Remap Alpha to Palette Texture (asset studio)

RemapAlphaToPalette.shader

// related thread https://forum.unity.com/threads/assetstudio-sprites-alpha8.1485684/
Shader "Unlit/RemapAlphaToPalette"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _PaletteTex ("PaletteTexture", 2D) = "white" {}
    }
    SubShader
    {
        // TODO cutout version
        Blend SrcAlpha OneMinusSrcAlpha
        Tags {"Queue" = "Transparent" "IgnoreProjector" = "True" "RenderType" = "Transparent"}
        LOD 100

        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;
            sampler2D _PaletteTex;
            float4 _MainTex_ST;

            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
            {
                fixed4 col = tex2D(_MainTex, i.uv);
                
                // use maintex alpha as index into palette
                fixed4 pal = tex2D(_PaletteTex, float2(col.a,0.5f));

                return pal;// *col.a;
            }
            ENDCG
        }
    }
}