Create an empty scene
=====================

.. currentmodule:: omni.isaac.orbit

This tutorial shows how to launch and control Isaac Sim simulator from a standalone Python script. It sets up an
empty scene in Orbit and introduces the two main classes used in the framework, :class:`app.AppLauncher` and
:class:`sim.SimulationContext`.

Please review `Isaac Sim Interface`_ and `Isaac Sim Workflows`_ prior to beginning this tutorial to get
an initial understanding of working with the simulator.


The Code
~~~~~~~~

The tutorial corresponds to the ``empty.py`` script in the ``orbit/source/standalone/tutorials/00_sim`` directory.

.. dropdown:: :fa:`eye,mr-1` Code for ``empty.py``

   .. literalinclude:: ../../../../source/standalone/tutorials/00_sim/empty.py
      :language: python
      :emphasize-lines: 18-30,34-38,44-48,51,56-58,71
      :linenos:


The Code Explained
~~~~~~~~~~~~~~~~~~

Launching the simulator
-----------------------

The first step when working with standalone Python scripts is to launch the simulation application.
This is necessary to do at the start since various dependency modules of Isaac Sim are only available
after the simulation app is running.

This can be done by importing the :class:`app.AppLauncher` class. This utility class wraps around
:class:`omni.isaac.kit.SimulationApp` class to launch the simulator. It provides mechanisms to
configure the simulator using command-line arguments and environment variables.

For this tutorial, we mainly look at adding the command-line options to a user-defined
:class:`argparse.ArgumentParser`. This is done by passing the parser instance to the
:meth:`app.AppLauncher.add_app_launcher_args` method, which appends different parameters
to it. These include launching the app headless, configuring different Livestream options,
and enabling off-screen rendering.

.. literalinclude:: ../../../../source/standalone/tutorials/00_sim/empty.py
   :language: python
   :lines: 18-30
   :linenos:
   :lineno-start: 18

Importing python modules
------------------------

Once the simulation app is running, it is possible to import different Python modules from
Isaac Sim and other libraries. Here we import two modules:

* `carb`_: A library that provides various microservices and diagnostics utilities in Omniverse.
* :mod:`omni.isaac.orbit.sim`: A sub-package in Orbit for all the core simulator-related operations.

.. literalinclude:: ../../../../source/standalone/tutorials/00_sim/empty.py
   :language: python
   :lines: 36-38
   :linenos:
   :lineno-start: 36


Configuring the simulation context
----------------------------------

When launching the simulator from a standalone script, the user has complete control over playing,
pausing and stepping the simulator. All these operations are handled through the **simulation
context**. It takes care of various timeline events and also configures the `physics scene`_ for
simulation.

In Orbit, the :class:`sim.SimulationContext` class inherits from Isaac Sim's
:class:`omni.isaac.core.simulation_context.SimulationContext` to allow configuring the simulation
through Python's `dataclass` object and handle certain intricacies of the simulation stepping.

For this tutorial, we set the physics and rendering time step to 0.01 seconds. This is done
by passing these quantities to the :class:`sim.SimulationCfg`, which is then used to create an
instance of the simulation context.

.. literalinclude:: ../../../../source/standalone/tutorials/00_sim/empty.py
   :language: python
   :lines: 44-48
   :linenos:
   :lineno-start: 44

Following the creation of the simulation context, we have only configured the physics acting on the
simulated scene. This includes the device to use for simulation, the gravity vector, and other advanced
solver parameters. There are now two main steps remaining to run the simulation:

1. Designing the simulation scene: Adding sensors, robots and other simulated objects
2. Running the simulation loop: Stepping the simulator, and setting and getting data from the simulator

In this tutorial, we look at Step 2 first for an empty scene to focus on the simulation control first.
In the following tutorials, we will look into Step 1 and working with simulation handles for interacting
with the simulator.

Running the simulation
----------------------

The first thing, after setting up the simulation scene, is to call the :meth:`sim.SimulationContext.reset`
method. This method plays the timeline and initializes the physics handles in the simulator. It must always
be called the first time before stepping the simulator. Otherwise, the simulation handles are not initialized
properly.

.. note::

   :meth:`sim.SimulationContext.reset` is different from :meth:`sim.SimulationContext.play` method as the latter
   only plays the timeline and does not initializes the physics handles.

After playing the simulation timeline, we set up a simple simulation loop where the simulator is stepped repeatedly
while the simulation app is running. The method :meth:`sim.SimulationContext.step` takes in as argument :attr:`render`,
which dictates whether the step includes updating the rendering-related events or not. By default, this flag is
set to True.

.. literalinclude:: ../../../../source/standalone/tutorials/00_sim/empty.py
   :language: python
   :lines: 56-58
   :linenos:
   :lineno-start: 56

Exiting the simulation
----------------------

Lastly, the simulation application is stopped and its window is closed by calling
:meth:`omni.isaac.kit.SimulationApp.close` method. We do this operation under a try-catch
statement to close the app gracefully in case an error happens.

.. literalinclude:: ../../../../source/standalone/tutorials/00_sim/empty.py
   :language: python
   :lines: 70-71
   :linenos:
   :lineno-start: 70


The Code Execution
~~~~~~~~~~~~~~~~~~

Now that we have gone through the code, let's run the script and see the result:

.. code-block:: bash

   ./orbit.sh -p source/standalone/tutorials/00_sim/empty.py


The simulation should be playing, and the stage should be rendering. To stop the simulation,
you can either close the window, or press ``Ctrl+C`` in the terminal.

Passing ``--help`` to the above script will show the different command-line arguments added
earlier by the :class:`app.AppLauncher` class. To run the script headless, you can execute the
following:

.. code-block:: bash

   ./orbit.sh -p source/standalone/tutorials/00_sim/empty.py --headless


Now that we have a basic understanding of how to run a simulation, let's move on to the
following tutorial where we will learn how to add assets to the stage.

.. _`Isaac Sim Interface`: https://docs.omniverse.nvidia.com/isaacsim/latest/introductory_tutorials/tutorial_intro_interface.html#isaac-sim-app-tutorial-intro-interface
.. _`Isaac Sim Workflows`: https://docs.omniverse.nvidia.com/isaacsim/latest/introductory_tutorials/tutorial_intro_workflows.html
.. _carb: https://docs.omniverse.nvidia.com/kit/docs/carbonite/latest/index.html
.. _`physics scene`: https://docs.omniverse.nvidia.com/prod_extensions/prod_extensions/ext_physics.html#physics-scene
