Skip to content

Latest commit

 

History

History
65 lines (45 loc) · 4.68 KB

axes_demo.md

File metadata and controls

65 lines (45 loc) · 4.68 KB
Raw

Axes

When you add the Axes plugin in RViz, it will default to attach itself to the <Fixed Frame>, i.e. the global frame. But what if we want to visualize a frame somewhere else?

In order to do that we need to create a new frame, which can be achieved in multiple way. By default, RViz uses the tf2 transformation library. There isn't any ROS2 specific documentation for tf2, but a lot of tutorials written for ROS1 can be found here.

The simplest way to add a new frame is to simply use the command line tool provided with the sub-package tf2_ros, namely: static_transform_publisher.

A command line utility for manually sending a transform.
Usage: static_transform_publisher x y z qx qy qz qw frame_id child_frame_id 
OR 
Usage: static_transform_publisher x y z yaw pitch roll frame_id child_frame_id

My intuition for quaternions isn't that sharp, so let's demonstrate using Euler angles:

ros2 run tf2_ros static_transform_publisher 2 0 0 0 1.5705 0 map new_frame

This will create a new frame, called new_frame, 2 units away from the parent frame map, while also being rotated ~$π,$rad (1.5705) around the $x$-axis. where:

  • yaw - Rotation about the the $z$-axis (blue)
  • pitch - Rotation about the $y$-axis (green)
  • roll - Rotation about the $x$-axis (red)

In RViz, you need to change the Reference Frame parameter of the Axes plugin to be new_frame.

If you also add a second Axes plugin that is referenced to the <Fixed Frame>, you should see something like this:

Using C++

The above example is probably of little use since we typically want to visualize things from our ROS nodes. So let's do the same thing as above, but with C++!

In order to do this we need to publish a tf2_msgs::msg::TFMessage on the topic /tf_static. From the documentation, we can see that this is a vector of geometry_msgs::msg::TransformStamped messages. Thankfully we can forgo the "trouble" of having to create a vector and simply use the StaticTransformBroadcaster class that tf2_ros provides.

All we have to do is:

  1. Create a geometry_msgs::msg::TransformStamped message.
  2. Create an instance of a StaticTransformBroadcaster
  3. Send the message using the sendTransform method.

You can find the code here.

Using Python

[TODO]

Troubleshooting

If you get strange garbled errors like below, then you have forgotten to add a dependency under ament_target_dependencies in the CMakeLists.txt. In the example below, it's because I removed tf2_ros.

--- stderr: rviz_demo                             
CMakeFiles/axes_demo.dir/src/axes_demo.cpp.o: In function `Axes::Axes(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
axes_demo.cpp:(.text._ZN4AxesC2ENSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES5_[_ZN4AxesC5ENSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES5_]+0x358): undefined reference to `tf2_ros::StaticTransformBroadcaster::sendTransform(geometry_msgs::msg::TransformStamped_<std::allocator<void> > const&)'
CMakeFiles/axes_demo.dir/src/axes_demo.cpp.o: In function `rclcpp::Publisher<tf2_msgs::msg::TFMessage_<std::allocator<void> >, std::allocator<void> >::Publisher(rclcpp::node_interfaces::NodeBaseInterface*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rclcpp::QoS const&, rclcpp::PublisherOptionsWithAllocator<std::allocator<void> > const&)':
axes_demo.cpp:(.text._ZN6rclcpp9PublisherIN8tf2_msgs3msg10TFMessage_ISaIvEEES4_EC2EPNS_15node_interfaces17NodeBaseInterfaceERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEERKNS_3QoSERKNS_29PublisherOptionsWithAllocatorIS4_EE[_ZN6rclcpp9PublisherIN8tf2_msgs3msg10TFMessage_ISaIvEEES4_EC5EPNS_15node_interfaces17NodeBaseInterfaceERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEERKNS_3QoSERKNS_29PublisherOptionsWithAllocatorIS4_EE]+0x6a): undefined reference to `rosidl_message_type_support_t const* rosidl_typesupport_cpp::get_message_type_support_handle<tf2_msgs::msg::TFMessage_<std::allocator<void> > >()'

Questions

One thing I do not understand is why tf2_ros needs to be added as a dependency, but not tf2? I am including <tf2/LinearMath/Quaternion.h>.

Acknowledgment

My code is heavily inspired by the code for the static_transform_broadcaster tool provided by the tf2_ros package. The code can be found here.