How to use QoS in Subscriber with message_filters in ROS2 and C++?

Question

I make a use of message-filters to merge 3 ROS2 topic into one and publish it. Im using ROS2 and humble and C++. So in the ROS2 need to declare QoS when subscribe to the topics. Here the code that works but without QoS.

#include <memory>
#include <string>
#include <cstring>

#include "rclcpp/rclcpp.hpp"
#include "geometry_msgs/msg/quaternion.hpp"
#include "sensor_msgs/msg/imu.hpp"
#include "geometry_msgs/msg/vector3.hpp"
#include "message_filters/subscriber.h"
#include "message_filters/time_synchronizer.h"
#include "geometry_msgs/msg/quaternion_stamped.hpp"
#include "geometry_msgs/msg/vector3_stamped.hpp"

#include "rclcpp/rclcpp.hpp"
#include "rmw/types.h"
#include "rclcpp/qos.hpp"

using std::placeholders::_1;
using std::placeholders::_2;
using std::placeholders::_3;


class ExactTimeSubscriber : public rclcpp::Node
{
public:
  ExactTimeSubscriber()
      : Node("exact_time_subscriber")
  {

    
     // Create publishers for the merged IMU topic`
    imu_pub_ = this->create_publisher<sensor_msgs::msg::Imu>("/robot/imu", 10);
    
    subscription_imu_1_.subscribe(this, "/robot/attitude",);
    subscription_imu_2_.subscribe(this, "/robot/angular_velocity");
    subscription_imu_3_.subscribe(this, "/robot/linear_acceleration");

    sync_ = std::make_shared<message_filters::TimeSynchronizer<geometry_msgs::msg::QuaternionStamped, geometry_msgs::msg::Vector3Stamped, geometry_msgs::msg::Vector3Stamped>>(subscription_imu_1_, subscription_imu_2_, subscription_imu_3_, 3);
    sync_->registerCallback(std::bind(&ExactTimeSubscriber::topic_callback, this,  _1, _2, _3));

  }

  
private:  
  void topic_callback(const geometry_msgs::msg::QuaternionStamped::ConstSharedPtr& msg1, const geometry_msgs::msg::Vector3Stamped::ConstSharedPtr& msg2, const geometry_msgs::msg::Vector3Stamped::ConstSharedPtr& msg3) const
  {
    // Create the IMU message and populate its fields
    sensor_msgs::msg::Imu imu_msg;
    imu_msg.header = msg1->header;
    imu_msg.orientation = msg1->quaternion;
    imu_msg.linear_acceleration = msg2->vector;
    imu_msg.angular_velocity = msg3->vector;

    // Publish the merged IMU message
    RCLCPP_INFO(this->get_logger(), "Mono_Inertial node started");
    imu_pub_->publish(imu_msg);

  }
  
  rclcpp::Publisher<sensor_msgs::msg::Imu>::SharedPtr imu_pub_;
  message_filters::Subscriber<geometry_msgs::msg::QuaternionStamped> subscription_imu_1_;
  message_filters::Subscriber<geometry_msgs::msg::Vector3Stamped> subscription_imu_2_;
  message_filters::Subscriber<geometry_msgs::msg::Vector3Stamped> subscription_imu_3_;

  std::shared_ptr<message_filters::TimeSynchronizer<geometry_msgs::msg::QuaternionStamped  , geometry_msgs::msg::Vector3Stamped, geometry_msgs::msg::Vector3Stamped>> sync_;
    
};

int main(int argc, char *argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<ExactTimeSubscriber>());
  rclcpp::shutdown();

  return 0;
}

Im not familiar with QoS. So not sure how to declare when subscribe to ROS2 topic. So my question is how to use it the QoS in this code?

Answer 1

I am able to set QoS to my subscriber functions with the codes that are posted below. However, I was not able to fire the TimeSynchronizer callback function. Maybe yours works with these QoS settings. (My environment is ROS2 Humble with Turtlebot3)

The header file for QoS:

    #include <rclcpp/qos.hpp>

In my private section:

    std::shared_ptr<message_filters::Subscriber<LaserScan>> my_sub_;

In the constructor function:

    ...
    rmw_qos_profile_t custom_qos = rmw_qos_profile_default;
    custom_qos.history=RMW_QOS_POLICY_HISTORY_KEEP_LAST;
    //custom_qos.history=RMW_QOS_POLICY_HISTORY_KEEP_ALL;
    custom_qos.depth=10;
    //custom_qos.reliability=RMW_QOS_POLICY_RELIABILITY_RELIABLE;
    custom_qos.reliability=RMW_QOS_POLICY_RELIABILITY_BEST_EFFORT;
    custom_qos.durability=RMW_QOS_POLICY_DURABILITY_VOLATILE;
    //custom_qos.durability=RMW_QOS_POLICY_DURABILITY_TRANSIENT_LOCAL;
    ...
    rclcpp::SubscriptionOptions sub_options;
    //sub_options.callback_group=callback_group_subs_;
    sub_options.callback_group=nullptr;
    sub_options.use_default_callbacks=false;
    sub_options.ignore_local_publications=false;
    ...
    my_sub_ = std::make_shared<message_filters::Subscriber<LaserScan>>(this, "/scan", custom_qos, sub_options);
    ...