skybox uv sphere shader

SkyboxUV.shader

// ref: https://qiita.com/aa_debdeb/items/2739745b3ab1a003d767
Shader "Skybox/UV"
{
    Properties
    {
        [NoScaleOffset] _Tex ("Tex", 2D) = "white" {}
        _Scale("Scale", float) = 1
    }

    SubShader
    {
        Tags
        {
            "RenderType"="Background"
            "Queue"="Background"
            "PreviewType"="SkyBox"
        }

        Pass
        {
            ZWrite Off
            Cull Off

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            static const float PI = 3.14159265f;
            sampler2D _Tex;
            float _Scale;

            struct appdata
            {
                float4 vertex : POSITION;
                float3 texcoord : TEXCOORD0;
            };

            struct v2f
            {
                float4 vertex : SV_POSITION;
                float3 texcoord : TEXCOORD0;
            };

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.texcoord = v.texcoord;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                float3 p = i.texcoord;
                float l_xz = length(float2(p.x, p.z));
                float y = (atan2(p.y, l_xz) / PI) + 0.5;
                float x = (atan2(p.x, p.z) / PI + 1) * 0.5;
                x = frac(x * _Scale);
                y = frac(y * _Scale);
                fixed4 col = tex2D(_Tex, float2(x,y));
                col.rgb *= col.a;
                return col;
            }
            ENDCG
        }
    }
}