Adding SetTopologyOptions(); needs work

Author: sherm1

multibody/plant/multibody_plant.cc

@@ -1266,6 +1266,19 @@ void MultibodyPlant<T>::RenameModelInstance(ModelInstanceIndex model_instance,
   }
 }
 
+template <typename T>
+void MultibodyPlant<T>::SetTopologyOptions(
+    internal::ForestBuildingOptions options,
+    std::optional<ModelInstanceIndex> model_instance) {
+  DRAKE_THROW_UNLESS(!is_finalized());
+  internal::LinkJointGraph& graph = mutable_tree().mutable_graph();
+  if (model_instance.has_value()) {
+    graph.SetForestBuildingOptions(*model_instance, options);
+  } else {
+    graph.SetGlobalForestBuildingOptions(options);
+  }
+}
+
 template <typename T>
 void MultibodyPlant<T>::Finalize() {
   // After finalizing the base class, tree is read-only.

multibody/plant/multibody_plant.h

@@ -1667,6 +1667,29 @@ class MultibodyPlant : public internal::MultibodyTreeSystem<T> {
   void RenameModelInstance(ModelInstanceIndex model_instance,
                            const std::string& name);
 
+  /// Sets options that affect the topology of the graph that will be
+  /// constructed to model this system when Finalize() is called. Options can be
+  /// set globally or for a particular ModelInstance. Global options are used
+  /// for any model elements that belong to model instances for which no
+  /// options have been set explicitly.
+  ///
+  /// | TopologyOption        | Description                         |
+  /// | --------------------- | ----------------------------------- |
+  /// | kDefault              | QuaternionFloatingJoint base bodies |
+  /// | kUseRpyFloatingJoints | RpyFloatingJoint base bodies        |
+  /// | kUseFixedBase         | WeldJoint base bodies               |
+  /// | kStatic               | Weld each body to World             |
+  ///
+  /// @param[in] options TopologyOptions, or-ed together
+  /// @param[in] model_instance (optional) the index of the model instance to
+  ///   which `options` is to be applied.
+  /// @note Options set for a ModelInstance are used _instead_ of the global
+  ///   options; they are not merged.
+  /// @throws std::exception if called after Finalize().
+  void SetTopologyOptions(
+      internal::ForestBuildingOptions options,
+      std::optional<ModelInstanceIndex> model_instance = {});
+
   /// This method must be called after all elements in the model (joints,
   /// bodies, force elements, constraints, etc.) are added and before any
   /// computations are performed.

multibody/tree/mobilizer.h

@@ -395,7 +395,7 @@ class Mobilizer : public MultibodyElement<T> {
   // Sets the default state stored with the mobilizer to some approximation of
   // this pose. It's up to the concrete mobilizer to figure out what to do.
   // (Only QuaternionFloatingMobilizer can represent this pose bit-exactly.) The
-  // result will affect MultibodyPlant::SetDefaultContext().
+  // result will affect CreateDefaultContext().
   // TODO(sherm1) Currently this is only implemented for 6dof mobilizers.
   //  It's still useful for other joints; broaden support.
   virtual void SetDefaultPosePair(const Eigen::Quaternion<double> q_FM,

multibody/tree/multibody_tree.h

@@ -959,6 +959,8 @@ class MultibodyTree {
     return link_joint_graph_;
   }
 
+  [[nodiscard]] LinkJointGraph& mutable_graph() { return link_joint_graph_; }
+
   [[nodiscard]] const SpanningForest& forest() const {
     DRAKE_ASSERT(graph().forest_is_valid());
     return graph().forest();