3 files changed, 11 insertions, 3 deletions
diff --git a/scene/resources/importer_mesh.cpp b/scene/resources/importer_mesh.cpp
index 742ac2bbd9..672581bbe2 100644
--- a/scene/resources/importer_mesh.cpp
+++ b/scene/resources/importer_mesh.cpp
@@ -452,6 +452,7 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, float p_normal_spli
 			new_indices.resize(index_count);
 
 			Vector<float> merged_normals_f32 = vector3_to_float32_array(merged_normals.ptr(), merged_normals.size());
+			const int simplify_options = SurfaceTool::SIMPLIFY_LOCK_BORDER;
 
 			size_t new_index_count = SurfaceTool::simplify_with_attrib_func(
 					(unsigned int *)new_indices.ptrw(),
@@ -460,6 +461,7 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, float p_normal_spli
 					sizeof(float) * 3, // Vertex stride
 					index_target,
 					max_mesh_error,
+					simplify_options,
 					&mesh_error,
 					merged_normals_f32.ptr(),
 					normal_weights.ptr(), 3);
diff --git a/scene/resources/surface_tool.cpp b/scene/resources/surface_tool.cpp
index 5a2b917b9a..16cc1c3370 100644
--- a/scene/resources/surface_tool.cpp
+++ b/scene/resources/surface_tool.cpp
@@ -1307,7 +1307,8 @@ Vector<int> SurfaceTool::generate_lod(float p_threshold, int p_target_index_coun
 	}
 
 	float error;
-	uint32_t index_count = simplify_func((unsigned int *)lod.ptrw(), (unsigned int *)index_array.ptr(), index_array.size(), vertices.ptr(), vertex_array.size(), sizeof(float) * 3, p_target_index_count, p_threshold, &error);
+	const int simplify_options = SIMPLIFY_LOCK_BORDER;
+	uint32_t index_count = simplify_func((unsigned int *)lod.ptrw(), (unsigned int *)index_array.ptr(), index_array.size(), vertices.ptr(), vertex_array.size(), sizeof(float) * 3, p_target_index_count, p_threshold, simplify_options, &error);
 	ERR_FAIL_COND_V(index_count == 0, lod);
 	lod.resize(index_count);
 
diff --git a/scene/resources/surface_tool.h b/scene/resources/surface_tool.h
index 00438c4a53..77318bb061 100644
--- a/scene/resources/surface_tool.h
+++ b/scene/resources/surface_tool.h
@@ -74,11 +74,16 @@ public:
 		SKIN_8_WEIGHTS
 	};
 
+	enum {
+		/* Do not move vertices that are located on the topological border (vertices on triangle edges that don't have a paired triangle). Useful for simplifying portions of the larger mesh. */
+		SIMPLIFY_LOCK_BORDER = 1 << 0, // From meshopt_SimplifyLockBorder
+	};
+
 	typedef void (*OptimizeVertexCacheFunc)(unsigned int *destination, const unsigned int *indices, size_t index_count, size_t vertex_count);
 	static OptimizeVertexCacheFunc optimize_vertex_cache_func;
-	typedef size_t (*SimplifyFunc)(unsigned int *destination, const unsigned int *indices, size_t index_count, const float *vertex_positions, size_t vertex_count, size_t vertex_positions_stride, size_t target_index_count, float target_error, float *r_error);
+	typedef size_t (*SimplifyFunc)(unsigned int *destination, const unsigned int *indices, size_t index_count, const float *vertex_positions, size_t vertex_count, size_t vertex_positions_stride, size_t target_index_count, float target_error, unsigned int options, float *r_error);
 	static SimplifyFunc simplify_func;
-	typedef size_t (*SimplifyWithAttribFunc)(unsigned int *destination, const unsigned int *indices, size_t index_count, const float *vertex_data, size_t vertex_count, size_t vertex_stride, size_t target_index_count, float target_error, float *result_error, const float *attributes, const float *attribute_weights, size_t attribute_count);
+	typedef size_t (*SimplifyWithAttribFunc)(unsigned int *destination, const unsigned int *indices, size_t index_count, const float *vertex_data, size_t vertex_count, size_t vertex_stride, size_t target_index_count, float target_error, unsigned int options, float *result_error, const float *attributes, const float *attribute_weights, size_t attribute_count);
 	static SimplifyWithAttribFunc simplify_with_attrib_func;
 	typedef float (*SimplifyScaleFunc)(const float *vertex_positions, size_t vertex_count, size_t vertex_positions_stride);
 	static SimplifyScaleFunc simplify_scale_func;