spiral shader

Shader "Unlit/NewUnlitShader"
{
	Properties
	{
		_MainTex ("Mask", 2D) = "white" {}
	}
	SubShader
	{
	Tags {"Queue"="Transparent" "IgnoreProjector"="True"}
    ZWrite Off
    Blend SrcAlpha OneMinusSrcAlpha

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag

			#include "UnityCG.cginc"

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

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

			sampler2D _MainTex;
			float4 _MainTex_ST;

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

			fixed4 frag (v2f i) : SV_Target
			{

				float2 st = i.uv;
    st *= 1.;
    st.x -= 0.5;
    st.y -= 0.5;
    float R = 3.5;
    float a = atan2(st.y, st.x);
    float r = length(st);
    a += 100.0 * r/R + 10.0*_Time.y;
    float y = r*sin(a);
    float col =  smoothstep(st.y - 0.1, st.y, y) - smoothstep(st.y, st.y + 0.01, y);

				col *= tex2D(_MainTex, i.uv);

				return float4(col,0,0, step(0.5, col));
			}
			ENDCG
		}
	}
}