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(visualizing-worlds)=
# Visualizing Worlds

This tutorial explains you how to visualize a world.
There are two recommended ways of doing it.
One light weight way through RVIZ2 and a more heavy weight way through simulation with multiverse.
Let's load a world first to get started.

```{code-cell} ipython3
import logging
import os

from importlib.resources import files
from pathlib import Path

from semantic_digital_twin.adapters.urdf import URDFParser 

logging.disable(logging.CRITICAL)
apartment = os.path.join(Path(files("semantic_digital_twin")).parent.parent, "resources", "urdf", "apartment.urdf")
world = URDFParser.from_file(apartment).parse()

```

For the RVIZ2 way, ROS2 and the TFPublisher is needed. A caveat of this approach is that you have to manage the lifecycle of a ROS2 node yourself.
We recommend to put the spinning into sperate threads and just shutdown the thread when exiting the system.

```{code-cell} ipython3
from semantic_digital_twin.adapters.ros.tf_publisher import TFPublisher
from semantic_digital_twin.adapters.ros.visualization.viz_marker import VizMarkerPublisher
import threading
import rclpy
rclpy.init()

node = rclpy.create_node("semantic_digital_twin")
thread = threading.Thread(target=rclpy.spin, args=(node,), daemon=True)
thread.start()

tf_publisher = TFPublisher(_world=world, node=node)
viz = VizMarkerPublisher(_world=world, node=node)
```

When you want to stop visualizing, you have to stop the visualizer and afterwards clean up ROS2.

```{code-cell} ipython3
node.destroy_node()
rclpy.shutdown()
```

The world can also be visualized directly through a running simulation.
Although this approach is computationally heavier, it provides the important advantage of enabling interaction with the environment. 
In addition to visualization, the physics engine can be used to simulate dynamics, contacts, and collisions.
Further details are provided in the [physics simulators](physics-simulators) section.

If you have followed the guide until here, you have probably noticed that we have used the RayTracer to visualize the world 
a few times. This is a convenient way of visualizing a world inside a notebook, like in these guides, but it is not recommended 
for normal usage.