mapping texture uvs to sphere for skybox

Question

I've been trying to learn shader coding, specifically procedural skyboxes. I've been using this guide here which is a shader graph tutorial and try to write out the steps in code. However, I'm stuck at the first part about fixing the texture UV's to wrap around a sphere as the current texture is warped:

Here is my current shader:

Shader "Unlit/TestSky"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 100
    Pass
    {
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        // make fog work
        #pragma multi_compile_fog

        #include "UnityCG.cginc"

        struct appdata
        {
            float4 vertex : POSITION;
            float2 uv : TEXCOORD0;
            float3 normal:NORMAL;
        };

        struct v2f
        {
            float2 uv : TEXCOORD0;
            UNITY_FOG_COORDS(1)
            float4 vertex : SV_POSITION;
            float3 normal :NORMAL;
        };

        sampler2D _MainTex;
        float4 _MainTex_ST;



        v2f vert (appdata v)
        {
            v2f o;
            o.vertex = normalize(UnityObjectToClipPos(v.vertex));
            o.normal = v.normal;
            o.uv = TRANSFORM_TEX(v.uv, _MainTex);
            UNITY_TRANSFER_FOG(o,o.vertex);
            return o;
        }
         #define PI 3.141592653589793
        fixed4 frag (v2f i) : SV_Target
        {
            float3 pos = i.vertex;
            float2 newUV;
            newUV.x = 0.5 + atan2(pos.z,pos.x)/PI*2;
            newUV.y = 0.5 - asin(pos.y)/PI;

            fixed4 col = tex2D(_MainTex, i.uv);


            UNITY_APPLY_FOG(i.fogCoord, col);
            return col;
        }
        ENDCG
    }
}

}

I also used wikipedias formula on wrapping uvs to a sphere:

However, when I go back to the Unity Editor all I see is this:

I tried using this solution:

Shader "Unlit/TestSky"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 100
    Pass
    {
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag
        // make fog work
        #pragma multi_compile_fog

        #include "UnityCG.cginc"

        struct appdata
        {
            float4 vertex : POSITION;
            float2 uv : TEXCOORD0;
            float3 normal:NORMAL;
        };

        struct v2f
        {
            UNITY_FOG_COORDS(1)
            float4 vertex : SV_POSITION;
            float3 normal :NORMAL;
            float3 direction : TEXCOORD0;
        };

        sampler2D _MainTex;
        float4 _MainTex_ST;



        v2f vert (appdata v)
        {
            v2f o;
            o.vertex = (UnityObjectToClipPos(v.vertex));
            o.direction = mul(unity_ObjectToWorld, v.vertex).xyz - _WorldSpaceCameraPos;
            return o;
        }
         #define PI 3.141592653589793
        fixed4 frag (v2f i) : SV_Target
        {
            float3 pos = normalize(i.direction);
            float2 newUV;
            newUV.x = 0.5 + atan2(pos.z,pos.x)/PI*2;
            newUV.y = 0.5 - asin(pos.y)/PI;

            float2 dx = ddx(newUV);
            float2 dy = ddy(newUV);
            float2 du = float2(dx.x, dy.x);
            du -= (abs(du) > 0.5f) * sign(du);
            dx.x = du.x;
            dy.x = du.y;

            // In case you want to rotate your view using the texture x-offset.
            newUV.x += _MainTex_ST.z;     

            // Sample the texture with our calculated UV & seam fixup.
            fixed4 col = tex2Dgrad(_MainTex, newUV, dx, dy);
            return col;
        }
        ENDCG
    }
}

}

But ended up getting this image:

Answer 1

You have mistranslated the formulas you found into code. You need to add parenthesis around the denominator in atan2(pos.z,pos.x)/PI*2, so change it to atan2(pos.z,pos.x)/(PI*2).