In the previous subsection, we created a TF broadcaster to publish the pose of a turtle to TF. In this subsection, we will create a TF listener that will use that TF.

We will create a tf_tutorials/src/ turtle_tf_listener.cpp file and add the following source code to it:

#include <ros/ros.h> 
#include <tf/transform_listener.h> 
#include <geometry_msgs/Twist.h> 
#include <turtlesim/Spawn.h> 
 
int main(int argc, char** argv){ 
  ros::init(argc, argv, "tf_listener"); 
 
  ros::NodeHandle node; 
 
  ros::service::waitForService("spawn"); 
  ros::ServiceClient add_turtle = 
    node.serviceClient<turtlesim::Spawn>("spawn"); 
  turtlesim::Spawn srv; 
  add_turtle.call(srv); 
 
  ros::Publisher turtle_vel = 
    node.advertise<geometry_msgs::Twist>("turtle2/cmd_vel", 10); 
 
  tf::TransformListener listener; 
 
  ros::Rate rate(10.0); 
  while (node.ok()){ 
    tf::StampedTransform transform; 
    try{ 
      listener.lookupTransform("/turtle2", "/turtle1", 
                               ros::Time(0), transform); 
    } 
    catch (tf::TransformException &ex) { 
      ROS_ERROR("%s",ex.what()); 
      ros::Duration(1.0).sleep(); 
      continue; 
    } 
 
    geometry_msgs::Twist vel_msg; 
    vel_msg.angular.z = 4.0 * atan2(transform.getOrigin().y(), 
                                    transform.getOrigin().x()); 
    vel_msg.linear.x = 0.5 * sqrt(pow(transform.getOrigin().x(), 2) + 
                                  pow(transform.getOrigin().y(), 2)); 
    turtle_vel.publish(vel_msg); 
 
    rate.sleep(); 
  } 
  return 0; 
}; 

Here, we will create a TransformListener object which will start receiving TF transformations over the ROS communication network, and will buffer them for up to 10 seconds by default:

try{ 
      listener.lookupTransform("/turtle2", "/turtle1", 
                               ros::Time(0), transform); 
    } 

In the preceding line of source code, we queried the listener for a specific transformation that will take four arguments:

• In the first two arguments, we will transform from the /turtle1 frame to the /turtle2 frame.
• In the next argument, the time at which the transformation is required, providing ros::Time(0) will set up the latest available transform.
• In the final argument, we will use the object to store the resulting transformation.