Unity C# Decode RGBM Texture

DecodeLightmap.shader

Shader "Unlit/DecodeLightmap"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags
        {
            "RenderType"="Opaque"
        }

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

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

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

            sampler2D _MainTex;
            float4 _MainTex_ST;

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

            #define LIGHTMAP_RGBM_MAX_GAMMA     half(5.0)
            #define LIGHTMAP_RGBM_MAX_LINEAR    half(34.493242)
            // #define LIGHTMAP_HDR_MULTIPLIER LIGHTMAP_RGBM_MAX_GAMMA
            // #define LIGHTMAP_HDR_EXPONENT   half(1.0)
            #define LIGHTMAP_HDR_MULTIPLIER LIGHTMAP_RGBM_MAX_LINEAR
            #define LIGHTMAP_HDR_EXPONENT   half(2.2)

            half4 frag(v2f i) : SV_Target
            {
                half4 decodeInstructions = half4(LIGHTMAP_HDR_MULTIPLIER, LIGHTMAP_HDR_EXPONENT, 0.0h, 0.0h);
                half3 col = DecodeLightmapRGBM(tex2D(_MainTex, i.uv), decodeInstructions);
                return half4(col, 1.0);
            }
            ENDCG
        }
    }
}

DecodeRGBM.cs

private static Texture2D DecodeLightmap(Texture2D lightmap)
{
    // Decode RGBM Reference
    // Library/PackageCache/[email protected]/ShaderLibrary/EntityLighting.hlsl LIGHTMAP_RGBM_MAX_GAMMA
    // UnityCG.cginc DecodeLightmapRGBM

    var tmpRT = RenderTexture.GetTemporary(lightmap.width, lightmap.height, 0, RenderTextureFormat.Default, lightmap.isDataSRGB ? RenderTextureReadWrite.sRGB : RenderTextureReadWrite.Linear);
    tmpRT.wrapMode = TextureWrapMode.Clamp;
    tmpRT.filterMode = FilterMode.Point;
    tmpRT.anisoLevel = 0;

    // Graphics.Blit(lightmap, tmpRT);

    // Shader Decode RGBM, Blit to RGB
    Graphics.Blit(lightmap, tmpRT, new Material(AssetDatabase.LoadAssetAtPath<Shader>("Assets/Tools/DecodeLightmap.shader")));

    var previous = RenderTexture.active;
    RenderTexture.active = tmpRT;

    var tmpTex = new Texture2D(lightmap.width, lightmap.height, TextureFormat.RGBA32, false, !lightmap.isDataSRGB);
    tmpTex.ReadPixels(new Rect(0, 0, tmpRT.width, tmpRT.height), 0, 0);
    tmpTex.Apply();

    // Decode RGBM, Calculate RGB
    // const float rgbmMaxGamma = 5.0f;
    // const float hdrExponent = 2.2f;
    // var rgbmMaxLinear = Mathf.Pow(rgbmMaxGamma, 2.2f);
    //
    // var pixels = tmpTex.GetPixels();
    // for (var i = 0; i < pixels.Length; i++)
    // {
    //     var pixel = pixels[i].linear;
    //     var m = rgbmMaxLinear * Mathf.Pow(pixel.a, hdrExponent);
    //     pixel.r *= m;
    //     pixel.g *= m;
    //     pixel.b *= m;
    //     pixel.a = 1;
    //     pixels[i] = pixel.gamma;
    // }
    //
    // tmpTex.SetPixels(pixels);
    // tmpTex.Apply();

    RenderTexture.active = previous;
    RenderTexture.ReleaseTemporary(tmpRT);

    return tmpTex;
}