Hello World

NVIDIA Omniverse™ Kit, the toolkit that NVIDIA Isaac Sim uses to build its applications, provides a Python interpreter for scripting. This means every single GUI command, as well as many additional functions are available as Python APIs. However, the learning curve for interfacing with Omniverse Kit using Pixar’s USD Python API is steep and steps are frequently tedious. Therefore we’ve provided a set of APIs that are designed to be used in robotics applications, APIs that abstract away the complexity of USD APIs and merge multiple steps into one for frequently performed tasks.

In this tutorial, we will present the concepts of Core APIs and how to use them. We will start with adding a cube to an empty stage, and we’ll build upon it to create a scene with multiple robots executing multiple tasks simultaneously, as seen below.

Learning Objectives

This tutorial series introduces the Core API. After this tutorial, you learn:

• Creating a World and Scene as defined by the Core API.

• How to add a rigid body to the Stage and simulate it using Python in NVIDIA Isaac Sim.

• The difference between running Python in an Extension Workflow vs a Standalone Workflow, and in Jupyter Notebook.

10-15 Minute Tutorial

Getting Started

Prerequisites

• Intermediate knowledge in Python and asynchronous programming is required for this tutorial.

• Please download and install Visual Studio Code prior to beginning this tutorial.

• Please review Getting Started Tutorials and Workflows prior to beginning this tutorial.

Begin by opening the Hello World example. First activate Windows > Examples > Robotics Examples which will open the Robotics Examples tab.

1. Click Robotics Examples > Hello World.

2. Verify that the window for the Hello World example extension is visible in the workspace.

3. Click the Open Source Code button to launch the source code for editing in Visual Studio Code.

4. Click the Open Containing Folder button to open the directory containing the example files.

This folder contains three files: hello_world.py, hello_world_extension.py, and __init__.py.

The hello_world.py script is where the logic of the application will be added, while the UI elements of the application will be added in hello_world_extension.py script and thus linked to the logic.

1. Click the LOAD button to load the World.

2. click File > New From Stage Template > Empty to create a new stage, click Don’t Save when prompted to save the current stage.

3. Click the LOAD button to load the World again.

4. Open hello_world.py and press “Ctrl+S” to use the hot-reload feature. You will notice that the menu disappears from the workspace (because it was restarted).

5. Open the example menu again and click the LOAD button.

Now you can begin adding to this example.

Code Overview

This example inherits from BaseSample, which is a boilerplate extension application that sets up the basics for every robotics extension application. The following are a few examples of the actions BaseSample performs:

1. Loading the world with its corresponding assets using a button.

2. Clearing the world when a new stage is created.

3. Resetting the world’s objects to their default states.

4. Handling hot reloading.

World is the core class that enables you to interact with the simulator in an easy and modular way. It handles many time-related events such as adding callbacks, stepping physics, resetting the scene, adding tasks (this will be covered later in Adding a Manipulator Robot), etc.

A world contains an instance of a Scene. The Scene class manages simulation assets of interest in the USD Stage. It provides an easy API to add, manipulate, inspect, and reset different USD assets in the stage.

from isaacsim.examples.interactive.base_sample import BaseSample #boiler plate of a robotics extension application

class HelloWorld(BaseSample):
    def __init__(self) -> None:
        super().__init__()
        return

    # This function is called to setup the assets in the scene for the first time
    # Class variables should not be assigned here, since this function is not called
    # after a hot-reload, its only called to load the world starting from an EMPTY stage
    def setup_scene(self):
        # A world is defined in the BaseSample, can be accessed everywhere EXCEPT __init__
        world = self.get_world()
        world.scene.add_default_ground_plane() # adds a default ground plane to the scene
        return

Singleton World

World is a Singleton, which means only one World can exist while running NVIDIA Isaac Sim. The code below demonstrates how to retrieve the current instance of the World across different files and extensions.

from isaacsim.examples.interactive.base_sample import BaseSample
from isaacsim.core.api import World

class HelloWorld(BaseSample):
    def __init__(self) -> None:
        super().__init__()
        return

    def setup_scene(self):
        world = World.instance()
        world.scene.add_default_ground_plane()
        return

Adding to the Scene

Use the Python API to add a cube as a rigid body to the scene.

from isaacsim.examples.interactive.base_sample import BaseSample
import numpy as np
# Can be used to create a new cube or to point to an already existing cube in stage.
from isaacsim.core.api.objects import DynamicCuboid

class HelloWorld(BaseSample):
    def __init__(self) -> None:
        super().__init__()
        return

    def setup_scene(self):
        world = self.get_world()
        world.scene.add_default_ground_plane()
        fancy_cube = world.scene.add(
            DynamicCuboid(
                prim_path="/World/random_cube", # The prim path of the cube in the USD stage
                name="fancy_cube", # The unique name used to retrieve the object from the scene later on
                position=np.array([0, 0, 1.0]), # Using the current stage units which is in meters by default.
                scale=np.array([0.5015, 0.5015, 0.5015]), # most arguments accept mainly numpy arrays.
                color=np.array([0, 0, 1.0]), # RGB channels, going from 0-1
            ))
        return

1. Press Ctrl+S to save the code and hot-reload NVIDIA Isaac Sim.

2. Open the menu again.

3. click File > New From Stage Template > Empty, then the LOAD button. You need to perform this action if you change anything in the setup_scene. Otherwise, you only need to press the LOAD button.

4. Press the PLAY button to start simulating the dynamic cube and see it falling.

Note

Every time the code is edited or changed, press Ctrl+S to save the code and hot-reload NVIDIA Isaac Sim.

Inspecting Object Properties

Print the world pose and velocity of the cube. The highlighted lines show how you can get the objects using the name and query their properties.

from isaacsim.examples.interactive.base_sample import BaseSample
import numpy as np
from isaacsim.core.api.objects import DynamicCuboid

class HelloWorld(BaseSample):
    def __init__(self) -> None:
        super().__init__()
        return

    def setup_scene(self):
        world = self.get_world()
        world.scene.add_default_ground_plane()
        fancy_cube = world.scene.add(
            DynamicCuboid(
                prim_path="/World/random_cube",
                name="fancy_cube",
                position=np.array([0, 0, 1.0]),
                scale=np.array([0.5015, 0.5015, 0.5015]),
                color=np.array([0, 0, 1.0]),
            ))
        return

    # Here we assign the class's variables
    # this function is called after load button is pressed
    # regardless starting from an empty stage or not
    # this is called after setup_scene and after
    # one physics time step to propagate appropriate
    # physics handles which are needed to retrieve
    # many physical properties of the different objects
    async def setup_post_load(self):
        self._world = self.get_world()
        self._cube = self._world.scene.get_object("fancy_cube")
        position, orientation = self._cube.get_world_pose()
        linear_velocity = self._cube.get_linear_velocity()
        # will be shown on terminal
        print("Cube position is : " + str(position))
        print("Cube's orientation is : " + str(orientation))
        print("Cube's linear velocity is : " + str(linear_velocity))
        return

Continuously Inspecting the Object Properties during Simulation

Print the world pose and velocity of the cube during simulation at every physics step executed. As mentioned in Workflows, in this workflow the application is running asynchronously and can’t control when to step physics. However, you can add callbacks to ensure certain things happen before certain events.

Add a physics callback as follows:

from isaacsim.examples.interactive.base_sample import BaseSample
import numpy as np
from isaacsim.core.api.objects import DynamicCuboid

class HelloWorld(BaseSample):
    def __init__(self) -> None:
        super().__init__()
        return

    def setup_scene(self):
        world = self.get_world()
        world.scene.add_default_ground_plane()
        fancy_cube = world.scene.add(
            DynamicCuboid(
                prim_path="/World/random_cube",
                name="fancy_cube",
                position=np.array([0, 0, 1.0]),
                scale=np.array([0.5015, 0.5015, 0.5015]),
                color=np.array([0, 0, 1.0]),
            ))
        return

    async def setup_post_load(self):
        self._world = self.get_world()
        self._cube = self._world.scene.get_object("fancy_cube")
        self._world.add_physics_callback("sim_step", callback_fn=self.print_cube_info) #callback names have to be unique
        return

    # here we define the physics callback to be called before each physics step, all physics callbacks must take
    # step_size as an argument
    def print_cube_info(self, step_size):
        position, orientation = self._cube.get_world_pose()
        linear_velocity = self._cube.get_linear_velocity()
        # will be shown on terminal
        print("Cube position is : " + str(position))
        print("Cube's orientation is : " + str(orientation))
        print("Cube's linear velocity is : " + str(linear_velocity))

Converting the Example to a Standalone Application

Note

• On windows use python.bat instead of python.sh

• The details of how python.sh works below are similar to how python.bat works

As mentioned in Workflows, in this workflow, the robotics application is started when launched from Python right away, and you can control when to step physics and rendering.

1. Open a new my_application.py file and add the following:

   #launch Isaac Sim before any other imports
   #default first two lines in any standalone application
   from isaacsim import SimulationApp
   simulation_app = SimulationApp({"headless": False}) # we can also run as headless.
   
   from isaacsim.core.api import World
   from isaacsim.core.api.objects import DynamicCuboid
   import numpy as np
   
   world = World()
   world.scene.add_default_ground_plane()
   fancy_cube =  world.scene.add(
       DynamicCuboid(
           prim_path="/World/random_cube",
           name="fancy_cube",
           position=np.array([0, 0, 1.0]),
           scale=np.array([0.5015, 0.5015, 0.5015]),
           color=np.array([0, 0, 1.0]),
       ))
   # Resetting the world needs to be called before querying anything related to an articulation specifically.
   # Its recommended to always do a reset after adding your assets, for physics handles to be propagated properly
   world.reset()
   for i in range(500):
       position, orientation = fancy_cube.get_world_pose()
       linear_velocity = fancy_cube.get_linear_velocity()
       # will be shown on terminal
       print("Cube position is : " + str(position))
       print("Cube's orientation is : " + str(orientation))
       print("Cube's linear velocity is : " + str(linear_velocity))
       # we have control over stepping physics and rendering in this workflow
       # things run in sync
       world.step(render=True) # execute one physics step and one rendering step
   
   simulation_app.close() # close Isaac Sim
   

2. Run it using ./python.sh ./exts/isaacsim.examples.interactive/isaacsim/examples/interactive/user_examples/my_application.py.

Summary

This tutorial covered the following topics:

1. Overview of the World and Scene classes.

2. Adding content to the Scene via Python.

3. Adding callbacks.

4. Accessing dynamic properties for objects.

5. The main differences in a standalone application.

Next Steps

Continue to Hello Robot to learn how to add a robot to the simulation.

Note

The next tutorials will be developed mainly using the extensions application workflow. However, conversion to other workflows is similar given what was covered in this tutorial.