In this video lecture, we will go over an example to help understand the API called lookup_transform, one of the most important and helpful transform APIs.

Correction: In the video, the instructor mentions that the lookup_transform API returns translation and rotation information separately. But that was the case in the tf package. In tf2_ros, the lookup_transform API actually returns a complete transform message which has the geometry_msgs/TransformStamped message type.

The example script comes with the hrwros_week5 package, in the scripts folder. You can navigate to this folder with roscd in a sourced CCS terminal. Let’s inspect the scripts in this folder using, for example, the Atom editor.

The robot squabblers from the start of this week actually used this API to find the relative positions of the TurtleBot. So, let’s take a look at the script file to find out how it was done.

• We have the necessary rospy and tf2_ros imports.
• There are two lists of strings, that actually contain the conversation lines between the robots.
• But let’s dive into the robot_squabblers() function:
  • It first initializes a ROS node called robot_squabblers.
  • It then creates a tf buffer, which we learned about in the last lecture. If you don’t remember exactly, don’t be afraid to go back and rewatch it if you need to! Since we don’t create any specific buffer length, it will go with the default length of 10 seconds.
  • Then, we associate the buffer to a tf listener, so that transform information on the tf topic is constantly updated to the tf buffer, as soon as these topic values become available.
  • Then, we have some helper variables for updating information regarding the transforms, and for displaying the little conversation in the console.
  • The meat is in the while loop:
    • We look up the transforms between the robot 1 and 2 base links, and the TurtleBot base link. The last argument is a way to indicate that we would like the last available information from the tf buffer.
    • The API returns both translation and rotation information separately, but in this script, we only use the translation information. This used to be the case in the tf package. With tf2_ros, the lookup_transform API returns a geometry_msgs/TransformStamped message which contains both translation and rotation information in one message.
    • The rest of the script assembles the text lines the robots speak, and prints them to the terminal.
    • The rate_pub and rate_listener helper variables help keep the conversation speed right: The robots will wait a bit between sentences, and wait until at least one bit of information is inside the tf buffer.
    • It is always good practice to put code that looks up transform information in a try-catch block, even if you check if there are messages available beforehand! This prevents warnings and errors.
    • Finally, keep in mind that the transform lookup API returns a transform message. This looks similar to a pose message, but is not the same!