diff --git a/src/libslic3r/TriangleSelector.cpp b/src/libslic3r/TriangleSelector.cpp
index 31d1b166a0..929d3094b4 100644
--- a/src/libslic3r/TriangleSelector.cpp
+++ b/src/libslic3r/TriangleSelector.cpp
@@ -290,7 +290,8 @@ void TriangleSelector::select_patch(int facet_start, std::unique_ptr<Cursor> &&c
         while (facet_idx < int(facets_to_check.size())) {
             int          facet = facets_to_check[facet_idx];
             const Vec3f& facet_normal = m_face_normals[m_triangles[facet].source_triangle];
-            float        world_normal_z = (trafo_no_translate.inverse() * facet_normal.cast<double>()).normalized().cast<float>().z();
+            Matrix3f     normal_matrix = static_cast<Matrix3f>(trafo_no_translate.matrix().block(0, 0, 3, 3).inverse().transpose().cast<float>());
+            float        world_normal_z = (normal_matrix* facet_normal).normalized().z();
             if (!visited[facet] && (highlight_by_angle_deg == 0.f || world_normal_z < highlight_angle_limit)) {
                 if (select_triangle(facet, new_state, triangle_splitting)) {
                     // add neighboring facets to list to be processed later
@@ -339,7 +340,8 @@ void TriangleSelector::seed_fill_select_triangles(const Vec3f &hit, int facet_st
         facet_queue.pop();
 
         const Vec3f &facet_normal = m_face_normals[m_triangles[current_facet].source_triangle];
-        float        world_normal_z = (trafo_no_translate.inverse() * facet_normal.cast<double>()).normalized().cast<float>().z();
+        Matrix3f     normal_matrix  = static_cast<Matrix3f>(trafo_no_translate.matrix().block(0, 0, 3, 3).inverse().transpose().cast<float>());
+        float        world_normal_z = (normal_matrix * facet_normal).normalized().z();
         if (!visited[current_facet] && (highlight_by_angle_deg == 0.f || world_normal_z < highlight_angle_limit)) {
             if (m_triangles[current_facet].is_split()) {
                 for (int split_triangle_idx = 0; split_triangle_idx <= m_triangles[current_facet].number_of_split_sides(); ++split_triangle_idx) {