diff --git a/src/admesh/stl.h b/src/admesh/stl.h
index fa0edec2b..9224b0459 100644
--- a/src/admesh/stl.h
+++ b/src/admesh/stl.h
@@ -184,10 +184,21 @@ extern void stl_mirror_xz(stl_file *stl);
 
 extern void stl_get_size(stl_file *stl);
 
+// the following function is not used
+/*
 template<typename T>
 extern void stl_transform(stl_file *stl, T *trafo3x4)
 {
-	for (uint32_t i_face = 0; i_face < stl->stats.number_of_facets; ++ i_face) {
+    Eigen::Matrix<T, 3, 3, Eigen::DontAlign> trafo3x3;
+    for (int i = 0; i < 3; ++i)
+    {
+        for (int j = 0; j < 3; ++j)
+        {
+            trafo3x3(i, j) = (i * 4) + j;
+        }
+    }
+    Eigen::Matrix<T, 3, 3, Eigen::DontAlign> r = trafo3x3.inverse().transpose();
+    for (uint32_t i_face = 0; i_face < stl->stats.number_of_facets; ++ i_face) {
 		stl_facet &face = stl->facet_start[i_face];
 		for (int i_vertex = 0; i_vertex < 3; ++ i_vertex) {
 			stl_vertex &v_dst = face.vertex[i_vertex];
@@ -196,21 +207,18 @@ extern void stl_transform(stl_file *stl, T *trafo3x4)
 			v_dst(1) = T(trafo3x4[4] * v_src(0) + trafo3x4[5] * v_src(1) + trafo3x4[6]  * v_src(2) + trafo3x4[7]);
 			v_dst(2) = T(trafo3x4[8] * v_src(0) + trafo3x4[9] * v_src(1) + trafo3x4[10] * v_src(2) + trafo3x4[11]);
 		}
-		stl_vertex &v_dst = face.normal;
-		stl_vertex  v_src = v_dst;
-		v_dst(0) = T(trafo3x4[0] * v_src(0) + trafo3x4[1] * v_src(1) + trafo3x4[2]  * v_src(2));
-		v_dst(1) = T(trafo3x4[4] * v_src(0) + trafo3x4[5] * v_src(1) + trafo3x4[6]  * v_src(2));
-		v_dst(2) = T(trafo3x4[8] * v_src(0) + trafo3x4[9] * v_src(1) + trafo3x4[10] * v_src(2));
-	}
+        face.normal = (r * face.normal.template cast<T>()).template cast<float>().eval();
+    }
 
 	stl_get_size(stl);
 }
+*/
 
 template<typename T>
 inline void stl_transform(stl_file *stl, const Eigen::Transform<T, 3, Eigen::Affine, Eigen::DontAlign>& t)
 {
-	const Eigen::Matrix<double, 3, 3, Eigen::DontAlign> r = t.matrix().template block<3, 3>(0, 0);
-	for (size_t i = 0; i < stl->stats.number_of_facets; ++ i) {
+    const Eigen::Matrix<T, 3, 3, Eigen::DontAlign> r = t.matrix().template block<3, 3>(0, 0).inverse().transpose();
+    for (size_t i = 0; i < stl->stats.number_of_facets; ++ i) {
 		stl_facet &f = stl->facet_start[i];
 		for (size_t j = 0; j < 3; ++j)
 			f.vertex[j] = (t * f.vertex[j].template cast<T>()).template cast<float>().eval();
@@ -223,12 +231,13 @@ inline void stl_transform(stl_file *stl, const Eigen::Transform<T, 3, Eigen::Aff
 template<typename T>
 inline void stl_transform(stl_file *stl, const Eigen::Matrix<T, 3, 3, Eigen::DontAlign>& m)
 {
-	for (size_t i = 0; i < stl->stats.number_of_facets; ++ i) {
+    const Eigen::Matrix<T, 3, 3, Eigen::DontAlign> r = m.inverse().transpose();
+    for (size_t i = 0; i < stl->stats.number_of_facets; ++ i) {
 		stl_facet &f = stl->facet_start[i];
 		for (size_t j = 0; j < 3; ++j)
 			f.vertex[j] = (m * f.vertex[j].template cast<T>()).template cast<float>().eval();
-		f.normal = (m * f.normal.template cast<T>()).template cast<float>().eval();
-	}
+        f.normal = (r * f.normal.template cast<T>()).template cast<float>().eval();
+    }
 
 	stl_get_size(stl);
 }