blah.shader

Shader "Custom/Circle" {
	Properties
	{
	_Scale("Scale", Float) = 1
	}
		SubShader{
			Blend SrcAlpha OneMinusSrcAlpha
			Pass{
			CGPROGRAM
	#pragma vertex vert
	#pragma fragment frag
	#pragma target 2.0

	#include "UnityCG.cginc"
	float _Scale;
	struct vertexInput {
		float4 vertex : POSITION;
		float4 texcoord0 : TEXCOORD0;
	};

	struct fragmentInput {
		float4 position : SV_POSITION;
		float4 texcoord0 : TEXCOORD0;
	};

	fragmentInput vert(vertexInput i) {
		fragmentInput o;
		o.position = mul(UNITY_MATRIX_MVP, i.vertex);
		o.texcoord0 = i.texcoord0;
		return o;
	}
	fixed4 frag(fragmentInput i) : SV_Target
	{
		float dx = 0.5 - i.texcoord0.x, dy = 0.5 - i.texcoord0.y;
		float dist = dx * dx + dy * dy;
		float distFromCenter = 0.25 - dist;  // positive when inside the circle
		if (distFromCenter > 0) {
			if (distFromCenter > 0.025)
				return 0;
			return 1;
		}
			
		return 0;
	}
		ENDCG
	}
	}
}