Tutorial: Import MJCF

Learning Objectives

This tutorial shows how to import a mjcf model and convert it to a USD in NVIDIA Isaac Sim. After this tutorial, you can use mjcf files in your pipeline while using NVIDIA Isaac Sim.

5-10 Minute Tutorial

Getting Started

Prerequisites

• Review the Getting Started Tutorials prior to beginning this tutorial.

Using the MJCF Importer

Let’s begin by importing an Ant MJCF from the Built in MJCF files that come with the extension.

1. Load the MJCF Importer extension, which should be automatically loaded when NVIDIA Isaac Sim opens and can be accessed from the File > Import menu. If not MJCF files are not listed in the import formats, go to Window > Extensions and enable isaacsim.asset.importer.mjcf.

2. In the file selection dialog box, navigate to the desired folder, and select the desired MJCF file. For this example, we will use the Humanoid nv_humanoid.xml file that comes with this extension, included in the extension assets. The easiest way to find it is:

   • Click on the folder icon beside AUTOLOAD to find the isaacsim.asset.importer.mjcf extension

   • Navigate to /data/mjcf and find nv_humanoid.xml.

3. Change the import options according to the your needs. Check Import Options for more information on the import options.

   

4. Click the Import button to add the robot to the stage.

   

The robot is now imported into the stage. You can now use it in your simulation. You can perform additional changes to the asset once it’s imported, such as adding sensors, changing materials, and updating the joint drives and configuration to achieve a more stable simulation. Robots are mapped as Articulations in the simulation, and for a complete guide in tuning articulations, refer to Articulation Stability Guide

Importing MJCF using Python

Let’s do the exact same things as we did before but with Python scripting instead.

1. Let’s begin by opening the Script Editor. Go to the top Menu Bar and click Window -> Script Editor

2. The window for the Script Editor should now be visible in the workspace.

3. Copy the following code into the Script Editor window.

   import omni.kit.commands
   from pxr import UsdLux, Sdf, Gf, UsdPhysics, PhysicsSchemaTools
   
   # create new stage
   omni.usd.get_context().new_stage()
   
   # setting up import configuration:
   status, import_config = omni.kit.commands.execute("MJCFCreateImportConfig")
   import_config.set_fix_base(False)
   import_config.set_make_default_prim(False)
   
   # Get path to extension data:
   ext_manager = omni.kit.app.get_app().get_extension_manager()
   ext_id = ext_manager.get_enabled_extension_id("isaacsim.asset.importer.mjcf")
   extension_path = ext_manager.get_extension_path(ext_id)
   
   # import MJCF
   omni.kit.commands.execute(
       "MJCFCreateAsset",
       mjcf_path=extension_path + "/data/mjcf/nv_ant.xml",
       import_config=import_config,
       prim_path="/ant"
   )
   
   # get stage handle
   stage = omni.usd.get_context().get_stage()
   
   # enable physics
   scene = UsdPhysics.Scene.Define(stage, Sdf.Path("/physicsScene"))
   
   # set gravity
   scene.CreateGravityDirectionAttr().Set(Gf.Vec3f(0.0, 0.0, -1.0))
   scene.CreateGravityMagnitudeAttr().Set(981.0)
   
   # add lighting
   distantLight = UsdLux.DistantLight.Define(stage, Sdf.Path("/DistantLight"))
   distantLight.CreateIntensityAttr(500)
   

4. Click the Run (Ctrl + Enter) button to import the Ant robot.

Summary

This tutorial covered the following topics:

1. Importing MJCF file using GUI

2. Importing MJCF file using Python

3. Create a Ground Plane

Further Learning

Checkout MJCF Importer Extension to learn more about the different configuration settings to import a mjcf in NVIDIA Isaac Sim.