lab4.md

@@ -122,8 +122,33 @@ for row in data["results"]["bindings"]:
   print(row)
 ```
 
-**Transformation** If you need to transform points from a frame to an other, you can use `tf.transform_listener`, look at `move_to_point.py` script from lab 2 for an example.
-You can also use the `transformPoint(dst_frame, point)` function to transform the points from the sensor frame to the global frame.
+**Transformation** If you need to transform points from a frame to an other, you can use `tf2`:
+
+
+```python
+
+import rclpy
+from rclpy.node import Node
+
+from tf2_ros import TransformException
+from tf2_ros.buffer import Buffer
+from tf2_ros.transform_listener import TransformListener
+
+# The tf_buffer and tf_listener needs to be kept alive and should be created in a constructor
+node = Node()
+tf_buffer = Buffer()
+tf_listener = TransformListener(self.tf_buffer, node)
+
+# To use them for point transformation
+
+try:
+    point_transformed = self.tf_buffer.transform(point, to_frame_rel)
+except TransformException as ex:
+    node.get_logger().info(f'Could not transform {point} to {to_frame_rel}: {ex}')
+    return
+```
+
+Where `to_frame_rel` is the destination frame, in our case, it should be `"map"`. `point` is the point stamped from the observation.
 
 
 ## C++
@@ -138,8 +163,26 @@ QJsonDocument doc = QJsonDocument::fromJson(QByteArray::fromStdString(result));
 
 You can check the API online for [QJsonDocument](https://doc.qt.io/qt-5/qjsondocument.html)
 
-**Transformation** If you need to transform points from a frame to an other, you can use `tf::transform_listener`, look at `ls_to_occ` program from lab 3 for an example.
-You can also use the `transformPoint(dst_frame, point)` function to transform the points from the sensor frame to the global frame.
+**Transformation** If you need to transform points from a frame to an other, you can use `tf2`:
+
+```c++
+
+// The tf_buffer and tf_listener needs to be kept alive and should be created in a constructor
+std::unique_ptr<tf2_ros::Buffer> tf_buffer_ = std::make_unique<tf2_ros::Buffer>(node->get_clock());
+std::shared_ptr<tf2_ros::TransformListener> tf_listener_ = std::make_shared<tf2_ros::TransformListener>(*tf_buffer_);
+
+// To use them for point transformation
+
+try {
+  point_transformed = tf_buffer_->tranform(point, to_frame_rel);
+} catch (const tf2::TransformException & ex) {
+  RCLCPP_INFO(node->get_logger(), "Could not transform %s to %s: %s",
+            point.header.frame_id.c_str(), to_frame_rel.c_str(), ex.what());
+  return;
+}
+```
+
+Where `to_frame_rel` is the destination frame, in our case, it should be `"map"`. `point` is the point stamped from the observation.
 
 # Visualisation