# Access the tf Transformation Tree in ROS

## Contents

## Introduction

The tf system in ROS keeps track of multiple coordinate frames and maintains the relationship between them in a tree structure. tf is distributed, so that the information about all coordinate frames is available to every node in the ROS network. MATLAB® allows you to access this transformation tree. This example familiarizes you with accessing the available coordinate frames, retrieving transformations between them, and transform points, vectors, and other entities between any two coordinate frames.

Prerequisites: docid:robotics_examples.example-ROSGettingStartedExample, docid:robotics_examples.example-ROSNetworkingExample

## Start up

- Initialize the ROS system.

rosinit

Initializing ROS master on http://bat5235win64:58071/. Initializing global node /matlab_global_node_56021 with NodeURI http://bat5235win64:58075/

To create a realistic environment for this example, use `exampleHelperROSStartTfPublisher` to broadcast several transformations. The transformations represent a camera that is mounted on a robot.

There are three coordinate frames that are defined in this transformation tree:

- the robot base frame (
`robot_base`) - the camera's mounting point (
`mounting_point`) - the optical center of the camera (
`camera_center`)

Two transforms are being published: the transformation from the robot base to the camera's mounting point and the transformation from the mounting point to the center of the camera.

exampleHelperROSStartTfPublisher

A visual representation of the three coordinate frames looks as follows.

Here, the x, y, and z axes of each frame are represented by red, green, and blue lines respectively. The parent-child relationship between the coordinate frames is shown through a brown arrow pointing from the child to its parent frame.

- Create a new transformation tree object with the
`rostf`function. You can use this object to access all available transformations and apply them to different entities.

tftree = rostf

tftree = TransformationTree with properties: AvailableFrames: {0x1 cell} LastUpdateTime: [0x1 Time] BufferTime: 10

Once the object is created, it starts receiving tf transformations and buffers them internally. You should keep the `tftree` variable in the workspace, so that it continues to receive data.

- Pause for a little bit to make sure that all transformations are received.

pause(1);

- You can see the names of all the available coordinate frames by accessing the
`AvailableFrames`property.

tftree.AvailableFrames

ans = 3x1 cell array {'camera_center' } {'mounting_point'} {'robot_base' }

This should show the three coordinate frames that describe the relationship between the camera, its mounting point, and the robot.

## Receive Transformations

Now that the transformations are available, you can inspect them. Any transformation is described by a ROS `geometry_msgs/TransformStamped` message and has a translational and rotational component.

- Retrieve the transformation that describes the relationship between the mounting point and the camera center. Use the
`getTransform`function to do that.

mountToCamera = getTransform(tftree, 'mounting_point', 'camera_center'); mountToCameraTranslation = mountToCamera.Transform.Translation quat = mountToCamera.Transform.Rotation mountToCameraRotationAngles = rad2deg(quat2eul([quat.W quat.X quat.Y quat.Z]))

mountToCameraTranslation = ROS Vector3 message with properties: MessageType: 'geometry_msgs/Vector3' X: 0 Y: 0 Z: 0.5000 Use showdetails to show the contents of the message quat = ROS Quaternion message with properties: MessageType: 'geometry_msgs/Quaternion' X: 0 Y: 0.7071 ...

Relative to the mounting point, the camera center is located 0.5 meters along the z-axis and is rotated by 90 degrees around the y-axis.

- To inspect the relationship between the robot base and the camera's mounting point, call
`getTransform`again.

baseToMount = getTransform(tftree, 'robot_base', 'mounting_point'); baseToMountTranslation = baseToMount.Transform.Translation baseToMountRotation = baseToMount.Transform.Rotation

baseToMountTranslation = ROS Vector3 message with properties: MessageType: 'geometry_msgs/Vector3' X: 1 Y: 0 Z: 0 Use showdetails to show the contents of the message baseToMountRotation = ROS Quaternion message with properties: MessageType: 'geometry_msgs/Quaternion' X: 0 Y: 0 ...

The mounting point is located at 1 meter along the robot base's x-axis.

## Apply Transformations

Assume now that you have a 3D point that is defined in the `camera_center` coordinate frame and you want to calculate what the point coordinates in the `robot_base` frame are.

- Use the
`waitForTransform`function to wait until the transformation between the`camera_center`and`robot_base`coordinate frames becomes available. This call blocks until the transform that takes data from`camera_center`to`robot_base`is valid and available in the transformation tree.

waitForTransform(tftree, 'robot_base', 'camera_center');

- Now define a point at position
`[3 1.5 0.2]`in the camera center's coordinate frame. You will subsequently transform this point into`robot_base`coordinates.

pt = rosmessage('geometry_msgs/PointStamped'); pt.Header.FrameId = 'camera_center'; pt.Point.X = 3; pt.Point.Y = 1.5; pt.Point.Z = 0.2;

- You can transform the point coordinates by calling the
`transform`function on the transformation tree object. Specify what the target coordinate frame of this transformation is. In this example, use`robot_base`.

```
tfpt = transform(tftree, 'robot_base', pt)
```

tfpt = ROS PointStamped message with properties: MessageType: 'geometry_msgs/PointStamped' Header: [1x1 Header] Point: [1x1 Point] Use showdetails to show the contents of the message

The transformed point `tfpt` has the following 3D coordinates.

tfpt.Point

ans = ROS Point message with properties: MessageType: 'geometry_msgs/Point' X: 1.2000 Y: 1.5000 Z: -2.5000 Use showdetails to show the contents of the message

Besides `PointStamped` messages, the `transform` function allows you to transform other entities like poses (`geometry_msgs/PoseStamped`), quaternions (`geometry_msgs/QuaternionStamped`), and point clouds (`sensor_msgs/PointCloud2`).

- If you want to store a transformation, you can retrieve it with the
`getTransform`function.

robotToCamera = getTransform(tftree, 'robot_base', 'camera_center')

robotToCamera = ROS TransformStamped message with properties: MessageType: 'geometry_msgs/TransformStamped' Header: [1x1 Header] ChildFrameId: 'camera_center' Transform: [1x1 Transform] Use showdetails to show the contents of the message

This transformation can be used to transform coordinates in the `camera_center` frame into coordinates in the `robot_base` frame.

robotToCamera.Transform.Translation robotToCamera.Transform.Rotation

ans = ROS Vector3 message with properties: MessageType: 'geometry_msgs/Vector3' X: 1 Y: 0 Z: 0.5000 Use showdetails to show the contents of the message ans = ROS Quaternion message with properties: MessageType: 'geometry_msgs/Quaternion' X: 0 Y: 0.7071 ...

## Send Transformations

You can also broadcast a new transformation to the ROS network.

Assume that you have a simple transformation that describes the offset of the `wheel` coordinate frame relative to the `robot_base` coordinate frame. The wheel is mounted -0.2 meters along the y-axis and -0.3 along the z-axis. The wheel has a relative rotation of 30 degrees around the y-axis.

- Create the corresponding
`geometry_msgs/TransformStamped`message that describes this transformation. The source coordinate frame,`wheel`, is placed to the`ChildFrameId`property. The target coordinate frame,`robot_base`, is added to the`Header.FrameId`property.

tfStampedMsg = rosmessage('geometry_msgs/TransformStamped'); tfStampedMsg.ChildFrameId = 'wheel'; tfStampedMsg.Header.FrameId = 'robot_base';

- The transformation itself is described in the
`Transform`property. It contains a`Translation`and a`Rotation`. Fill in the values that are listed above.

- The
`Rotation`is described as a quaternion. To convert the 30 degree rotation around the y-axis to a quaternion, you can use the`axang2quat`function. The y-axis is described by the`[0 1 0]`vector and 30 degrees can be converted to radians with the`deg2rad`function.

tfStampedMsg.Transform.Translation.X = 0; tfStampedMsg.Transform.Translation.Y = -0.2; tfStampedMsg.Transform.Translation.Z = -0.3; quatrot = axang2quat([0 1 0 deg2rad(30)]) tfStampedMsg.Transform.Rotation.W = quatrot(1); tfStampedMsg.Transform.Rotation.X = quatrot(2); tfStampedMsg.Transform.Rotation.Y = quatrot(3); tfStampedMsg.Transform.Rotation.Z = quatrot(4);

quatrot = 0.9659 0 0.2588 0

- Use
`rostime`to retrieve the current system time and use that to timestamp the transformation. This lets the recipients know at which point in time this transformation was valid.

```
tfStampedMsg.Header.Stamp = rostime('now');
```

- Use the
`sendTransform`function to broadcast this transformation.

sendTransform(tftree, tfStampedMsg)

The new `wheel` coordinate frame is now also in the list of available coordinate frames.

tftree.AvailableFrames

ans = 4x1 cell array {'camera_center' } {'mounting_point'} {'robot_base' } {'wheel' }

The final visual representation of all four coordinate frames looks as follows.

You can see that the `wheel` coordinate frame has the translation and rotation that you specified in sending the transformation.

## Stop Example Publisher and Shut Down ROS Network

- Stop the example transformation publisher.

exampleHelperROSStopTfPublisher

- Shut down the ROS master and delete the global node.

rosshutdown

Shutting down global node /matlab_global_node_56021 with NodeURI http://bat5235win64:58075/ Shutting down ROS master on http://bat5235win64:58071/.