Unity shader实现遮罩效果

Shader "Custom/Mask" {
 Properties {
 _MainTex ("Base (RGB)", 2D) = "white" {}//目标图片，即需要被遮罩的图片
 _MaskLayer("Culling Mask",2D) = "white"{}//混合的图片，设置为白色的图片，任何颜色与白色混合，其颜色不变
 _Cutoff("Alpha cutoff",Range(0,1)) = 0
 }
 SubShader {
 Tags { 
  "Queue"="Transparent" 
 }//渲染队列设置为 以从后往前的顺序渲染透明物体
 Lighting off //关闭光照
 ZWrite off //关闭深度缓存
 Blend off //关闭混合
 AlphaTest GEqual[_Cutoff] //启用alpha测试

 Pass{
  SetTexture[_MaskLayer]{combine texture}//混合贴图
  //混合贴图，previous为放置在前一序列这样在进行AlphaTest的时候会以这个图片为主来进行混合
  SetTexture[_MainTex]{combine texture,previous}
 } 
 }
}

Shader "Custom/CircleMask" {
  Properties {
    _MainTex ("MainTex", 2D) = "white" {}
    _MaskTex ("MaskTex", 2D) = "white" {}
    [HideInInspector]_Cutoff ("Alpha cutoff", Range(0,1)) = 0.5
    //MASK SUPPORT ADD
    _StencilComp ("Stencil Comparison", Float) = 8
    _Stencil ("Stencil ID", Float) = 0
    _StencilOp ("Stencil Operation", Float) = 0
    _StencilWriteMask ("Stencil Write Mask", Float) = 255
    _StencilReadMask ("Stencil Read Mask", Float) = 255
    _ColorMask ("Color Mask", Float) = 15
    //MASK SUPPORT END
  }
  SubShader {
    Tags {
      "IgnoreProjector"="True"
      "Queue"="Transparent"
      "RenderType"="Transparent"
    }
    //MASK SUPPORT ADD
    Stencil
    {
      Ref [_Stencil]
      Comp [_StencilComp]
      Pass [_StencilOp]
      ReadMask [_StencilReadMask]
      WriteMask [_StencilWriteMask]
    }
    ColorMask [_ColorMask]
    //MASK SUPPORT END
    Pass {
      Name "FORWARD"
      Tags {
        "LightMode"="ForwardBase"
      }
      Blend SrcAlpha OneMinusSrcAlpha
      ZWrite Off
      CGPROGRAM
      #pragma vertex vert
      #pragma fragment frag
      #define UNITY_PASS_FORWARDBASE
      #include "UnityCG.cginc"
      #pragma multi_compile_fwdbase
      #pragma only_renderers d3d9 d3d11 glcore gles
      #pragma target 3.0
      uniform sampler2D _MainTex; uniform float4 _MainTex_ST;
      uniform sampler2D _MaskTex; uniform float4 _MaskTex_ST;
      struct VertexInput {
        float4 vertex : POSITION;
        float2 texcoord0 : TEXCOORD0;
      };
      struct VertexOutput {
        float4 pos : SV_POSITION;
        float2 uv0 : TEXCOORD0;
      };
      VertexOutput vert (VertexInput v) {
        VertexOutput o = (VertexOutput)0;
        o.uv0 = v.texcoord0;
        o.pos = UnityObjectToClipPos( v.vertex );
        return o;
      }
      float4 frag(VertexOutput i) : COLOR {
////// Lighting:
        float4 _MainTex_var = tex2D(_MainTex,TRANSFORM_TEX(i.uv0, _MainTex));
        float3 finalColor = _MainTex_var.rgb;
        float4 _MaskTex_var = tex2D(_MaskTex,TRANSFORM_TEX(i.uv0, _MaskTex));
        return fixed4(finalColor,_MaskTex_var.a);
      }
      ENDCG
    }
  }
  FallBack "Diffuse"
}