使用带时间戳的数据类型与``tf2_ros::MessageFilter``

**目标：**了解如何使用``tf2_ros::MessageFilter``处理带时间戳的数据类型。

教程级别： 中级

时间: 10分钟

背景

本教程解释了如何在tf2中使用传感器数据。一些传感器数据的真实世界示例包括：

• 摄像头，包括单目和双目

• 激光扫描

假设创建了一个名为“turtle3”的新海龟，它没有良好的里程计，但有一台高空摄像头跟踪它的位置，并将其作为与“world”坐标系相关的“PointStamped”消息发布。

“turtle1”想知道相对于自己，"turtle3"在哪里。

为了实现这一点，“turtle1”必须订阅发布“turtle3”姿态的主题，等待直到可以将其转换为所需坐标系，然后执行相应操作。为了简化此过程，“tf2_ros::MessageFilter”非常有用。“tf2_ros::MessageFilter”将订阅具有头信息的任何ROS 2消息，并将其缓存，直到可以将其转换为目标坐标系。

任务

1、编写发布PointStamped消息的广播节点

在本教程中，我们将设置一个演示应用程序，其中包含一个用Python编写的节点，用于广播“turtle3”的“PointStamped”位置消息。

首先，让我们创建源文件。

进入我们在之前教程中创建的“learning_tf2_py”包的“src/learning_tf2_py/learning_tf2_py”目录，并通过输入以下命令下载示例传感器消息广播器代码：

LinuxmacOSWindows

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

使用您偏爱的文本编辑器打开该文件。

from geometry_msgs.msg import PointStamped
from geometry_msgs.msg import Twist

import rclpy
from rclpy.node import Node

from turtlesim.msg import Pose
from turtlesim.srv import Spawn


class PointPublisher(Node):

    def __init__(self):
        super().__init__('turtle_tf2_message_broadcaster')

        # Create a client to spawn a turtle
        self.spawner = self.create_client(Spawn, 'spawn')
        # Boolean values to store the information
        # if the service for spawning turtle is available
        self.turtle_spawning_service_ready = False
        # if the turtle was successfully spawned
        self.turtle_spawned = False
        # if the topics of turtle3 can be subscribed
        self.turtle_pose_cansubscribe = False

        self.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 = True
            else:
                if self.result.done():
                    self.get_logger().info(
                        f'Successfully spawned {self.result.result().name}')
                    self.turtle_spawned = True
                else:
                    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/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)
                self.turtle_spawning_service_ready = True
            else:
                # Check if the service is ready
                self.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 = 1.0
        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)


def main():
    rclpy.init()
    node = PointPublisher()
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass

    rclpy.shutdown()

1.1 检查代码

现在让我们来看一下代码。首先，在“on_timer”回调函数中，我们通过异步调用“turtlesim”的“Spawn”服务来生成“turtle3”，并在准备好龟生成服务时将其位置初始化为（4，2，0）。

# 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)

之后，该节点发布“turtle3/cmd_vel”主题、“turtle3/turtle_point_stamped”主题，并订阅“turtle3/pose”主题，并在每次接收到消息时运行回调函数“handle_turtle_pose”。

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)

最后，在回调函数“handle_turtle_pose”中，我们初始化“turtle3”的“Twist”消息并发布它们，这将使“turtle3”沿着一个圆形运动。然后，我们使用传入的“Pose”消息填充“turtle3”的“PointStamped”消息并发布它们。

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)

1.2 编写启动文件

为了运行此演示，我们需要在包 learning_tf2_py 的 launch 子目录中创建一个名为 turtle_tf2_sensor_message.launch.py 的启动文件：

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch_ros.actions import Node


def 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',
        ),
    ])

1.3 添加入口点

为了让 ros2 run 命令能够运行你的节点，你必须在 setup.py``（位于 ``src/learning_tf2_py 目录中）中添加入口点。

在``'console_scripts':``括号之间添加以下行：

'turtle_tf2_message_broadcaster = learning_tf2_py.turtle_tf2_message_broadcaster:main',

1.4 构建

在你的工作空间的根目录中运行 rosdep，以检查缺少的依赖项。

LinuxmacOSWindows

rosdep install -i --from-path src --rosdistro humble -y

然后我们可以构建该包：

LinuxmacOSWindows

colcon build --packages-select learning_tf2_py

2 编写消息过滤器/监听器节点

现在，为了可靠地获取 turtle3 在 turtle1 坐标系下的实时流 PointStamped 数据，我们将创建消息过滤器/监听器节点的源文件。

进入之前教程中创建的``learning_tf2_cpp`` package。在``src/learning_tf2_cpp/src``目录下，通过输入以下命令下载文件``turtle_tf2_message_filter.cpp``：

LinuxmacOSWindows

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

使用您偏爱的文本编辑器打开该文件。

#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"
#endif

using namespace std::chrono_literals;

class PoseDrawer : public rclcpp::Node
{
public:
  PoseDrawer()
  : Node("turtle_tf2_pose_drawer")
  {
    // Declare and acquire `target_frame` parameter
    target_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 exception
    auto 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 available
    tf2_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 caught
        this->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;
}

2.1 检查代码

首先，您必须包含``tf2_ros``包中的``tf2_ros::MessageFilter``头文件，以及先前使用过的``tf2``和``ros2``相关头文件。

#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

其次，需要有``tf2_ros::Buffer``、``tf2_ros::TransformListener``和``tf2_ros::MessageFilter``的持久实例。

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_;

第三，ROS 2中的``message_filters::Subscriber``必须使用该主题进行初始化。并且``tf2_ros::MessageFilter``必须使用该``Subscriber``对象进行初始化。``MessageFilter``构造函数中的其他值得注意的参数是``target_frame``和回调函数。目标框架是它将确保``canTransform``成功的框架。回调函数是数据准备就绪时将调用的函数。

PoseDrawer()
: Node("turtle_tf2_pose_drawer")
{
  // Declare and acquire `target_frame` parameter
  target_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 exception
  auto 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 available
  tf2_filter_->registerCallback(&PoseDrawer::msgCallback, this);
}

最后，当数据准备就绪时，回调方法将调用``tf2_buffer_->transform``并将输出打印到控制台。

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 caught
        this->get_logger(), "Failure %s\n", ex.what());
    }
  }

2.2 添加依赖项

在构建``learning_tf2_cpp``包之前，请在该包的``package.xml``文件中添加另外两个依赖项：

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

2.3 CMakeLists.txt

在``CMakeLists.txt``文件中，在现有依赖项下面添加两行：

find_package(message_filters REQUIRED)
find_package(tf2_geometry_msgs REQUIRED)

以下行将处理ROS版本之间的差异：

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_HEADERS
  NAMES tf2_geometry_msgs.hpp
  PATHS ${_include_dirs}
  NO_CACHE
  PATH_SUFFIXES tf2_geometry_msgs
)

之后，添加可执行文件，并将其命名为``turtle_tf2_message_filter``，稍后将在``ros2 run``中使用。

add_executable(turtle_tf2_message_filter src/turtle_tf2_message_filter.cpp)
ament_target_dependencies(
  turtle_tf2_message_filter
  geometry_msgs
  message_filters
  rclcpp
  tf2
  tf2_geometry_msgs
  tf2_ros
)

if(EXISTS ${TF2_CPP_HEADERS})
  target_compile_definitions(turtle_tf2_message_filter PUBLIC -DTF2_CPP_HEADERS)
endif()

最后，在其他现有节点下面添加``install(TARGETS…)``部分，以便``ros2 run``可以找到您的可执行文件：

install(TARGETS
  turtle_tf2_message_filter
  DESTINATION lib/${PROJECT_NAME})

2.4 构建

在你的工作空间的根目录中运行 rosdep，以检查缺少的依赖项。

LinuxmacOSWindows

rosdep install -i --from-path src --rosdistro humble -y

现在打开一个新的终端，导航到您的工作空间的根目录，并使用以下命令重新构建软件包：

LinuxmacOSWindows

colcon build --packages-select learning_tf2_cpp

打开一个新终端，进入你的工作空间的根目录，并源化设置文件：

LinuxmacOSWindows

. install/setup.bash

3 运行

首先，我们需要通过启动启动文件``turtle_tf2_sensor_message.launch.py``来运行多个节点（包括PointStamped消息的广播节点）：

ros2 launch learning_tf2_py turtle_tf2_sensor_message.launch.py

这将在``turtlesim``窗口中带来两只乌龟，其中``turtle3``沿着一个圆圈移动，而``turtle1``起初不动。但是你可以在另一个终端中运行``turtle_teleop_key``节点来驱动``turtle1``移动：

ros2 run turtlesim turtle_teleop_key

现在，如果你回显主题``turtle3/turtle_point_stamped``：

ros2 topic echo /turtle3/turtle_point_stamped

那么将会有如下输出：

header:
  stamp:
    sec: 1629877510
    nanosec: 902607040
  frame_id: world
point:
  x: 4.989276885986328
  y: 3.073937177658081
  z: 0.0
---
header:
  stamp:
    sec: 1629877510
    nanosec: 918389395
  frame_id: world
point:
  x: 4.987966060638428
  y: 3.089883327484131
  z: 0.0
---
header:
  stamp:
    sec: 1629877510
    nanosec: 934186680
  frame_id: world
point:
  x: 4.986400127410889
  y: 3.105806589126587
  z: 0.0
---

当演示运行时，打开另一个终端并运行消息过滤器/监听器节点：

ros2 run learning_tf2_cpp turtle_tf2_message_filter

如果运行正确，你应该会看到像下面这样的流式数据：

[INFO] [1630016162.006173900] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.493231 y:-2.961614 z:0.000000

[INFO] [1630016162.006291983] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.472169 y:-3.004742 z:0.000000

[INFO] [1630016162.006326234] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.479420 y:-2.990479 z:0.000000

[INFO] [1630016162.006355644] [turtle_tf2_pose_drawer]: Point of turtle3 in frame of turtle1: x:-6.486441 y:-2.976102 z:0.000000

总结

在本教程中，您学习了如何在tf2中使用传感器数据/消息。具体来说，您学习了如何在一个话题上发布“PointStamped”消息，以及如何监听该话题并使用“tf2_ros::MessageFilter”转换“PointStamped”消息的帧。