Unity3D UGUI Shader画一个圆环

Shader "Unlit/NewUnlitShader"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Color("Tint", Color) = (,,,)

        _StencilComp("Stencil Comparison", Float) = 

        _Stencil("Stencil ID", Float) = 

        _StencilOp("Stencil Operation", Float) = 

        _StencilWriteMask("Stencil Write Mask", Float) = 

        _StencilReadMask("Stencil Read Mask", Float) = 

        _ColorMask("Color Mask", Float) = 

        _Width("_Width", Float) = 

    }
    SubShader
    {
            Tags
        {
            "Queue" = "Transparent"
            "IgnoreProjector" = "True"
            "RenderType" = "Transparent"
            "PreviewType" = "Plane"
        }
        Cull Off
        Lighting Off

        Fog{ Mode Off }
        Blend SrcAlpha OneMinusSrcAlpha

        Stencil
        {
            Ref[_Stencil]
            Comp[_StencilComp]
            Pass[_StencilOp]
            ReadMask[_StencilReadMask]
            WriteMask[_StencilWriteMask]
        }
        ColorMask[_ColorMask]

        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;
            float _Width;
            float4 _Color;

            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
            {
                float d = distance(i.uv,float2(0.5,0.5));
                if (d < 0.5- / _Width)
                {
                    discard;
                }
                if (d > 0.5)
                {
                    discard;
                }
                fixed4 col = tex2D(_MainTex, i.uv);
                return col*_Color;
            }
            ENDCG
        }
    }
}