Added an another helper method.

game/addons/mesh_data_resource_editor/MIDGizmo.gd

@@ -383,12 +383,41 @@ func add_quad() -> void:
 	if _mdr:
 		MDRMeshUtils.add_quad(_mdr)
 
+func is_verts_equal(v0 : Vector3, v1 : Vector3) -> bool:
+	return is_equal_approx(v0.x, v1.x) && is_equal_approx(v0.y, v1.y) && is_equal_approx(v0.z, v1.z)
+
+func add_triangle_to_edge(edge : int) -> void:
+#	var ps : PoolIntArray = _handle_to_vertex_map[edge]
+#
+#	if ps.size() == 0:
+#		return
+#
+#	var v0 : Vector3 = _vertices[ps[0]]
+#	var v1 : Vector3 = Vector3()
+#	var found_index : int = -1
+#
+#	for j in range(1, ps.size()):
+#		v1 = _vertices[j]
+#
+#		if !is_verts_equal(v0, v1):
+#			found_index = j
+#			break
+#
+#	if found_index == -1:
+#		return
+#
+	
+	#should_flip_reflected_triangle
+	
+	pass
+
 func add_triangle_at() -> void:
 	if selection_mode == SelectionMode.SELECTION_MODE_VERTEX:
 		#todo
 		pass
 	elif selection_mode == SelectionMode.SELECTION_MODE_EDGE:
-		#should_flip_reflected_triangle
+		for i in _selected_points:
+			add_triangle_to_edge(i)
 	else:
 		add_triangle()
 		

game/addons/mesh_data_resource_editor/utilities/mdred_mesh_utils.gd

@@ -1,6 +1,13 @@
 tool
 extends Object
 
+# Appends a triangle to the mesh. It's created from miroring v2 to the ev0, and ev1 edge
+static func append_triangle_to_tri_edge(mdr : MeshDataResource, ev0 : Vector3, ev1 : Vector3, v2 : Vector3) -> void:
+	var vref : Vector3 = reflect_vertex(ev0, ev1, v2)
+	var should_flip : bool = should_flip_reflected_triangle(ev0, ev1, v2)
+	
+	add_triangle_at(mdr, ev0, ev1, vref, should_flip)
+
 static func add_triangle_at(mdr : MeshDataResource, v0 : Vector3, v1 : Vector3, v2 : Vector3, flip : bool = false) -> void:
 	var st : SurfaceTool = SurfaceTool.new()