Fix T96430: new OBJ exporter wrong normals for non-uniform scale, and wrong face order for negative scale

The new 3.1+ OBJ exporter did not have correct logic when faced with
non-uniform & mirrored (negative on odd number of axes) object scale:

- Normals were not transformed correctly (should use inverse transpose of the matrix),
 and were not normalized,
- Face order was not "flipped" when transform has negative scale on odd number of axes
 (visible when using "face orientation" viewport overlay).

Cherry-picked from 8aa365745a78, with minor conflict fixes.

Reviewed By: Howard Trickey

Differential Revision: https://developer.blender.org/D14343

1 files changed, 6 insertions, 0 deletions

diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
index f3ace140006..3ec670d2203 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
@@ -73,6 +73,8 @@ class OBJMesh : NonCopyable {
    * object's world transform matrix.
    */
   float world_and_axes_transform_[4][4];
+  float world_and_axes_normal_transform_[3][3];
+  bool mirrored_transform_;
 
   /**
    * Total UV vertices in a mesh's texture map.
@@ -113,6 +115,10 @@ class OBJMesh : NonCopyable {
   int tot_uv_vertices() const;
   int tot_normal_indices() const;
   int tot_edges() const;
+  bool is_mirrored_transform() const
+  {
+    return mirrored_transform_;
+  }
 
   /**
    * \return Total materials in the object.