float tilesX = max(AtlasGridSize.x, 1.0);
float tilesY = max(AtlasGridSize.y, 1.0);
float TileCount = AtlasGridSize.x * AtlasGridSize.y;
float Index = round((TileCount - 1) * Noise);
// Clamp index
float safeIndex = max(Index, 0.0);
float tileX = fmod(safeIndex, tilesX);
float tileY = floor(safeIndex / tilesX);
float2 tileID = float2(tileX, tileY);
// Build 64x64 grid
float2 grid = max(NoiseGridSize, float2(1.0, 1.0));
float2 baseUV = UV * grid;
float2 cellID = floor(baseUV);
float2 cellUV = frac(baseUV);// Declare rot outside so it's accessible at the end
int rot = 0;// Apply rotation only if enabled
if (UseTextureRotation > 0.5)
{
    float2 p = cellID;
    p = frac(p * 0.3183099 + float2(0.71, 0.113));
    p *= 17.0;
    float rand = frac(p.x * p.y * (p.x + p.y));
    rot = (int)(rand * 4.0);
    if (rot == 1)
    {
        cellUV = float2(cellUV.y, 1.0 - cellUV.x);
    }
    else if (rot == 2)
    {
        cellUV = 1.0 - cellUV;
    }
    else if (rot == 3)
    {
        cellUV = float2(1.0 - cellUV.y, cellUV.x);
    }
}

// Slight padding to avoid bleeding
//cellUV = cellUV * 0.98 + 0.01;// Atlas UV

float2 atlasUV;
atlasUV.x = (tileX + cellUV.x) / tilesX;
atlasUV.y = (tileY + cellUV.y) / tilesY;// RG = atlas UV, B = rotation index for normal map correction
return float3(atlasUV, float(rot));