TF2

For TF2, there is no need to support multiple data types since TF2’s API exclusively uses geometry_msgs::msg::TransformStamped. On the Slicer side, the classes vtkMRMLROS2Tf2BroadcasterNode and vtkMRMLROS2Tf2LookupNode support both vtkMatrix4x4 and vtkMRMLTransformNode.

TF2 lookups use a TF2 buffer to store all the TF2 messages (broadcasts) sent by all the ROS nodes. For the SlicerROS2 module, we decided to add a TF2 buffer as a private data member of the vtkMRMLROS2NodeNode since most users will never need a direct access to the TF2 buffer. The TF2 lookups are performed when the node node is spun.

Broadcasts

To create a new TF2 broadcaster, one should use the MRML ROS 2 node method vtkMRMLROS2NodeNode::CreateAndAddTf2BroadcasterNode. This method takes two parameters:

• the parent ID (std::string)

• the child ID (std::string)

Broadcasters are triggered by calling the Broadcast method. It is also possible to set the TF2 broadcast as an observer for an existing vtkMRMLTransformNode using the method ObserveTransformNode. The broadcast will then automatically occur when the observed transform node is modified.

PythonC++

rosLogic = slicer.util.getModuleLogic('ROS2')
rosNode = rosLogic.GetDefaultROS2Node()
broadcaster = rosNode.CreateAndAddTf2BroadcasterNode('Parent', 'Child')
# Broadcast a 4x4 matrix
broadcastedMat = vtk.vtkMatrix4x4()
broadcastedMat.SetElement(0, 3, 66.0) # Set a default value
broadcaster.Broadcast(broadcastedMat)

To remove the broadcaster node, use the method vtkMRMLROS2NodeNode::RemoveAndDeleteTf2BroadcasterNode. This method takes two parameters:

• the parent ID (std::string)

• the child ID (std::string)

Lookups

To create a new TF2 lookup, one should use the MRML ROS 2 node method vtkMRMLROS2NodeNode::CreateAndAddTf2LookupNode. This method takes two parameters:

• the parent ID (std::string)

• the child ID (std::string)

The class vtkMRMLROS2Tf2LookupNode is derived from vtkMRMLTransformNode so it can be used as any other transformation in the MRML scene.

Lookup nodes get updated when the ROS 2 node is spun. Users can set their own callback to act on updated transformations using an observer on the MRML ROS subscriber node. The last transformation received can be retrieved using GetMatrixTransformToParent.

PythonC++

rosLogic = slicer.util.getModuleLogic('ROS2')
rosNode = rosLogic.GetDefaultROS2Node()
# Note that this next line will give you a repeated error if there is nothing broadcasting to Parent -> Child
lookupNode = rosNode.CreateAndAddTf2LookupNode('Parent', 'Child')
# get the transform "manually"
lookupMat = lookupNode.GetMatrixTransformToParent()
# or use an observer
observerId = lookupNode.AddObserver(slicer.vtkMRMLTransformNode.TransformModifiedEvent, observer.Callback)

To remove the lookup node, the method vtkMRMLROS2NodeNode::RemoveAndDeleteTf2LookupNode. This method takes two parameters:

• the parent ID (std::string)

• the child ID (std::string)