Fix blocky mesh model didn't use materials directly in the mesh resource

meshers/blocky/voxel_blocky_model.cpp

@@ -186,6 +186,15 @@ Ref<Material> VoxelBlockyModel::get_material_override(int index) const {
 	return _surface_params[index].material_override;
 }
 
+bool VoxelBlockyModel::has_material_override() const {
+	for (const SurfaceParams &sp : _surface_params) {
+		if (sp.material_override.is_valid()) {
+			return true;
+		}
+	}
+	return false;
+}
+
 void VoxelBlockyModel::set_mesh_collision_enabled(int surface_index, bool enabled) {
 	// TODO Can't check for `_surface_count` instead, because there is no guarantee about the order in which Godot will
 	// set properties when loading the resource. The mesh could be set later, so we can't know the number of surfaces.
@@ -265,12 +274,12 @@ void VoxelBlockyModel::bake(BakedData &baked_data, bool bake_tangents, MaterialI
 	for (unsigned int surface_index = 0; surface_index < model.surface_count; ++surface_index) {
 		if (surface_index < _surface_count) {
 			const SurfaceParams &surface_params = _surface_params[surface_index];
-			const Ref<Material> material = surface_params.material_override;
-
 			BakedData::Surface &surface = model.surfaces[surface_index];
 
-			const unsigned int material_index = materials.get_or_create_index(material);
-			surface.material_id = material_index;
+			if (surface_params.material_override.is_valid()) {
+				const unsigned int material_index = materials.get_or_create_index(surface_params.material_override);
+				surface.material_id = material_index;
+			}
 
 			surface.collision_enabled = surface_params.collision_enabled;
 		}

meshers/blocky/voxel_blocky_model.h

@@ -139,6 +139,7 @@ class VoxelBlockyModel : public Resource {
 
 	void set_material_override(int index, Ref<Material> material);
 	Ref<Material> get_material_override(int index) const;
+	bool has_material_override() const;
 
 	void set_mesh_collision_enabled(int surface_index, bool enabled);
 	bool is_mesh_collision_enabled(int surface_index) const;

meshers/blocky/voxel_blocky_model_cube.cpp

@@ -119,12 +119,16 @@ void bake_cube_geometry(
 		const VoxelBlockyModelCube &config,
 		VoxelBlockyModel::BakedData &baked_data,
 		Vector2i p_atlas_size,
+		VoxelBlockyModel::MaterialIndexer &material_indexer,
 		bool bake_tangents
 ) {
 	const float height = config.get_height();
 
 	baked_data.model.surface_count = 1;
 	VoxelBlockyModel::BakedData::Surface &surface = baked_data.model.surfaces[0];
+	// The only way to specify matererials in this model is via "material overrides", since there is no base mesh.
+	// Even if none are specified, we should at least index the "empty" material.
+	surface.material_id = material_indexer.get_or_create_index(config.get_material_override(0));
 
 	for (unsigned int side = 0; side < Cube::SIDE_COUNT; ++side) {
 		VoxelBlockyModel::BakedData::SideSurface &side_surface = surface.sides[side];
@@ -198,7 +202,7 @@ void bake_cube_geometry(
 
 void VoxelBlockyModelCube::bake(BakedData &baked_data, bool bake_tangents, MaterialIndexer &materials) const {
 	baked_data.clear();
-	bake_cube_geometry(*this, baked_data, _atlas_size_in_tiles, bake_tangents);
+	bake_cube_geometry(*this, baked_data, _atlas_size_in_tiles, materials, bake_tangents);
 	VoxelBlockyModel::bake(baked_data, bake_tangents, materials);
 }
 
@@ -211,7 +215,9 @@ Ref<Mesh> VoxelBlockyModelCube::get_preview_mesh() const {
 
 	VoxelBlockyModel::BakedData baked_data;
 	baked_data.color = get_color();
-	bake_cube_geometry(*this, baked_data, _atlas_size_in_tiles, bake_tangents);
+	StdVector<Ref<Material>> materials;
+	MaterialIndexer material_indexer{ materials };
+	bake_cube_geometry(*this, baked_data, _atlas_size_in_tiles, material_indexer, bake_tangents);
 
 	Ref<Mesh> mesh = make_mesh_from_baked_data(baked_data, bake_tangents);