tf教程（三）：Writing a tf listener

Writing a tf listener (C++)

Description:  This  tutorial (辅导的)  teaches you how to use tf to get access to  frame (设计)   transformations (转化) .

Tutorial Level:  BEGINNER

Next Tutorial:  Adding a frame  (C++)  

目录

1. How to create a tf listener
   1. The Code
   2. The Code Explained
2. Running the listener
3. Checking the results

catkin rosbuild

In the previous tutorials we created a tf broadcaster to publish the pose(姿势) of a turtle(龟) to tf. In this tutorial we'll create a tf listener to start using tf.

How to create a tf listener

Let's first create the source files. Go to the package we created in the previous tutorial:

 $ roscd learning_tf

The Code

Fire up your favorite editor and paste(张贴) the following code into a new file called src/turtle_tf_listener.cpp.

https://raw.github.com/ros/geometry_tutorials/hydro-devel/turtle_tf/src/turtle_tf_listener.cpp

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#include /ros.h>
#include /transform_listener.h>
#include /Twist.h>
#include /Spawn.h>

int main(int argc, char** argv){
  ros::init(argc, argv, "my_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;
};

If you get an error "Lookup would require extrapolation(外推法) into the past" while running, you can try this alternative(供选择的) code to call the listener:

try {
    listener.waitForTransform(destination_frame, original_frame, ros::Time(0), ros::Duration(10.0) );
    listener.lookupTransform(destination_frame, original_frame, ros::Time(0), transform);
} catch (tf::TransformException ex) {
    ROS_ERROR("%s",ex.what());
}

The Code Explained

Now, let's take a look at the code that is relevant(有关的) to publishing the turtle(龟) pose(造成) to tf.

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   2 #include /transform_listener.h>
   3 

The tf package provides an implementation(实现) of a TransformListener to help make the task of receiving transforms(改变)easier. To use the TransformListener, we need to include the tf/transform_listener.h header file.

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  20   tf::TransformListener listener;

Here, we create a TransformListener object. Once the listener is created, it starts receiving tf transformations(转化) over the wire, and buffers them for up to 10 seconds. The TransformListener object should be scoped(审视) to persist(存留)otherwise it's cache will be unable to fill and almost every query will fail. A common method is to make the TransformListener object a member variable(变量) of a class.

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  25     try{
  26       listener.lookupTransform("/turtle2", "/turtle1",
  27                                ros::Time(0), transform);
  28     }

Here, the real work is done, we query the listener for a specific(特殊的) transformation(转化). Let's take a look at the four arguments:

1. We want the transform(改变) from this frame(框架) ...
2. ... to this frame.

3. The time at which we want to transform. Providing ros::Time(0) will just get us the latest available transform.

4. The object in which we store the resulting transform.

All this is wrapped(包) in a try-catch block to catch possible exceptions(例外).

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  35     geometry_msgs::Twist vel_msg;
  36     vel_msg.angular.z = 4.0 * atan2(transform.getOrigin().y(),
  37                                     transform.getOrigin().x());
  38     vel_msg.linear.x = 0.5 * sqrt(pow(transform.getOrigin().x(), 2) +

Here, the transform is used to calculate(计算) new linear(线的) and angular(有角的) velocities(速率) for turtle(龟)2, based on its distance and angle from turtle1. The new velocity is published in the topic "turtle2/cmd_vel" and the sim will use that to update turtle2's movement.

Running the listener

Now that we created the code, lets compile(编译) it first. Open the CMakeLists.txt file, and add the following line on the bottom:

add_executable(turtle_tf_listener src/turtle_tf_listener.cpp)
target_link_libraries(turtle_tf_listener ${catkin_LIBRARIES})

Build your package at the top folder of your catkin workspace:

  $ catkin_make

If everything went well, you should have a binary(二进制的) file called turtle_tf_listener in your devel/lib/learning_tf folder.

If so, we're ready add it the launch(发射) file for this demo. With your text editor, open the launch file called start_demo.launch, and merge(合并) the node block below inside the  block:

  
    ...
              name="listener" />
  

First, make sure you stopped the launch file from the previous tutorial(辅导的) (use ctrl-c). Now you're ready to start your full turtle(龟) demo:

 $ roslaunch learning_tf start_demo.launch

You should see the turtlesim with two turtles.

Checking the results

To see if things work, simply drive around the first turtle(龟) using the arrow keys (make sure your terminal window is active, not your simulator(模拟器) window), and you'll see the second turtle following the first one!

When the turtlesim starts up you may see:

• [ERROR] 1253915565.300572000: Frame id /turtle2 does not exist! When trying to transform between /turtle1 and /turtle2.
  [ERROR] 1253915565.401172000: Frame id /turtle2 does not exist! When trying to transform between /turtle1 and /turtle2.

This happens because our listener is trying to compute the transform(改变) before messages about turtle 2 have been received because it takes a little time to spawn(产卵) in turtlesim and start broadcasting a tf frame(框架).

Now you're ready to move on to the next tutorial(辅导的), where you'll learn how to add a frame (Python) (C++)