<POINT CLOUD TRANSPORT TUTORIAL>

ROS2 v0.1.

Contents

• Writing a Simple Publisher
  • Code of the Publisher
  • Code Explained
  • Example of Running the Publisher
• Writing a Simple Subscriber
  • Code of the Subscriber
  • Code Explained
  • Example of Running the Subscriber
• Using Publishers And Subscribers With Plugins
  • Running the Publisher and Subsriber
  • Changing the Transport Used
  • Changing Transport Behavior
• Implementing Custom Plugins

Writing a Simple Publisher

In this section, we'll see how to create a publisher node, which opens a ROS 2 bag and publishes PointCloud2 messages from it.

This tutorial assumes that you have created your workspace containing <point_cloud_transport> and <point_cloud_transport_plugins>

Before we start, change to the directory, clone this repository, and unzip the example rosbag in the resources folder:

$ cd ~/<point_cloud_transport_ws>/src
$ git clone https://github.com/ros-perception/point_cloud_transport_tutorial
$ cd point_cloud_transport_tutorial
$ tar -C resources/ -xvf resources/rosbag2_2023_08_05-16_08_51.tar.xz
$ cd ~/<point_cloud_transport_ws>
$ colcon build --merge-install --event-handlers console_direct+

Code of the Publisher

Take a look at my_publisher.cpp

#include <point_cloud_transport/point_cloud_transport.hpp>

// for reading rosbag
#include <ament_index_cpp/get_package_share_directory.hpp>
#include <rclcpp/serialization.hpp>
#include <rclcpp/rclcpp.hpp>
#include <rosbag2_cpp/reader.hpp>
#include <rosbag2_cpp/storage_options.hpp>
#include <rosbag2_cpp/converter_interfaces/serialization_format_converter.hpp>
#include <sensor_msgs/msg/point_cloud2.hpp>

int main(int argc, char ** argv)
{
  rclcpp::init(argc, argv);

  auto node = std::make_shared<rclcpp::Node>("point_cloud_publisher");

  point_cloud_transport::PointCloudTransport pct(node);
  point_cloud_transport::Publisher pub = pct.advertise("pct/point_cloud", 100);

  const std::string bagged_cloud_topic = "/point_cloud";
  const std::string shared_directory = ament_index_cpp::get_package_share_directory(
    "point_cloud_transport_tutorial");
  const std::string bag_file = shared_directory + "/resources/rosbag2_2023_08_05-16_08_51";

  // boiler-plate to tell rosbag2 how to read our bag
  rosbag2_storage::StorageOptions storage_options;
  storage_options.uri = bag_file;
  storage_options.storage_id = "mcap";
  rosbag2_cpp::ConverterOptions converter_options;
  converter_options.input_serialization_format = "cdr";
  converter_options.output_serialization_format = "cdr";

  // open the rosbag
  rosbag2_cpp::readers::SequentialReader reader;
  reader.open(storage_options, converter_options);

  sensor_msgs::msg::PointCloud2 cloud_msg;
  rclcpp::Serialization<sensor_msgs::msg::PointCloud2> cloud_serialization;
  while (reader.has_next() && rclcpp::ok()) {
    // get serialized data
    auto serialized_message = reader.read_next();
    rclcpp::SerializedMessage extracted_serialized_msg(*serialized_message->serialized_data);
    if (serialized_message->topic_name == bagged_cloud_topic) {
      // deserialize and convert to  message
      cloud_serialization.deserialize_message(&extracted_serialized_msg, &cloud_msg);
      // publish the message
      pub.publish(cloud_msg);
      rclcpp::spin_some(node);
      rclcpp::sleep_for(std::chrono::milliseconds(100));
    }
  }
  reader.close();

  node.reset();
  rclcpp::shutdown();
}

Code of Publisher Explained

Now we'll break down the code piece by piece.

Header for including <point_cloud_transport>:

#include <point_cloud_transport/point_cloud_transport.hpp>

Creates PointCloudTransport instance and initializes it with our Node shared pointer. Methods of PointCloudTransport can later be used to create point cloud publishers and subscribers similar to how methods of Node are used to create generic publishers and subscribers.

point_cloud_transport::PointCloudTransport pct(node);

Uses PointCloudTransport method to create a publisher on base topic "pct/point_cloud". Depending on whether more plugins are built, additional (per-plugin) topics derived from the base topic may also be advertised. The second argument is the size of our publishing queue.

point_cloud_transport::Publisher pub = pct.advertise("pct/point_cloud", 10);

Publishes sensor_msgs::PointCloud2 message from the specified rosbag:

  sensor_msgs::msg::PointCloud2 cloud_msg;
  //... rosbag boiler plate to populate cloud_msg ...
  // publish the message
  pub.publish(cloud_msg);
  // spin the node...
  rclcpp::spin_some(node);
  // repeat...

Example of Running the Publisher

To run my_publisher.cpp open terminal in the root of workspace and run the following:

$ source install/setup.bash
$ ros2 run point_cloud_transport_tutorial publisher_test

Writing a Simple Subscriber

In this section, we'll see how to create a subscriber node, which receives PointCloud2 messages and prints the number of points in them.

Code of the Subscriber

Take a look at my_subscriber.cpp:

#include <point_cloud_transport/point_cloud_transport.hpp>
#include <rclcpp/rclcpp.hpp>
#include <sensor_msgs/msg/point_cloud2.hpp>

int main(int argc, char ** argv)
{
  rclcpp::init(argc, argv);
  auto node = std::make_shared<rclcpp::Node>("point_cloud_subscriber");

  point_cloud_transport::PointCloudTransport pct(node);
  point_cloud_transport::Subscriber pct_sub = pct.subscribe(
    "pct/point_cloud", 100,
    [node](const sensor_msgs::msg::PointCloud2::ConstSharedPtr & msg)
    {
      RCLCPP_INFO_STREAM(
        node->get_logger(),
        "Message received, number of points is: " << msg->width * msg->height);
    }, {});

  RCLCPP_INFO_STREAM(node->get_logger(), "Waiting for point_cloud message...");

  rclcpp::spin(node);

  rclcpp::shutdown();

  return 0;
}

Code of Subscriber Explained

Now we'll break down the code piece by piece.

Header for including <point_cloud_transport>:

#include <point_cloud_transport/point_cloud_transport.hpp>

Initializes the ROS node:

rclcpp::init(argc, argv);
auto node = rclcpp::Node::make_shared("point_cloud_subscriber");

Creates PointCloudTransport instance and initializes it with our Node. Methods of PointCloudTransport can later be used to create point cloud publishers and subscribers similar to how methods of NodeHandle are used to create generic publishers and subscribers.

point_cloud_transport::PointCloudTransport pct(node);

Uses PointCloudTransport method to create a subscriber on base topic "pct/point_cloud". The second argument is the size of our subscribing queue. The third argument tells the subscriber to execute lambda function whenever a message is received.

  point_cloud_transport::Subscriber pct_sub = pct.subscribe(
    "pct/point_cloud", 100,
    [node](const sensor_msgs::msg::PointCloud2::ConstSharedPtr & msg)
    {
      RCLCPP_INFO_STREAM(
        node->get_logger(),
        "draco message received, number of points is: " << msg->width * msg->height);
    }, {});

Select a specific transport

Or you can select a specific transport using the TransportHint class. Creates a TransportHint shared pointer. This is how to tell the subscriber that we want to subscribe to a particular transport (in this case "pct/point_cloud/draco"), rather than the raw "pct/point_cloud" topic.

  auto transport_hint = std::make_shared<point_cloud_transport::TransportHints>("draco");

Uses PointCloudTransport method to create a subscriber on base topic "pct/point_cloud" and add the transport_hint variable as the last argument.

  auto transport_hint = std::make_shared<point_cloud_transport::TransportHints>("draco");
  point_cloud_transport::Subscriber pct_sub = pct.subscribe(
    "pct/point_cloud", 100,
    [node](const sensor_msgs::msg::PointCloud2::ConstSharedPtr & msg)
    {
      RCLCPP_INFO_STREAM(
        node->get_logger(),
        "draco message received, number of points is: " << msg->width * msg->height);
    }, {}, transport_hint.get());

Example of Running the Subscriber

To run my_subscriber.cpp, open terminal in the root of workspace and run the following:

$ source install/setup.bash
$ ros2 run point_cloud_transport_tutorial subscriber_test --ros-args -p point_cloud_transport:=draco

The point_cloud_transport parameter is read by the point_cloud_transport library. The complexity of the parameters is hidden in the library.

Using Publishers And Subscribers With Plugins

Running the Publisher and Subsriber

Now we can run the Publisher/Subsriber nodes. To run both start two terminal tabs and enter commands:

$ source install/setup.bash
$ ros2 run point_cloud_transport_tutorial subscriber_test

And in the second tab:

$ source install/setup.bash
$ ros2 run point_cloud_transport_tutorial publisher_test
$ # or choose which plugin you want load (a.k.a. whitelist them).
$ ros2 run point_cloud_transport_tutorial publisher_test --ros-args -p pct.point_cloud.enable_pub_plugins:=["point_cloud_transport/draco"]

If both nodes are running properly, you should see the subscriber node start printing out messages similar to:

Message received, number of points is: XXX

To list the topics, which are being published and subscribed to, enter command:

$ ros2 topic list

The output should look similar to this:

Published topics:
 * /parameter_events [rcl_interfaces/msg/ParameterEvent] 3 publishers
 * /pct/point_cloud [sensor_msgs/msg/PointCloud2] 1 publisher
 * /pct/point_cloud/draco [point_cloud_interfaces/msg/CompressedPointCloud2] 1 publisher
 * /pct/point_cloud/zlib [point_cloud_interfaces/msg/CompressedPointCloud2] 1 publisher
 * /rosout [rcl_interfaces/msg/Log] 3 publishers

Subscribed topics:
 * /parameter_events [rcl_interfaces/msg/ParameterEvent] 2 subscribers
 * /pct/point_cloud/draco [point_cloud_interfaces/msg/CompressedPointCloud2] 1 subscriber

To display the ROS computation graph, enter command:

$ ros2 run rqt_graph rqt_graph

You should see a graph similar to this:

Changing the Transport Used

To check which plugins are built on your machine, enter command:

$ ros2 run point_cloud_transport list_transports

You should see output similar to:

Declared transports:
point_cloud_transport/draco
point_cloud_transport/raw

Details:
----------
"point_cloud_transport/draco"
 - Provided by package: draco_point_cloud_transport
 - Publisher:
      This plugin publishes a CompressedPointCloud2 using KD tree compression.

 - Subscriber:
      This plugin decompresses a CompressedPointCloud2 topic.

----------
"point_cloud_transport/raw"
 - Provided by package: point_cloud_transport
 - Publisher:
            This is the default publisher. It publishes the PointCloud2 as-is on the base topic.

 - Subscriber:
            This is the default pass-through subscriber for topics of type sensor_msgs/PointCloud2.

Shut down your publisher node and restart it. If you list the published topics again and have <point_cloud_transport_plugins> installed, you should see:

 * /pct/point_cloud/draco [draco_point_cloud_transport/CompressedPointCloud2] 1 publisher

ros2 run point_cloud_transport_tutorial my_subscriber --ros-args -r __node:=draco_listener -p point_cloud_transport:=<point_cloud_transport_type>
ros2 run point_cloud_transport_tutorial my_subscriber --ros-args -r __node:=draco_listener -p point_cloud_transport:=draco

If we check the node graph again:

rqt_graph

Changing Transport Behavior

For a particular transport, we may want to tweak settings such as compression level and speed, quantization of particular attributes of point cloud, etc. Transport plugins can expose such settings through rqt_reconfigure. For example, /point_cloud_transport/draco/ allows you to change multiple parameters of the compression on the fly.

For now let's adjust the position quantization. By default, "draco" transport uses quantization of 14 bits, allowing 16384 distinquishable positions in each axis; let's change it to 8 bits (256 positions):

$ ros2 run rqt_reconfigure rqt_reconfigure

Now pick /pct/point_cloud/draco in the drop-down menu and move the quantization_POSITION slider down to 8. If you visualize the messages, such as in RVIZ, you should be able to see the level of detail of the point cloud drop.

Dynamic Reconfigure has updated the dynamically reconfigurable parameter /pct/point_cloud/draco/quantization_POSITION. You can verify this by running:

ros2 param get /point_cloud_subscriber /pct/point_cloud/draco/quantization_POSITION

This should display 8.

Full explanation of the reconfigure parameters and an example of how to use them can be found at <point_cloud_transport_plugins> repository.

Whitelist point cloud transport

This allows you to specify plugins you do want to load (a.k.a. whitelist them).

ros2 run point_cloud_transport_tutorial publisher_test --ros-args -p pct.point_cloud.enable_pub_plugins:=["point_cloud_transport/zlib"]