Fix T96470 new obj exporter writing material groups

The logic in the code was _completely different_ from the documentation
and what the python exporter in 3.0 did. The new code assumed that
"export material groups" meant "append material name to the object name",
and was only ever kicking in when the "export object groups" option was
also checked. But the proper behavior (as in 3.0 exporter & the online docs),
is to emit g objectname_materialname before each usemtl line. Which is something entirely else.

Cherry picked from b9123b80 (D14349) with minor conflict fixes.

source/blender/editors/io/io_obj.c

@@ -359,7 +359,7 @@ void WM_OT_obj_export(struct wmOperatorType *ot)
                   "export_material_groups",
                   false,
                   "Export Material Groups",
-                  "Append mesh name and material name to object name, separated by a '_'");
+                  "Generate an OBJ group for each part of a geometry using a different material");
   RNA_def_boolean(
       ot->srna,
       "export_vertex_groups",

source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc

@@ -178,31 +178,13 @@ void OBJWriter::write_mtllib_name(const StringRefNull mtl_filepath) const
   file_handler_->write<eOBJSyntaxElement::mtllib>(mtl_file_name);
 }
 
-void OBJWriter::write_object_group(const OBJMesh &obj_mesh_data) const
-{
-  /* "o object_name" is not mandatory. A valid .OBJ file may contain neither
-   * "o name" nor "g group_name". */
-  BLI_assert(export_params_.export_object_groups);
-  if (!export_params_.export_object_groups) {
-    return;
-  }
-  const std::string object_name = obj_mesh_data.get_object_name();
-  const char *object_mesh_name = obj_mesh_data.get_object_mesh_name();
-  const char *object_material_name = obj_mesh_data.get_object_material_name(0);
-  if (export_params_.export_materials && export_params_.export_material_groups &&
-      object_material_name) {
-    file_handler_->write<eOBJSyntaxElement::object_group>(object_name + "_" + object_mesh_name +
-                                                          "_" + object_material_name);
-    return;
-  }
-  file_handler_->write<eOBJSyntaxElement::object_group>(object_name + "_" + object_mesh_name);
-}
-
 void OBJWriter::write_object_name(const OBJMesh &obj_mesh_data) const
 {
   const char *object_name = obj_mesh_data.get_object_name();
   if (export_params_.export_object_groups) {
-    write_object_group(obj_mesh_data);
+    const std::string object_name = obj_mesh_data.get_object_name();
+    const char *mesh_name = obj_mesh_data.get_object_mesh_name();
+    file_handler_->write<eOBJSyntaxElement::object_group>(object_name + "_" + mesh_name);
     return;
   }
   file_handler_->write<eOBJSyntaxElement::object_name>(object_name);
@@ -278,13 +260,14 @@ int16_t OBJWriter::write_poly_material(const OBJMesh &obj_mesh_data,
     file_handler_->write<eOBJSyntaxElement::poly_usemtl>(MATERIAL_GROUP_DISABLED);
     return current_mat_nr;
   }
-  if (export_params_.export_object_groups) {
-    write_object_group(obj_mesh_data);
-  }
   const char *mat_name = matname_fn(current_mat_nr);
   if (!mat_name) {
     mat_name = MATERIAL_GROUP_DISABLED;
   }
+  if (export_params_.export_material_groups) {
+    const std::string object_name = obj_mesh_data.get_object_name();
+    file_handler_->write<eOBJSyntaxElement::object_group>(object_name + "_" + mat_name);
+  }
   file_handler_->write<eOBJSyntaxElement::poly_usemtl>(mat_name);
 
   return current_mat_nr;

source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh

@@ -65,10 +65,6 @@ class OBJWriter : NonMovable, NonCopyable {
    * Write object's name or group.
    */
   void write_object_name(const OBJMesh &obj_mesh_data) const;
-  /**
-   * Write an object's group with mesh and/or material name appended conditionally.
-   */
-  void write_object_group(const OBJMesh &obj_mesh_data) const;
   /**
    * Write file name of Material Library in .OBJ file.
    */

source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc

@@ -205,11 +205,6 @@ const Material *OBJMesh::get_object_material(const int16_t mat_nr) const
    */
   Object *obj = const_cast<Object *>(&export_object_eval_);
   const Material *r_mat = BKE_object_material_get(obj, mat_nr + 1);
-#ifdef DEBUG
-  if (!r_mat) {
-    std::cerr << "Material not found for mat_nr = " << mat_nr << std::endl;
-  }
-#endif
   return r_mat;
 }
 

source/blender/io/wavefront_obj/tests/obj_exporter_tests.cc

@@ -479,4 +479,17 @@ TEST_F(obj_exporter_regression_test, all_objects)
                                _export.params);
 }
 
+TEST_F(obj_exporter_regression_test, all_objects_mat_groups)
+{
+  OBJExportParamsDefault _export;
+  _export.params.forward_axis = OBJ_AXIS_Y_FORWARD;
+  _export.params.up_axis = OBJ_AXIS_Z_UP;
+  _export.params.export_smooth_groups = true;
+  _export.params.export_material_groups = true;
+  compare_obj_export_to_golden("io_tests/blend_scene/all_objects.blend",
+                               "io_tests/obj/all_objects_mat_groups.obj",
+                               "io_tests/obj/all_objects_mat_groups.mtl",
+                               _export.params);
+}
+
 }  // namespace blender::io::obj