Alpha Erode Particle Shader Unity

Shader "Maki/Particle/AlphaErode"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _AlphaScale("Alpha Scale", float) = 1
        _MainColor("Main Color", color) = (1,1,1,1)
    }
    SubShader
    {
        Tags {
            "IgnoreProjector"="True"
            "Queue"="Transparent"
            "RenderType"="Transparent"
        }

        Pass
        {
            zwrite off

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag


            #include "UnityCG.cginc"

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

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

            sampler2D _MainTex;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                o.color = v.color;
                o.color.a = 1 - o.color.a;

                return o;
            }

            float _AlphaScale;
            half4 _MainColor;

            fixed4 frag (v2f i) : SV_Target
            {
                fixed col = tex2D(_MainTex, i.uv * _AlphaScale);
                clip(col - i.color.a);
                return _MainColor * i.color;

            }
            ENDCG
        }
    }
}