Added better handling for exporting meshes with negative scaling

The exporter now corrects the face normals of meshes with negative
scaling that would otherwise be inverted on export. The face orientation
should now always match what is seen in the viewport.

io_scene_psk_psa/psk/builder.py

@@ -150,6 +150,8 @@ def build_psk(context, options: PskBuildOptions) -> PskBuildResult:
 
     for object_index, input_mesh_object in enumerate(input_objects.mesh_objects):
 
+        should_flip_normals = False
+
         # MATERIALS
         material_indices = [material_names.index(material_slot.material.name) for material_slot in input_mesh_object.material_slots]
 
@@ -177,8 +179,16 @@ def build_psk(context, options: PskBuildOptions) -> PskBuildResult:
             mesh_object.matrix_world = input_mesh_object.matrix_world
 
             scale = (input_mesh_object.scale.x, input_mesh_object.scale.y, input_mesh_object.scale.z)
-            if any(map(lambda x: x < 0, scale)):
+
-                result.warnings.append(f'Mesh "{input_mesh_object.name}" has negative scaling which may result in inverted normals.')
+            # Negative scaling in Blender results in inverted normals after the scale is applied. However, if the scale
+            # is not applied, the normals will appear unaffected in the viewport. The evaluated mesh data used in the
+            # export will have the scale applied, but this behavior is not obvious to the user.
+            #
+            # In order to have the exporter be as WYSIWYG as possible, we need to check for negative scaling and invert
+            # the normals if necessary. If two axes have negative scaling and the third has positive scaling, the
+            # normals will be correct. We can detect this by checking if the number of negative scaling axes is odd. If
+            # it is, we need to invert the normals of the mesh by swapping the order of the vertices in each face.
+            should_flip_normals = sum(1 for x in scale if x < 0) % 2 == 1
 
             # Copy the vertex groups
             for vertex_group in input_mesh_object.vertex_groups:
@@ -233,6 +243,7 @@ def build_psk(context, options: PskBuildOptions) -> PskBuildResult:
 
         # FACES
         poly_groups, groups = mesh_data.calc_smooth_groups(use_bitflags=True)
+        psk_face_start_index = len(psk.faces)
         for f in mesh_data.loop_triangles:
             face = Psk.Face()
             face.material_index = material_indices[f.material_index]
@@ -242,6 +253,11 @@ def build_psk(context, options: PskBuildOptions) -> PskBuildResult:
             face.smoothing_groups = poly_groups[f.polygon_index]
             psk.faces.append(face)
 
+        if should_flip_normals:
+            # Invert the normals of the faces.
+            for face in psk.faces[psk_face_start_index:]:
+                face.wedge_indices[0], face.wedge_indices[2] = face.wedge_indices[2], face.wedge_indices[0]
+
         # WEIGHTS
         if armature_object is not None:
             armature_data = typing.cast(Armature, armature_object.data)