Limit Color Pallete Using Shader

Question

Basically, what I want to do is take a primary input image texture (which is a render texture in-game) and a secondary pallete texture (contains X number of colors, and is X by 1 pixels in size). What I want to do is replace all of the pixels in the image with the most similar color in the color pallete texture. I made a stab at producing the effect, but at the moment, it only seems to return the first color in the pallete. Below is my (nonfunctional) version. I'm new to shaders, so sorry for the potentially very sloppy/basic shader code.

Shader "Custom/Limiter" {
Properties
{
    _MainTex ("Texture", 2D) = "white" {}
    _PalTex ("Pallete", 2D) = "white" {}
}
SubShader
{
    // No culling or depth
    Cull Off ZWrite Off ZTest Always

    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;
        };

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

        sampler2D _MainTex;
        sampler2D _PalTex;

        fixed4 frag (v2f i) : SV_Target
        {
            float4 baseCol = tex2D(_MainTex, i.uv);
            float4 palCol = tex2D(_PalTex, 0);
            float palDiff = 1000;

            for (int p = 0; p < 37; p++) {
                float baseColDiff = abs(tex2D(_PalTex, p).r - baseCol.r) + abs(tex2D(_PalTex, p).g - baseCol.g) + abs(tex2D(_PalTex, p).b - baseCol.b);
                palDiff = min(palDiff, baseColDiff);
                palCol = (abs(palDiff - baseColDiff) < 0.001) ? tex2D(_PalTex, p) : palCol;
            }

            return palCol;
        }
        ENDCG
    }
}
}

Any help would be greatly appreciated! I feel like it's a relatively simple problem to solve, but I couldn't find any examples of existing shaders like it, or any resources describing it.

Edit: This shader works... sort of. It has been picking the nearest color, but it has only been picking two yellow colors out of the pallete. I tried removing those colors, and it then chose to use only an orange. I don't know if this will help anyone, but I thought it was at least mildly interesting.

Answer 1

What I would do is loop through each palette color, looking for the closest one to the base color. Something like:

float4 bestColor = float4(0, 0, 0, 0);
float3 bestDiff = float3(1000, 1000, 1000);
for (float i = 0; i < X /*number of colors in palette*/; ++i) {
    float4 palCol = tex2D(_PalText, float2(i, 0));
    float3 diff = abs(baseCol.rgb, palCol.rgb);
    if (length(diff) < length(bestDiff)) {
        bestDiff = diff;
        bestColor = palCol;
    }
}
return bestColor;

(Apologies for any typos)

Answer 2

Ok, so I ended up getting the answer anyway. Here's a GitHub repository that does the effect by using lists of colors. Not sure why it works when this shader doesn't, but it works. https://github.com/oxysoft/RetroSuite3D

And here's the shader in that repository doing the job. Don't directly copy-paste it, as it has to use a script to assign the colors to it. https://github.com/oxysoft/RetroSuite3D/blob/master/Assets/Shaders/Custom/RetroPixelMax.shader