Fix T96470 new obj exporter writing material groups

This is patch D14349 from Aras Pranckevicius.

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.

5 files changed, 21 insertions, 36 deletions

diff --git a/source/blender/editors/io/io_obj.c b/source/blender/editors/io/io_obj.c
index 8e380e3f2bc..df15191916a 100644
--- a/source/blender/editors/io/io_obj.c
+++ b/source/blender/editors/io/io_obj.c
@@ -342,7 +342,7 @@ 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",
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
index 1668ce3269c..3f9fc3b1dbb 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
@@ -174,34 +174,14 @@ void OBJWriter::write_mtllib_name(const StringRefNull mtl_filepath) const
   fh.write_to_file(outfile_);
 }
 
-void OBJWriter::write_object_group(FormatHandler<eFileType::OBJ> &fh,
-                                   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) {
-    fh.write<eOBJSyntaxElement::object_group>(object_name + "_" + object_mesh_name + "_" +
-                                              object_material_name);
-  }
-  else {
-    fh.write<eOBJSyntaxElement::object_group>(object_name + "_" + object_mesh_name);
-  }
-}
-
 void OBJWriter::write_object_name(FormatHandler<eFileType::OBJ> &fh,
                                   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(fh, 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();
+    fh.write<eOBJSyntaxElement::object_group>(object_name + "_" + mesh_name);
     return;
   }
   fh.write<eOBJSyntaxElement::object_name>(object_name);
@@ -363,13 +343,14 @@ void OBJWriter::write_poly_elements(FormatHandler<eFileType::OBJ> &fh,
           buf.write<eOBJSyntaxElement::poly_usemtl>(MATERIAL_GROUP_DISABLED);
         }
         else {
-          if (export_params_.export_object_groups) {
-            write_object_group(buf, obj_mesh_data);
-          }
           const char *mat_name = matname_fn(mat);
           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();
+            fh.write<eOBJSyntaxElement::object_group>(object_name + "_" + mat_name);
+          }
           buf.write<eOBJSyntaxElement::poly_usemtl>(mat_name);
         }
       }
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh
index 50de8ad3282..96f7d434338 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.hh
@@ -67,10 +67,6 @@ class OBJWriter : NonMovable, NonCopyable {
    */
   void write_object_name(FormatHandler<eFileType::OBJ> &fh, const OBJMesh &obj_mesh_data) const;
   /**
-   * Write an object's group with mesh and/or material name appended conditionally.
-   */
-  void write_object_group(FormatHandler<eFileType::OBJ> &fh, const OBJMesh &obj_mesh_data) const;
-  /**
    * Write file name of Material Library in .OBJ file.
    */
   void write_mtllib_name(const StringRefNull mtl_filepath) const;
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc
index 5be68565054..7d3e3cd84fd 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc
@@ -203,11 +203,6 @@ 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;
 }
 
diff --git a/source/blender/io/wavefront_obj/tests/obj_exporter_tests.cc b/source/blender/io/wavefront_obj/tests/obj_exporter_tests.cc
index e96a7d28561..b8fecfb25f3 100644
--- a/source/blender/io/wavefront_obj/tests/obj_exporter_tests.cc
+++ b/source/blender/io/wavefront_obj/tests/obj_exporter_tests.cc
@@ -511,4 +511,17 @@ 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