5 files changed, 42 insertions, 11 deletions
diff --git a/modules/gltf/doc_classes/GLTFState.xml b/modules/gltf/doc_classes/GLTFState.xml
index 1dbd89aed8..6c2f488c1c 100644
--- a/modules/gltf/doc_classes/GLTFState.xml
+++ b/modules/gltf/doc_classes/GLTFState.xml
@@ -7,6 +7,14 @@
 	<tutorials>
 	</tutorials>
 	<methods>
+		<method name="add_used_extension">
+			<return type="void" />
+			<param index="0" name="extension_name" type="String" />
+			<param index="1" name="required" type="bool" />
+			<description>
+				Appends an extension to the list of extensions used by this GLTF file during serialization. If [param required] is true, the extension will also be added to the list of required extensions. Do not run this in [method GLTFDocumentExtension._export_post], as that stage is too late to add extensions. The final list is sorted alphabetically.
+			</description>
+		</method>
 		<method name="get_accessors">
 			<return type="GLTFAccessor[]" />
 			<description>
diff --git a/modules/gltf/gltf_document.cpp b/modules/gltf/gltf_document.cpp
index 53cf7285f9..436fd2a1fc 100644
--- a/modules/gltf/gltf_document.cpp
+++ b/modules/gltf/gltf_document.cpp
@@ -198,7 +198,7 @@ Error GLTFDocument::_serialize(Ref<GLTFState> state, const String &p_path) {
 	}
 
 	/* STEP SERIALIZE EXTENSIONS */
-	err = _serialize_extensions(state);
+	err = _serialize_gltf_extensions(state);
 	if (err != OK) {
 		return Error::FAILED;
 	}
@@ -219,9 +219,9 @@ Error GLTFDocument::_serialize(Ref<GLTFState> state, const String &p_path) {
 	return OK;
 }
 
-Error GLTFDocument::_serialize_extensions(Ref<GLTFState> state) const {
-	Array extensions_used;
-	Array extensions_required;
+Error GLTFDocument::_serialize_gltf_extensions(Ref<GLTFState> state) const {
+	Vector<String> extensions_used = state->extensions_used;
+	Vector<String> extensions_required = state->extensions_required;
 	if (!state->lights.is_empty()) {
 		extensions_used.push_back("KHR_lights_punctual");
 	}
@@ -230,9 +230,11 @@ Error GLTFDocument::_serialize_extensions(Ref<GLTFState> state) const {
 		extensions_required.push_back("KHR_texture_transform");
 	}
 	if (!extensions_used.is_empty()) {
+		extensions_used.sort();
 		state->json["extensionsUsed"] = extensions_used;
 	}
 	if (!extensions_required.is_empty()) {
+		extensions_required.sort();
 		state->json["extensionsRequired"] = extensions_required;
 	}
 	return OK;
@@ -6917,12 +6919,17 @@ Error GLTFDocument::append_from_file(String p_path, Ref<GLTFState> r_state, uint
 
 Error GLTFDocument::_parse_gltf_extensions(Ref<GLTFState> state) {
 	ERR_FAIL_NULL_V(state, ERR_PARSE_ERROR);
-	if (state->json.has("extensionsRequired") && state->json["extensionsRequired"].get_type() == Variant::ARRAY) {
-		Array extensions_required = state->json["extensionsRequired"];
-		if (extensions_required.find("KHR_draco_mesh_compression") != -1) {
-			ERR_PRINT("glTF2 extension KHR_draco_mesh_compression is not supported.");
-			return ERR_UNAVAILABLE;
-		}
+	if (state->json.has("extensionsUsed")) {
+		Vector<String> ext_array = state->json["extensionsUsed"];
+		state->extensions_used = ext_array;
+	}
+	if (state->json.has("extensionsRequired")) {
+		Vector<String> ext_array = state->json["extensionsRequired"];
+		state->extensions_required = ext_array;
+	}
+	if (state->extensions_required.find("KHR_draco_mesh_compression") != -1) {
+		ERR_PRINT("glTF2 extension KHR_draco_mesh_compression is not supported.");
+		return ERR_UNAVAILABLE;
 	}
 	return OK;
 }
diff --git a/modules/gltf/gltf_document.h b/modules/gltf/gltf_document.h
index b3ed786a39..750d3d403e 100644
--- a/modules/gltf/gltf_document.h
+++ b/modules/gltf/gltf_document.h
@@ -265,7 +265,7 @@ private:
 	Dictionary _serialize_texture_transform_uv2(Ref<BaseMaterial3D> p_material);
 	Error _serialize_version(Ref<GLTFState> state);
 	Error _serialize_file(Ref<GLTFState> state, const String p_path);
-	Error _serialize_extensions(Ref<GLTFState> state) const;
+	Error _serialize_gltf_extensions(Ref<GLTFState> state) const;
 
 public:
 	// https://www.itu.int/rec/R-REC-BT.601
diff --git a/modules/gltf/gltf_state.cpp b/modules/gltf/gltf_state.cpp
index 85bac446cc..a23fb39503 100644
--- a/modules/gltf/gltf_state.cpp
+++ b/modules/gltf/gltf_state.cpp
@@ -31,6 +31,7 @@
 #include "gltf_state.h"
 
 void GLTFState::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("add_used_extension", "extension_name", "required"), &GLTFState::add_used_extension);
 	ClassDB::bind_method(D_METHOD("get_json"), &GLTFState::get_json);
 	ClassDB::bind_method(D_METHOD("set_json", "json"), &GLTFState::set_json);
 	ClassDB::bind_method(D_METHOD("get_major_version"), &GLTFState::get_major_version);
@@ -112,6 +113,17 @@ void GLTFState::_bind_methods() {
 	ADD_PROPERTY(PropertyInfo(Variant::ARRAY, "animations", PROPERTY_HINT_NONE, "", PROPERTY_USAGE_STORAGE | PROPERTY_USAGE_INTERNAL | PROPERTY_USAGE_EDITOR), "set_animations", "get_animations"); // Vector<Ref<GLTFAnimation>>
 }
 
+void GLTFState::add_used_extension(const String &p_extension_name, bool p_required) {
+	if (!extensions_used.has(p_extension_name)) {
+		extensions_used.push_back(p_extension_name);
+	}
+	if (p_required) {
+		if (!extensions_required.has(p_extension_name)) {
+			extensions_required.push_back(p_extension_name);
+		}
+	}
+}
+
 Dictionary GLTFState::get_json() {
 	return json;
 }
diff --git a/modules/gltf/gltf_state.h b/modules/gltf/gltf_state.h
index 6b2d1ca228..791431f376 100644
--- a/modules/gltf/gltf_state.h
+++ b/modules/gltf/gltf_state.h
@@ -78,6 +78,8 @@ class GLTFState : public Resource {
 	Vector<int> root_nodes;
 	Vector<Ref<GLTFTexture>> textures;
 	Vector<Ref<Texture2D>> images;
+	Vector<String> extensions_used;
+	Vector<String> extensions_required;
 
 	Vector<Ref<GLTFSkin>> skins;
 	Vector<Ref<GLTFCamera>> cameras;
@@ -97,6 +99,8 @@ protected:
 	static void _bind_methods();
 
 public:
+	void add_used_extension(const String &p_extension, bool p_required = false);
+
 	Dictionary get_json();
 	void set_json(Dictionary p_json);