
  }

  _mainMaterial.SetInt("_SmoothLength", _smoothLength);
  _mainMaterial.SetColor("_DarkColor", _darkColor);
  _mainMaterial.SetInt("_ItemCnt", _itemDatas.Length);
  _mainMaterial.SetVectorArray("_Item", _itemDatas);

  Graphics.Blit(source, destination, _mainMaterial);
 }
}