【Unity】RenderTextureを使ってオブジェクトにアウトラインを付ける

Shader "Custom/MaronOutlineShader"
{
    Properties
    {
        [NoScaleOffset]_MainTex ("Render Texture", 2D) = "white" {}
        _SubTex ("Pattern Texture", 2D) = "white" {}
        _Color ("Main Color", Color) = (1,1,1,1)
        _Outline ("Outline thickness", Range (0,1)) = 0.1
        _DrawNum ("Drawing times", Range (2,32)) = 2
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "Queue" = "Transparent" }
        LOD 100
        ZWrite off
        Blend srcAlpha OneMinusSrcAlpha

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            // make fog work
            #pragma multi_compile_fog

            #include "UnityCG.cginc"

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

            struct v2f
            {
                float2 uv : TEXCOORD0;
                UNITY_FOG_COORDS(1)
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;
            float4 _MainTex_TexelSize;
            sampler2D _SubTex;
            float4 _SubTex_ST;
            float4 _SubTex_TexelSize;
            float4 _Color;
            float _Outline;
            int _DrawNum;

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

            float2 PolarToCartesian(float r, float theta)
            {
                float x = r * cos(theta);
                float y = r * sin(theta);
                return float2(x, y);
            }

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 col = tex2D(_MainTex, i.uv);

                float2 tiling_offset_uv = float2(i.uv.xy *  _SubTex_ST *float2(1, _MainTex_TexelSize.x/_MainTex_TexelSize.y) +  _SubTex_ST.zw);
                fixed4 subcol = tex2D(_SubTex, tiling_offset_uv);

                float2 outline = _Outline*0.1;

                float PI = 3.14159265358979323;
                int n = _DrawNum;
                float theta;
                for(int j = 0; j < 2 * n; j++)
                {
                    theta = PI * j / n;
                    col += tex2D(_MainTex, i.uv + PolarToCartesian(outline, theta)*float2(_MainTex_TexelSize.x/_MainTex_TexelSize.y, 1));               
                }
                //境界をなめらかに
                col = smoothstep(0.5, 0.51, col);
                // apply fog
                UNITY_APPLY_FOG(i.fogCoord, col);




                //一定以下のalphaはゼロにする
                _Color.a *= step(1-col.a, 0.01);

                return subcol*_Color;
            }
            ENDCG
        }
    }
}

for(int j = 0; j < 2 * n; j++)
{
    theta = PI * j / n;
    col += tex2D(_MainTex, i.uv + PolarToCartesian(outline, theta)*float2(_MainTex_TexelSize.x/_MainTex_TexelSize.y, 1));               
}

float2 PolarToCartesian(float r, float theta)
{
    float x = r * cos(theta);
    float y = r * sin(theta);
    return float2(x, y);
}