Introduction
In the world of robotics and simulation, accurately representing real-world objects in virtual environments is crucial. Whether it’s for designing robots, running simulations, or visualizing interactions, choosing the right file format ensures seamless integration between software tools and real-world applications. Various formats have been developed, each catering to specific needs such as physics-based simulations, kinematic modeling, and 3D visualization. This blog explores these file formats, their applications, and how tools like ROS and Isaac Sim can be leveraged for robotics development.
Understanding File Formats for Robotics and Simulation
Unified Robot Description Format (URDF)
Ease of Building: URDF is an XML-based format widely used within the Robot Operating System (ROS) ecosystem. Creating URDF files manually can be labor-intensive and error-prone. However, tools like the ACDC4Robot plugin for Autodesk Fusion 360 have been developed to automate the conversion of CAD models into URDF, simplifying the building process. For example, using ACDC4Robot, a designer can create a robot model in Fusion 360 and export it directly to a URDF file, ready for simulation in ROS.
Ease of Exporting: The availability of dedicated exporters in popular CAD programs facilitates straightforward exporting of URDF files. This integration enables users to design in a familiar environment and seamlessly generate URDF models for simulation and testing.
URDF with Rviz
Simulation Description Format (SDFormat)
Ease of Building: SDFormat, also XML-based, extends the capabilities of URDF by supporting more complex features like closed-loop kinematic chains and comprehensive environment descriptions. Building SDFormat files can be more complex due to these extended features. However, tools like the ACDC4Robot plugin for Fusion 360 have been developed to automate the conversion of CAD models into SDFormat, simplifying the building process. For instance, a designer can model a robot in Fusion 360 and use ACDC4Robot to export it as an SDFormat file for use in simulators like Gazebo.
Ease of Exporting: With plugins like ACDC4Robot, exporting SDFormat files from CAD models becomes more accessible, allowing users to design in CAD software and export directly to SDFormat for simulation purposes.
SDF File in Gazebo
Universal Scene Description (USD) and OpenUSD
Ease of Building: USD, developed by Pixar, is a powerful framework for describing and composing 3D scenes. Building USD files can be complex due to its extensive features and capabilities. However, its adoption in the industry has led to the development of various tools and plugins that facilitate the creation and manipulation of USD files.
Ease of Exporting: Many modern 3D modeling applications, such as Autodesk Maya and Blender, have developed plugins or native support for exporting models to USD, streamlining the workflow for users familiar with these tools.
USD File in Nvidia Isaacsim
MuJoCo XML Format (MJCF)
Ease of Building: MJCF is the native format for the MuJoCo physics engine, designed for detailed modeling of robots and biomechanics. Building MJCF files often requires manual coding, which can be intricate and time-consuming. To alleviate this, some workflows involve designing the model in CAD software, exporting to an intermediary format like URDF, and then converting to MJCF. For example, a robot model created in Fusion 360 can be exported to URDF using ACDC4Robot and then converted to MJCF for simulation in MuJoCo.
Ease of Exporting: Direct exporters for MJCF are less common. Therefore, users often rely on conversion tools to transform models from other formats (e.g., URDF) into MJCF, adding an extra step to the workflow.
COLLADA (Collaborative Design Activity)
Ease of Building: COLLADA is an interchange format that enables the transfer of 3D assets between various graphics software applications. Many 3D modeling tools support COLLADA, allowing users to build models within their preferred software and export to the COLLADA format.
Ease of Exporting: The widespread support for COLLADA in numerous 3D modeling applications makes exporting relatively straightforward. Users can design models in software like Blender or Autodesk 3ds Max and export directly to COLLADA without the need for additional plugins.
Other Simulation Platforms and File Format Usage
Apart from ROS and Isaac Sim, other popular simulation platforms utilize these file formats:
Gazebo
- Preferred Formats: SDFormat (primary), URDF (with some limitations)
- Why? Supports detailed environment and sensor modeling, making it ideal for physics-based simulations.
MuJoCo
- Preferred Formats: MJCF (native), URDF (converted)
- Why? Optimized for physics-based simulations with real-time dynamics.
NVIDIA Isaac Sim
- Preferred Formats: USD (native), URDF (converted)
- Why? Leverages high-performance rendering and AI-driven simulation.
Webots
- Preferred Formats: VRML, X3D, URDF
- Why? Webots supports multiple formats for ease of use in educational and research applications.
Unity and Unreal Engine
- Preferred Formats: USD, FBX, COLLADA
- Why? Used for real-time visualization and interactive simulations.
Choosing the Right Format for Your Needs
Selecting the appropriate format depends on your project’s goals:
- For ROS-based Robotics Development: Use URDF or SDFormat.
- For High-Fidelity Simulations: Use USD with Isaac Sim.
- For Physics-Based Research: Use MJCF with MuJoCo.
- For 3D Asset Interchange: Use COLLADA for visualization and data sharing.
- For Game Engine Integration: Use USD or FBX for Unity/Unreal.
Interoperability between formats is often required. A robot designed in CAD might be exported to COLLADA for visualization, converted to URDF for ROS-based control, and further adapted for simulation in SDFormat or USD.
MuJoCo
- Preferred Formats: MJCF (native), URDF (converted)
- Why? Optimized for physics-based simulations with real-time dynamics.
NVIDIA Isaac Sim
- Preferred Formats: USD (native), URDF (converted)
- Why? Leverages high-performance rendering and AI-driven simulation.
Webots
- Preferred Formats: VRML, X3D, URDF
- Why? Webots supports multiple formats for ease of use in educational and research applications.
Unity and Unreal Engine
- Preferred Formats: USD, FBX, COLLADA
- Why? Used for real-time visualization and interactive simulations.
Choosing the Right Format for Your Needs
Selecting the appropriate format depends on your project’s goals:
- For ROS-based Robotics Development: Use URDF or SDFormat.
- For High-Fidelity Simulations: Use USD with Isaac Sim.
- For Physics-Based Research: Use MJCF with MuJoCo.
- For 3D Asset Interchange: Use COLLADA for visualization and data sharing.
- For Game Engine Integration: Use USD or FBX for Unity/Unreal.
Interoperability between formats is often required. A robot designed in CAD might be exported to COLLADA for visualization, converted to URDF for ROS-based control, and further adapted for simulation in SDFormat or USD.