Creating a Layer

In this tutorial, you will create a layer that draws a published geometry_msgs::Point onto a published image. The instructions are similar to the Creating and Using Plugins (C++) tutorial.

1. Create a Package

Create a new empty package in your workspace src directory:

ros2 pkg create geometry_msgs_layers --build-type ament_cmake --dependencies rqt_image_overlay_layer geometry_msgs --library-name point_layer

The command:

Created a package called geometry_msgs_layers
Added code to CMakeLists.txt of the new package, to generate a library called point_layer
Generated files include/geometry_msgs_layers/point_layer.hpp and src/point_layer.cpp

2. Write point_layer.hpp

Open the generated include/geometry_msgs_layers/point_layer.hpp file in your favorite editor, and paste the following instead of it:

#ifndef GEOMETRY_MSGS_LAYERS__POINT_LAYER_HPP_
#define GEOMETRY_MSGS_LAYERS__POINT_LAYER_HPP_

#include "geometry_msgs_layers/visibility_control.h"
#include "rqt_image_overlay_layer/plugin.hpp"
#include "geometry_msgs/msg/point.hpp"

namespace geometry_msgs_layers
{

class PointLayer : public rqt_image_overlay_layer::Plugin<geometry_msgs::msg::Point>
{
protected:
  void overlay(
    QPainter & painter,
    const geometry_msgs::msg::Point & msg) override;
};

}  // namespace geometry_msgs_layers

#endif  // GEOMETRY_MSGS_LAYERS__POINT_LAYER_HPP_

Your PointLayer class must inherit the class rqt_image_overlay_layer::Plugin<T> where T is the msg type you are displaying in the layer (ie. geometry_msgs::msg::Point).

3. Write point_layer.cpp

Open the generated src/point_layer.cpp file in your favorite editor, and paste the following instead of it:

#include <QPainter>
#include "geometry_msgs_layers/point_layer.hpp"

namespace geometry_msgs_layers
{

void PointLayer::overlay(
  QPainter & painter,
  const geometry_msgs::msg::Point & msg)
{
  painter.translate(msg.x, msg.y);

  painter.save();
  QPen pen = painter.pen();
  pen.setCapStyle(Qt::RoundCap);
  pen.setWidth(20);
  painter.setPen(pen);
  painter.drawPoint(0, 0);
  painter.restore();

  painter.translate(-30, -20);
  QString str = "(%1, %2)";
  painter.drawText(0, 0, str.arg(msg.x).arg(msg.y));
}

}  // namespace geometry_msgs_layers

#include "pluginlib/class_list_macros.hpp"

PLUGINLIB_EXPORT_CLASS(geometry_msgs_layers::PointLayer, rqt_image_overlay_layer::PluginInterface)

The implementation of point_layer.cpp consists of drawing a black point and drawing the coordinate as text above it.

The arguments to the PLUGINLIB_EXPORT_CLASS macro, are:

The fully-qualified type of the layer class, in this case, geometry_msgs_layers::PointLayer.
The fully-qualified type of the base class, this is always rqt_image_overlay_layer::PluginInterface

Important

The base class is rqt_image_overlay_layer::PluginInterface, which is a non-templated in-direct parent class. The direct parent class rqt_image_overlay_layer::Plugin<T> cannot be a base class for plugins because it is a template class.

4. Plugin Declaration XML

A Plugin Declaration XML file must be created to store meta-data about the package.

In the package, create plugins.xml with the following code:

<library path="point_layer">
  <class type="geometry_msgs_layers::PointLayer" base_class_type="rqt_image_overlay_layer::PluginInterface">
    <description>This is an rqt_image_overlay layer for geometry_msgs::Point</description>
  </class>
</library>

Tip

See Plugin Declaration XML from the official ROS2 tutorials to get familiar with the XML tags.

5. CMake Plugin Declaration

CMake Plugin Declaration is required file for the package to be recognised as an rqt_image_overlay_layer plugin.

In your package’s CMakeLists.txt, add the pluginlib_export_plugin_description_file line after the existing ament_target_dependencies line as follows:

ament_target_dependencies(
  point_layer
  "rqt_image_overlay_layer"
  "geometry_msgs"
)

pluginlib_export_plugin_description_file(rqt_image_overlay_layer plugins.xml)

Important

The first argument to pluginlib_export_plugin_description_file (ie. rqt_image_overlay_layer) is the plugin category your layer belongs to, not the name of your layer.

6. Build and Run

Navigate back to the root of your workspace, and build your new package:

colcon build --packages-select geometry_msgs_layers

In a new terminal, source your workspace, and either run rqt, or rqt_image_overlay:

ros2 run rqt_image_overlay rqt_image_overlay

You should be able to see your new layer when you go to add a layer, as following:

7. Testing

In a fresh terminal, publish a point (100.0, 200.0) on topic point by running:

ros2 topic pub point geometry_msgs/msg/Point "
x: 100.0
y: 200.0
z: 0.0"

In rqt_image_overlay, add a geometry_msgs_layer::PointLayer, and set the image topic and set the plugin’s topic to point. You should see the point layer over the image, as below:

Congratulations! You now know how to visualize any custom ros msg topic onto an image!