Inventory2/assets/shaders/ReplacePalette.cs

public partial class ReplacePalette : Node3D
{
	readonly struct TextureInfo(float min, float max)
	{
		public float MinBrightness { get; } = min;
		public float MaxBrightness { get; } = max;
	}

	static Shader _paletteShader;
	static Dictionary<Texture2D, TextureInfo> _textureCache = [];

	[Export] public Texture2D Palette { get; set; }

	public override void _Ready()
	{
		_paletteShader ??= Load<Shader>("res://assets/shaders/palette_swap.gdshader");
		ReplaceShaderRecursive(this, Palette);
	}

	static void ReplaceShaderRecursive(Node node, Texture2D palette)
	{
		if (node is MeshInstance3D mesh) {
			var oldMaterial = (StandardMaterial3D)mesh.Mesh.SurfaceGetMaterial(0);
			var texture = oldMaterial.AlbedoTexture;
			var info = GetOrCreateTextureInfo(texture);

			var material = new ShaderMaterial { Shader = _paletteShader };
			material.SetShaderParameter("palette", palette);
			material.SetShaderParameter("albedo_texture", texture);
			material.SetShaderParameter("min_brightness", info.MinBrightness);
			material.SetShaderParameter("max_brightness", info.MaxBrightness);
			mesh.MaterialOverride = material;
		}

		foreach (var child in node.GetChildren())
			ReplaceShaderRecursive(child, palette);
	}

	static TextureInfo GetOrCreateTextureInfo(Texture2D texture)
	{
		if (!_textureCache.TryGetValue(texture, out var info)) {
			var minBrightness = 1.0f;
			var maxBrightness = 0.0f;

			var image = texture.GetImage();
			image.Convert(Image.Format.Rgba8);
			var data = image.GetData().AsSpan();
			for (var i = 0; i < data.Length; i += 4) {
				var rgba = data.Slice(i, 4);
				if (rgba[3] <= byte.MaxValue / 2) continue;
				var brightness = Max(rgba[0], Max(rgba[1], rgba[2])) / (float)byte.MaxValue;
				minBrightness = Min(minBrightness, brightness);
				maxBrightness = Max(maxBrightness, brightness);
			}
			// for (var x = 0; x < image.GetWidth(); x++)
			// for (var y = 0; y < image.GetHeight(); y++) {
			// 	var color = image.GetPixel(x, y);
			// 	// var brightness = Max(color.R, Max(color.G, color.B));
			// 	color.ToHsv(out _, out _, out var brightness);
			// 	minBrightness = Min(minBrightness, brightness);
			// 	maxBrightness = Max(maxBrightness, brightness);
			// }

			info = new(minBrightness, maxBrightness);
			_textureCache.Add(texture, info);
		}
		return info;
	}
}