Fix T95360, new 3.1 obj exporter losing nurbs curve "endpoint".

The new wavefront .obj exporter in 3.1 was producing slightly invalid parm line syntax (missing u), and was not setting first/last N params to zeroes and ones for curves with "endpoint" flag properly.

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

@@ -352,12 +352,24 @@ void OBJWriter::write_nurbs_curve(const OBJCurve &obj_nurbs_data) const
 
     /**
      * In `parm u 0 0.1 ..` line:, (total control points + 2) equidistant numbers in the
-     * parameter range are inserted.
+     * parameter range are inserted. However for curves with endpoint flag,
+     * first degree+1 numbers are zeroes, and last degree+1 numbers are ones
      */
+    const short flagsu = obj_nurbs_data.get_nurbs_flagu(spline_idx);
+    const bool cyclic = flagsu & CU_NURB_CYCLIC;
+    const bool endpoint = !cyclic && (flagsu & CU_NURB_ENDPOINT);
     file_handler_->write<eOBJSyntaxElement::nurbs_parameter_begin>();
     for (int i = 1; i <= total_control_points + 2; i++) {
-      file_handler_->write<eOBJSyntaxElement::nurbs_parameters>(1.0f * i /
-                                                                (total_control_points + 2 + 1));
+      float parm = 1.0f * i / (total_control_points + 2 + 1);
+      if (endpoint) {
+        if (i <= nurbs_degree) {
+          parm = 0;
+        }
+        else if (i > total_control_points + 2 - nurbs_degree) {
+          parm = 1;
+        }
+      }
+      file_handler_->write<eOBJSyntaxElement::nurbs_parameters>(parm);
     }
     file_handler_->write<eOBJSyntaxElement::nurbs_parameter_end>();
 

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

@@ -168,7 +168,7 @@ constexpr FormattingSyntax syntax_elem_to_formatting(const eOBJSyntaxElement key
       return {"curv 0.0 1.0", 0, is_type_string_related<T...>};
     }
     case eOBJSyntaxElement::nurbs_parameter_begin: {
-      return {"parm 0.0", 0, is_type_string_related<T...>};
+      return {"parm u 0.0", 0, is_type_string_related<T...>};
     }
     case eOBJSyntaxElement::nurbs_parameters: {
       return {" %f", 1, is_type_float<T...>};

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

@@ -102,4 +102,10 @@ int OBJCurve::get_nurbs_degree(const int spline_index) const
   return nurb->orderu - 1;
 }
 
+short OBJCurve::get_nurbs_flagu(const int spline_index) const
+{
+  const Nurb *const nurb = static_cast<Nurb *>(BLI_findlink(&export_curve_->nurb, spline_index));
+  return nurb->flagu;
+}
+
 }  // namespace blender::io::obj

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

@@ -61,6 +61,10 @@ class OBJCurve : NonCopyable {
    * Get the degree of the NURBS spline at the given index.
    */
   int get_nurbs_degree(int spline_index) const;
+  /**
+   * Get the U flags (CU_NURB_*) of the NURBS spline at the given index.
+   */
+  short get_nurbs_flagu(int spline_index) const;
 
  private:
   /**

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

@@ -378,6 +378,17 @@ TEST_F(obj_exporter_regression_test, nurbs_as_nurbs)
       "io_tests/blend_geometry/nurbs.blend", "io_tests/obj/nurbs.obj", "", _export.params);
 }
 
+TEST_F(obj_exporter_regression_test, nurbs_curves_as_nurbs)
+{
+  OBJExportParamsDefault _export;
+  _export.params.forward_axis = OBJ_AXIS_Y_FORWARD;
+  _export.params.up_axis = OBJ_AXIS_Z_UP;
+  _export.params.export_materials = false;
+  _export.params.export_curves_as_nurbs = true;
+  compare_obj_export_to_golden(
+      "io_tests/blend_geometry/nurbs_curves.blend", "io_tests/obj/nurbs_curves.obj", "", _export.params);
+}
+
 TEST_F(obj_exporter_regression_test, nurbs_as_mesh)
 {
   OBJExportParamsDefault _export;