前言

        此篇将介绍如何利用tf2来使用传感器数据（如单声道和立体声摄像机以及雷达）。

        假设我们创建了一只海龟叫turtle3，它的里程计不大好用，为了监视turtle3的活动轨迹，有台头顶摄像机被安装到该海龟的背上（负碑的赑屃），并且实时发布相对于世界坐标系的PointStamped消息（包含位姿和时间）。

        有只叫turtle1的海龟想要知道turtle3相对自己的位姿(在turtle1坐标系中)。

        于是乎turtle1订阅了摄像机发布的关于turtle3位姿的主题，并等待可用的转换数据以便执行其他操作。为了方便实现这个目标，我们可以利用tf2_ros::MessageFilter。

        tf2_ros::MessageFilter会订阅任何带有头（header）的消息并缓存起来，直到可以将该消息从源坐标系变换到目标坐标系为止。

动动手

        需要实现两个节点，一个python实现一个C++实现，其中python的实现需要在learning_tf2_py功能包下，如果前期一直用的learning_tf2_cpp包而没有此包的话，请在工作空间根路径的src下执行如下命令：

$ros2 pkg create --build-type ament_python --license Apache-2.0 -- learning_tf2_py

写一个PointStamped消息的广播节点

        我们先实现一个广播turtle3 PointStamped位置消息的节点代码（python）。

        进入learning_tf2_py包路径下src/learning_tf2_py/learning_tf2_py，执行如下命令下载传感器消息广播节点的例子代码turtle_tf2_message_broadcaster.py：

$wget https://raw.githubusercontent.com/ros/geometry_tutorials/ros2/turtle_tf2_py/turtle_tf2_py/turtle_tf2_message_broadcaster.py

内容如下：

from geometry_msgs.msg import PointStamped
from geometry_msgs.msg import Twistimport rclpy
from rclpy.node import Nodefrom turtlesim.msg import Pose
from turtlesim.srv import Spawnclass PointPublisher(Node):def __init__(self):super().__init__('turtle_tf2_message_broadcaster')# Create a client to spawn a turtleself.spawner = self.create_client(Spawn, 'spawn')# Boolean values to store the information# if the service for spawning turtle is availableself.turtle_spawning_service_ready = False# if the turtle was successfully spawnedself.turtle_spawned = False# if the topics of turtle3 can be subscribedself.turtle_pose_cansubscribe = Falseself.timer = self.create_timer(1.0, self.on_timer)def on_timer(self):if self.turtle_spawning_service_ready:if self.turtle_spawned:self.turtle_pose_cansubscribe = Trueelse:if self.result.done():self.get_logger().info(f'Successfully spawned {self.result.result().name}')self.turtle_spawned = Trueelse:self.get_logger().info('Spawn is not finished')else:if self.spawner.service_is_ready():# Initialize request with turtle name and coordinates# Note that x, y and theta are defined as floats in turtlesim/srv/Spawnrequest = Spawn.Request()request.name = 'turtle3'request.x = 4.0request.y = 2.0request.theta = 0.0# Call requestself.result = self.spawner.call_async(request)self.turtle_spawning_service_ready = Trueelse:# Check if the service is readyself.get_logger().info('Service is not ready')if self.turtle_pose_cansubscribe:self.vel_pub = self.create_publisher(Twist, 'turtle3/cmd_vel', 10)self.sub = self.create_subscription(Pose, 'turtle3/pose', self.handle_turtle_pose, 10)self.pub = self.create_publisher(PointStamped, 'turtle3/turtle_point_stamped', 10)def handle_turtle_pose(self, msg):vel_msg = Twist()vel_msg.linear.x = 1.0vel_msg.angular.z = 1.0self.vel_pub.publish(vel_msg)ps = PointStamped()ps.header.stamp = self.get_clock().now().to_msg()ps.header.frame_id = 'world'ps.point.x = msg.xps.point.y = msg.yps.point.z = 0.0self.pub.publish(ps)def main():rclpy.init()node = PointPublisher()try:rclpy.spin(node)except KeyboardInterrupt:passrclpy.shutdown()

代码分析

# Initialize request with turtle name and coordinates
# Note that x, y and theta are defined as floats in turtlesim/srv/Spawn
request = Spawn.Request()
request.name = 'turtle3'
request.x = 4.0
request.y = 2.0
request.theta = 0.0
# Call request
self.result = self.spawner.call_async(request)

        on_timer回调函数中，我们通过异步调用turtlesim中的Spawn服务孵化出turtle3，并给予turtle3初始位置(4, 2, 0)。

self.vel_pub = self.create_publisher(Twist, '/turtle3/cmd_vel', 10)
self.sub = self.create_subscription(Pose, '/turtle3/pose', self.handle_turtle_pose, 10)
self.pub = self.create_publisher(PointStamped, '/turtle3/turtle_point_stamped', 10)

        之后节点发布主题/turtle3/cmd_vel及主题/turtle3/turtle_point_stamped数据，并订阅了/turtle3/pose主题，当进来消息后会调用handle_turtle_pose回调函数来处理这些消息。

vel_msg = Twist()
vel_msg.linear.x = 1.0
vel_msg.angular.z = 1.0
self.vel_pub.publish(vel_msg)ps = PointStamped()
ps.header.stamp = self.get_clock().now().to_msg()
ps.header.frame_id = 'world'
ps.point.x = msg.x
ps.point.y = msg.y
ps.point.z = 0.0
self.pub.publish(ps)

        最后，在回调函数handle_turtle_pose中，我们初始化了turtle3的Twist类型消息（半径为1米的圆周运动）并发布了它们，接着我们将接收到的Pose消息解析并填充到PointStamped消息中，最后发布了这个PointStamped类型消息。

写一个启动文件

        在learning_tf2_py包中的launch文件夹内创建turtle_tf2_sensor_message_launch.py文件用来运行我们的例子。

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch_ros.actions import Nodedef generate_launch_description():return LaunchDescription([DeclareLaunchArgument('target_frame', default_value='turtle1',description='Target frame name.'),Node(package='turtlesim',executable='turtlesim_node',name='sim',output='screen'),Node(package='turtle_tf2_py',executable='turtle_tf2_broadcaster',name='broadcaster1',parameters=[{'turtlename': 'turtle1'}]),Node(package='turtle_tf2_py',executable='turtle_tf2_broadcaster',name='broadcaster2',parameters=[{'turtlename': 'turtle3'}]),Node(package='turtle_tf2_py',executable='turtle_tf2_message_broadcaster',name='message_broadcaster',),])

添加入口点

        我们必须在src/learning_tf2_py路径下的setup.py文件中添加入口点才能让ros2 run命令启动我们的节点。将下面语句添加到'console_scripts':括弧内：

'turtle_tf2_message_broadcaster = learning_tf2_py.turtle_tf2_message_broadcaster:main',

        另外为了使ros2 launch能够通过包名找到上面的launch文件，我们还需添加相关引入模块及安装信息（否则就会提示在share下找不到launch文件，因为那个路径下根本就未安装成功）：

import os
from glob import glob
from setuptools import find_packages, setuppackage_name = 'launch_tutorial'setup(# Other parameters ...data_files=[# ... Other data files# Include all launch files.(os.path.join('share', package_name, 'launch'), glob(os.path.join('launch', '*launch.[pxy][yma]*')))]
)

        完整的setup.py信息如下： 

import os
from glob import glob
from setuptools import find_packages, setuppackage_name = 'learning_tf2_py'setup(name=package_name,version='0.0.0',packages=find_packages(exclude=['test']),data_files=[('share/ament_index/resource_index/packages',['resource/' + package_name]),('share/' + package_name, ['package.xml']),(os.path.join('share', package_name, 'launch'), glob(os.path.join('launch', '*launch.[pxy][yma]*')))],install_requires=['setuptools'],zip_safe=True,maintainer='mike',maintainer_email='mike@todo.todo',description='TODO: Package description',license='Apache-2.0',tests_require=['pytest'],entry_points={'console_scripts': ['turtle_tf2_message_broadcaster = learning_tf2_py.turtle_tf2_message_broadcaster:main',],},
)

构建

        进入工作空间根路径，分别执行如下命令进行依赖检查和最终包的构建工作。

$rosdep install -i --from-path src --rosdistro iron -y

$colcon build --packages-select learning_tf2_py

写一个消息过滤器/监听器节点

        现在，为了可靠地在turtle1的坐标系下获取turtle3的流式PointStamped数据，我们将创建消息过滤器/监听器节点的源文件。

        进入src/learning_tf2_cpp/src路径，执行下面的命令下载turtle_tf2_message_filter.cpp文件：

$wget https://raw.githubusercontent.com/ros/geometry_tutorials/ros2/turtle_tf2_cpp/src/turtle_tf2_message_filter.cpp

内容如下：

#include <chrono>
#include <memory>
#include <string>#include "geometry_msgs/msg/point_stamped.hpp"
#include "message_filters/subscriber.h"
#include "rclcpp/rclcpp.hpp"
#include "tf2_ros/buffer.h"
#include "tf2_ros/create_timer_ros.h"
#include "tf2_ros/message_filter.h"
#include "tf2_ros/transform_listener.h"
#ifdef TF2_CPP_HEADERS#include "tf2_geometry_msgs/tf2_geometry_msgs.hpp"
#else#include "tf2_geometry_msgs/tf2_geometry_msgs.h"
#endifusing namespace std::chrono_literals;class PoseDrawer : public rclcpp::Node
{
public:PoseDrawer(): Node("turtle_tf2_pose_drawer"){// Declare and acquire `target_frame` parametertarget_frame_ = this->declare_parameter<std::string>("target_frame", "turtle1");std::chrono::duration<int> buffer_timeout(1);tf2_buffer_ = std::make_shared<tf2_ros::Buffer>(this->get_clock());// Create the timer interface before call to waitForTransform,// to avoid a tf2_ros::CreateTimerInterfaceException exceptionauto timer_interface = std::make_shared<tf2_ros::CreateTimerROS>(this->get_node_base_interface(),this->get_node_timers_interface());tf2_buffer_->setCreateTimerInterface(timer_interface);tf2_listener_ =std::make_shared<tf2_ros::TransformListener>(*tf2_buffer_);point_sub_.subscribe(this, "/turtle3/turtle_point_stamped");tf2_filter_ = std::make_shared<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>>(point_sub_, *tf2_buffer_, target_frame_, 100, this->get_node_logging_interface(),this->get_node_clock_interface(), buffer_timeout);// Register a callback with tf2_ros::MessageFilter to be called when transforms are availabletf2_filter_->registerCallback(&PoseDrawer::msgCallback, this);}private:void msgCallback(const geometry_msgs::msg::PointStamped::SharedPtr point_ptr){geometry_msgs::msg::PointStamped point_out;try {tf2_buffer_->transform(*point_ptr, point_out, target_frame_);RCLCPP_INFO(this->get_logger(), "Point of turtle3 in frame of turtle1: x:%f y:%f z:%f\n",point_out.point.x,point_out.point.y,point_out.point.z);} catch (const tf2::TransformException & ex) {RCLCPP_WARN(// Print exception which was caughtthis->get_logger(), "Failure %s\n", ex.what());}}std::string target_frame_;std::shared_ptr<tf2_ros::Buffer> tf2_buffer_;std::shared_ptr<tf2_ros::TransformListener> tf2_listener_;message_filters::Subscriber<geometry_msgs::msg::PointStamped> point_sub_;std::shared_ptr<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>> tf2_filter_;
};int main(int argc, char * argv[])
{rclcpp::init(argc, argv);rclcpp::spin(std::make_shared<PoseDrawer>());rclcpp::shutdown();return 0;
}

代码分析

#include "geometry_msgs/msg/point_stamped.hpp"
#include "message_filters/subscriber.h"
#include "rclcpp/rclcpp.hpp"
#include "tf2_ros/buffer.h"
#include "tf2_ros/create_timer_ros.h"
#include "tf2_ros/message_filter.h"
#include "tf2_ros/transform_listener.h"
#ifdef TF2_CPP_HEADERS#include "tf2_geometry_msgs/tf2_geometry_msgs.hpp"
#else#include "tf2_geometry_msgs/tf2_geometry_msgs.h"
#endif

        首先需包含tf2_ros::MessageFilter、tf2、ros2等相关头文件，以使能相关API。

std::string target_frame_;
std::shared_ptr<tf2_ros::Buffer> tf2_buffer_;
std::shared_ptr<tf2_ros::TransformListener> tf2_listener_;
message_filters::Subscriber<geometry_msgs::msg::PointStamped> point_sub_;
std::shared_ptr<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>> tf2_filter_;

        其次声明有关tf2_ros::Buffer、tf2_ros::TransformListener及tf2_ros::MessageFilter的全局变量。

PoseDrawer()
: Node("turtle_tf2_pose_drawer")
{// Declare and acquire `target_frame` parametertarget_frame_ = this->declare_parameter<std::string>("target_frame", "turtle1");std::chrono::duration<int> buffer_timeout(1);tf2_buffer_ = std::make_shared<tf2_ros::Buffer>(this->get_clock());// Create the timer interface before call to waitForTransform,// to avoid a tf2_ros::CreateTimerInterfaceException exceptionauto timer_interface = std::make_shared<tf2_ros::CreateTimerROS>(this->get_node_base_interface(),this->get_node_timers_interface());tf2_buffer_->setCreateTimerInterface(timer_interface);tf2_listener_ =std::make_shared<tf2_ros::TransformListener>(*tf2_buffer_);point_sub_.subscribe(this, "/turtle3/turtle_point_stamped");tf2_filter_ = std::make_shared<tf2_ros::MessageFilter<geometry_msgs::msg::PointStamped>>(point_sub_, *tf2_buffer_, target_frame_, 100, this->get_node_logging_interface(),this->get_node_clock_interface(), buffer_timeout);// Register a callback with tf2_ros::MessageFilter to be called when transforms are availabletf2_filter_->registerCallback(&PoseDrawer::msgCallback, this);
}

        第三，ROS 2中的message_filters::Subscriber必须使用主题（"/turtle3/turtle_point_stamped"）进行初始化。同时，tf2_ros::MessageFilter也必须使用那个Subscriber对象(point_sub_)进行初始化。在MessageFilter构造函数中值得注意的其他参数包括目标帧（target_frame）和回调函数（callback）。目标帧是确保canTransform能够成功执行的目标坐标系。当数据准备就绪时，回调函数（msgCallback）就会被调用。

private:void msgCallback(const geometry_msgs::msg::PointStamped::SharedPtr point_ptr){geometry_msgs::msg::PointStamped point_out;try {tf2_buffer_->transform(*point_ptr, point_out, target_frame_);RCLCPP_INFO(this->get_logger(), "Point of turtle3 in frame of turtle1: x:%f y:%f z:%f\n",point_out.point.x,point_out.point.y,point_out.point.z);} catch (const tf2::TransformException & ex) {RCLCPP_WARN(// Print exception which was caughtthis->get_logger(), "Failure %s\n", ex.what());}}

        最后，在回调函数msgCallback中，当数据准备好的时候会调用transform函数将数据转换为目标坐标系视角下的对应数据，并会将结果数据输出到控制台。

package,xml添加依赖

        我们需要增加下面两行内容到package.xml:

<depend>message_filters</depend>
<depend>tf2_geometry_msgs</depend>

CMakeLists.txt

        同样，CMakeLists.txt文件也需添加下面两行内容：

find_package(message_filters REQUIRED)
find_package(tf2_geometry_msgs REQUIRED)

        下面内容是为了处理不同版本的ROS：

if(TARGET tf2_geometry_msgs::tf2_geometry_msgs)get_target_property(_include_dirs tf2_geometry_msgs::tf2_geometry_msgs INTERFACE_INCLUDE_DIRECTORIES)
else()set(_include_dirs ${tf2_geometry_msgs_INCLUDE_DIRS})
endif()find_file(TF2_CPP_HEADERSNAMES tf2_geometry_msgs.hppPATHS ${_include_dirs}NO_CACHEPATH_SUFFIXES tf2_geometry_msgs
)

        接着，我们还需加上下面内容，我们将消息过滤器/监听器节点可执行文件命名为turtle_tf2_message_filter：

add_executable(turtle_tf2_message_filter src/turtle_tf2_message_filter.cpp)
ament_target_dependencies(turtle_tf2_message_filtergeometry_msgsmessage_filtersrclcpptf2tf2_geometry_msgstf2_ros
)if(EXISTS ${TF2_CPP_HEADERS})target_compile_definitions(turtle_tf2_message_filter PUBLIC -DTF2_CPP_HEADERS)
endif()

        最后再加上安装信息，使ros2 run命令能够根据路径找到可执行文件：

install(TARGETSturtle_tf2_message_filterDESTINATION lib/${PROJECT_NAME})

构建

        执行依赖检查和最终构建：

$rosdep install -i --from-path src --rosdistro iron -y

$colcon build --packages-select learning_tf2_cpp

运行

        新开一个终端，进入工作空间根路径，source下环境（. install/setup.bash），首先启动几个节点（PointStamped消息的广播节点）：

$ros2 launch learning_tf2_py turtle_tf2_sensor_message_launch.py

        如果上述命令提示找不到turtle_tf2_sensor_message_launch.py文件（原因及解决方法见上文setup.py文件的修改，主要是未添加安装信息，如cmake中的install()一样）也可以直接进入launch路径执行，如下图所示。 

        启动完成后会有两只小海龟，turtle3在做圆周运动，turtle1静止不动，我们可以再开启一个终端执行如下命令控制turtle1:

$ros2 run turtlesim turtle_teleop_key

        我们可以订阅查看下turtle3/turtle_point_stamped主题的消息：

$ros2 topic echo /turtle3/turtle_point_stamped

        这些都完成之后，我们再运行下最后构建的消息过滤器/监听器节点：

$ros2 run learning_tf2_cpp turtle_tf2_message_filter

        如果一切OK，我们会在终端看到下面的信息（turtle3在turtle1坐标系中的位姿数据）：

本篇完。