Simple masking radial shader

MaskingShader.shader

Shader "Hidden/NewImageEffectShader"
{
	Properties
	{
		_MainTex ("Texture", 2D) = "white" {}
        _MaskTex ("Mask", 2D) = "white" {}
        _MaskValue ("Mask Value", range(0,1)) = 0.0
	}
	SubShader
	{
		// No culling or depth
		Cull Off ZWrite Off ZTest Always
        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;
			};

			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);
				o.uv = v.uv;
				return o;
			}
			
			sampler2D _MainTex;
            sampler2D _MaskTex;
            float _MaskValue;

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

                float mask = (atan2(i.uv.y*2-1, i.uv.x*2-1)/UNITY_PI)/2+0.5;
                float stepValue = step(mask, _MaskValue);
                fixed4 value = stepValue;
                source.a = (source.a * stepValue);
                
                source.rgb = source * value;


				return source;
			}
			ENDCG
		}
	}
}