Fix normal direction when exporting STL (#6406)

The export function does not depend on Model/ModelObject::mesh() family of functions,
changing them might break the already too brittle code.

src/libslic3r/Model.cpp

@@ -833,18 +833,6 @@ indexed_triangle_set ModelObject::raw_indexed_triangle_set() const
     return out;
 }
 
-// Non-transformed (non-rotated, non-scaled, non-translated) sum of all object volumes.
-TriangleMesh ModelObject::full_raw_mesh() const
-{
-    TriangleMesh mesh;
-    for (const ModelVolume *v : this->volumes)
-    {
-        TriangleMesh vol_mesh(v->mesh());
-        vol_mesh.transform(v->get_matrix());
-        mesh.merge(vol_mesh);
-    }
-    return mesh;
-}
 
 const BoundingBoxf3& ModelObject::raw_mesh_bounding_box() const
 {

src/libslic3r/Model.hpp

@@ -289,8 +289,6 @@ public:
     TriangleMesh raw_mesh() const;
     // The same as above, but producing a lightweight indexed_triangle_set.
     indexed_triangle_set raw_indexed_triangle_set() const;
-    // Non-transformed (non-rotated, non-scaled, non-translated) sum of all object volumes.
-    TriangleMesh full_raw_mesh() const;
     // A transformed snug bounding box around the non-modifier object volumes, without the translation applied.
     // This bounding box is only used for the actual slicing.
     const BoundingBoxf3& raw_bounding_box() const;

src/libslic3r/PrintObject.cpp

@@ -2207,6 +2207,7 @@ std::vector<ExPolygons> PrintObject::slice_volumes(
             TriangleMesh vol_mesh(model_volume.mesh());
             vol_mesh.transform(model_volume.get_matrix(), true);
             mesh.merge(vol_mesh);
+            mesh.repair(false);
         }
         if (mesh.stl.stats.number_of_facets > 0) {
             mesh.transform(m_trafo, true);

src/libslic3r/TriangleMesh.cpp

@@ -357,6 +357,10 @@ void TriangleMesh::transform(const Transform3d& t, bool fix_left_handed)
     its_transform(its, t);
 	if (fix_left_handed && t.matrix().block(0, 0, 3, 3).determinant() < 0.) {
 		// Left handed transformation is being applied. It is a good idea to flip the faces and their normals.
+        // As for the assert: the repair function would fix the normals, reversing would
+        // break them again. The caller should provide a mesh that does not need repair.
+        // The repair call is left here so things don't break more than they were.
+        assert(this->repaired);
         this->repair(false);
         stl_reverse_all_facets(&stl);
         this->its.clear();
@@ -369,7 +373,8 @@ void TriangleMesh::transform(const Matrix3d& m, bool fix_left_handed)
     stl_transform(&stl, m);
     its_transform(its, m);
     if (fix_left_handed && m.determinant() < 0.) {
-        // Left handed transformation is being applied. It is a good idea to flip the faces and their normals.
+        // See comments in function above.
+        assert(this->repaired);
         this->repair(false);
         stl_reverse_all_facets(&stl);
         this->its.clear();

src/slic3r/GUI/GLCanvas3D.cpp

@@ -1,7 +1,6 @@
 #include "libslic3r/libslic3r.h"
 #include "GLCanvas3D.hpp"
 
-#include "admesh/stl.h"
 #include "libslic3r/ClipperUtils.hpp"
 #include "libslic3r/PrintConfig.hpp"
 #include "libslic3r/GCode/ThumbnailData.hpp"

src/slic3r/GUI/Plater.cpp

@@ -5085,6 +5085,30 @@ void Plater::export_stl(bool extended, bool selection_only)
     if (selection_only && (obj_idx == -1 || selection.is_wipe_tower()))
         return;
 
+    // Following lambda generates a combined mesh for export with normals pointing outwards.
+    auto mesh_to_export = [](const ModelObject* mo, bool instances) -> TriangleMesh {
+        TriangleMesh mesh;
+        for (const ModelVolume *v : mo->volumes)
+            if (v->is_model_part()) {
+                TriangleMesh vol_mesh(v->mesh());
+                vol_mesh.repair();
+                vol_mesh.transform(v->get_matrix(), true);
+                mesh.merge(vol_mesh);
+            }
+        mesh.repair();
+        if (instances) {
+            TriangleMesh vols_mesh(mesh);
+            mesh = TriangleMesh();
+            for (const ModelInstance *i : mo->instances) {
+                TriangleMesh m = vols_mesh;
+                m.transform(i->get_matrix(), true);
+                mesh.merge(m);
+            }
+        }
+        mesh.repair();
+        return mesh;
+    };
+
     TriangleMesh mesh;
     if (p->printer_technology == ptFFF) {
         if (selection_only) {
@@ -5092,20 +5116,21 @@ void Plater::export_stl(bool extended, bool selection_only)
             if (selection.get_mode() == Selection::Instance)
             {
                 if (selection.is_single_full_object())
-                    mesh = model_object->mesh();
+                    mesh = mesh_to_export(model_object, true);
                 else
-                    mesh = model_object->full_raw_mesh();
+                    mesh = mesh_to_export(model_object, false);
             }
             else
             {
                 const GLVolume* volume = selection.get_volume(*selection.get_volume_idxs().begin());
                 mesh = model_object->volumes[volume->volume_idx()]->mesh();
-                mesh.transform(volume->get_volume_transformation().get_matrix());
+                mesh.transform(volume->get_volume_transformation().get_matrix(), true);
                 mesh.translate(-model_object->origin_translation.cast<float>());
             }
         }
         else {
-            mesh = p->model.mesh();
+            for (const ModelObject *o : p->model.objects)
+                mesh.merge(mesh_to_export(o, true));
         }
     }
     else