GLTF: Allow parsing glTF files without nodes

Author: aaronfranke

modules/gltf/gltf_document.cpp

@@ -565,7 +565,9 @@ String GLTFDocument::_gen_unique_bone_name(Ref<GLTFState> p_state, const GLTFSke
 
 Error GLTFDocument::_parse_scenes(Ref<GLTFState> p_state) {
 	p_state->unique_names.insert("Skeleton3D"); // Reserve skeleton name.
-	ERR_FAIL_COND_V(!p_state->json.has("scenes"), ERR_FILE_CORRUPT);
+	if (!p_state->json.has("scenes")) {
+		return OK; // No scenes.
+	}
 	const Array &scenes = p_state->json["scenes"];
 	int loaded_scene = 0;
 	if (p_state->json.has("scene")) {
@@ -577,10 +579,11 @@ Error GLTFDocument::_parse_scenes(Ref<GLTFState> p_state) {
 	if (scenes.size()) {
 		ERR_FAIL_COND_V(loaded_scene >= scenes.size(), ERR_FILE_CORRUPT);
 		const Dictionary &scene_dict = scenes[loaded_scene];
-		ERR_FAIL_COND_V(!scene_dict.has("nodes"), ERR_UNAVAILABLE);
-		const Array &nodes = scene_dict["nodes"];
-		for (int j = 0; j < nodes.size(); j++) {
-			p_state->root_nodes.push_back(nodes[j]);
+		if (scene_dict.has("nodes")) {
+			const Array &nodes = scene_dict["nodes"];
+			for (const Variant &node : nodes) {
+				p_state->root_nodes.push_back(node);
+			}
 		}
 		// Determine what to use for the scene name.
 		if (scene_dict.has("name") && !String(scene_dict["name"]).is_empty() && !((String)scene_dict["name"]).begins_with("Scene")) {
@@ -597,7 +600,9 @@ Error GLTFDocument::_parse_scenes(Ref<GLTFState> p_state) {
 }
 
 Error GLTFDocument::_parse_nodes(Ref<GLTFState> p_state) {
-	ERR_FAIL_COND_V(!p_state->json.has("nodes"), ERR_FILE_CORRUPT);
+	if (!p_state->json.has("nodes")) {
+		return OK; // No nodes to parse.
+	}
 	const Array &nodes = p_state->json["nodes"];
 	for (int i = 0; i < nodes.size(); i++) {
 		Ref<GLTFNode> node;
@@ -8676,6 +8681,7 @@ Node *GLTFDocument::_generate_scene_node_tree(Ref<GLTFState> p_state) {
 	// Generate the node tree.
 	Node *single_root;
 	if (p_state->extensions_used.has("GODOT_single_root")) {
+		ERR_FAIL_COND_V_MSG(p_state->nodes.is_empty(), nullptr, "glTF: Single root file has no nodes. This glTF file is invalid.");
 		if (_naming_version < 2) {
 			_generate_scene_node_compat_4pt4(p_state, 0, nullptr, nullptr);
 		} else {
@@ -8843,50 +8849,53 @@ Error GLTFDocument::write_to_filesystem(Ref<GLTFState> p_state, const String &p_
 }
 
 Node *GLTFDocument::generate_scene(Ref<GLTFState> p_state, float p_bake_fps, bool p_trimming, bool p_remove_immutable_tracks) {
-	Ref<GLTFState> state = p_state;
-	ERR_FAIL_COND_V(state.is_null(), nullptr);
-	ERR_FAIL_INDEX_V(0, state->root_nodes.size(), nullptr);
+	ERR_FAIL_COND_V(p_state.is_null(), nullptr);
+	// The glTF file must have nodes, and have some marked as root nodes, in order to generate a scene.
+	if (p_state->nodes.is_empty()) {
+		WARN_PRINT("glTF: This glTF file has no nodes, the generated Godot scene will be empty.");
+	}
+	// Now that we know that we have glTF nodes, we can begin generating a scene from the parsed glTF data.
 	Error err = OK;
 	p_state->set_bake_fps(p_bake_fps);
-	Node *root = _generate_scene_node_tree(state);
-	ERR_FAIL_NULL_V(root, nullptr);
-	_process_mesh_instances(state, root);
-	if (state->get_create_animations() && state->animations.size()) {
-		AnimationPlayer *ap = memnew(AnimationPlayer);
-		root->add_child(ap, true);
-		ap->set_owner(root);
-		for (int i = 0; i < state->animations.size(); i++) {
-			_import_animation(state, ap, i, p_trimming, p_remove_immutable_tracks);
-		}
-	}
-	for (KeyValue<GLTFNodeIndex, Node *> E : state->scene_nodes) {
+	Node *godot_root_node = _generate_scene_node_tree(p_state);
+	ERR_FAIL_NULL_V(godot_root_node, nullptr);
+	_process_mesh_instances(p_state, godot_root_node);
+	if (p_state->get_create_animations() && p_state->animations.size()) {
+		AnimationPlayer *anim_player = memnew(AnimationPlayer);
+		godot_root_node->add_child(anim_player, true);
+		anim_player->set_owner(godot_root_node);
+		for (int i = 0; i < p_state->animations.size(); i++) {
+			_import_animation(p_state, anim_player, i, p_trimming, p_remove_immutable_tracks);
+		}
+	}
+	for (KeyValue<GLTFNodeIndex, Node *> E : p_state->scene_nodes) {
 		ERR_CONTINUE(!E.value);
 		for (Ref<GLTFDocumentExtension> ext : document_extensions) {
 			ERR_CONTINUE(ext.is_null());
 			Dictionary node_json;
-			if (state->json.has("nodes")) {
-				Array nodes = state->json["nodes"];
+			if (p_state->json.has("nodes")) {
+				Array nodes = p_state->json["nodes"];
 				if (0 <= E.key && E.key < nodes.size()) {
 					node_json = nodes[E.key];
 				}
 			}
-			Ref<GLTFNode> gltf_node = state->nodes[E.key];
+			Ref<GLTFNode> gltf_node = p_state->nodes[E.key];
 			err = ext->import_node(p_state, gltf_node, node_json, E.value);
 			ERR_CONTINUE(err != OK);
 		}
 	}
-	ImporterMeshInstance3D *root_importer_mesh = Object::cast_to<ImporterMeshInstance3D>(root);
+	ImporterMeshInstance3D *root_importer_mesh = Object::cast_to<ImporterMeshInstance3D>(godot_root_node);
 	if (unlikely(root_importer_mesh)) {
-		root = GLTFDocumentExtensionConvertImporterMesh::convert_importer_mesh_instance_3d(root_importer_mesh);
+		godot_root_node = GLTFDocumentExtensionConvertImporterMesh::convert_importer_mesh_instance_3d(root_importer_mesh);
 		memdelete(root_importer_mesh);
 	}
 	for (Ref<GLTFDocumentExtension> ext : document_extensions) {
 		ERR_CONTINUE(ext.is_null());
-		err = ext->import_post(p_state, root);
+		err = ext->import_post(p_state, godot_root_node);
 		ERR_CONTINUE(err != OK);
 	}
-	ERR_FAIL_NULL_V(root, nullptr);
-	return root;
+	ERR_FAIL_NULL_V(godot_root_node, nullptr);
+	return godot_root_node;
 }
 
 Error GLTFDocument::append_from_scene(Node *p_node, Ref<GLTFState> p_state, uint32_t p_flags) {