FullScreenStencil

Shader "Unlit/FullScreenStencil"
{
    Properties {
        _Color ("Color", Color) = (0.5,0.5,0.5,0.2)
    }
    SubShader
    {
        Tags { "Queue"="Transparent" "RenderType"="Transparent" }
        LOD 100


        Pass
        {
            Blend SrcAlpha OneMinusSrcAlpha
            Cull off
            ZTest off
            ZWrite off
            Stencil {
                Ref 1
                Comp equal
            }

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag alpha:blend

            #include "UnityCG.cginc"

            fixed4 _Color;
            struct appdata
            {
                float4 vertex : POSITION;
            };

            struct v2f
            {
                float4 vertex : SV_POSITION;
            };

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

            fixed4 frag (v2f i) : SV_Target
            {
                return _Color;
            }
            ENDCG
        }
    }
}