sdformat-mjcf 0.1.0

Tool to convert between SDFormat and MJCF

sdformat-mjcf

This Python package allows bidirectional conversion between SDFormat and MJCF to share worlds and robot models. We have created a command line tool to convert between these two formats. It takes as input an SDFormat file that works in Gazebo Sim and produces as output a MJCF file that works in Mujoco with approximately equivalent results; and vice versa.

Install sdformat-mjcf

To start development, create a python3 virtual environment, upgrade pip and install dm-control

python3 -m venv path/to/venv --system-site-packages
. path/to/venv/bin/activate

pip install -U pip
pip install dm-control

Install python3-ignition-math7 and python3-sdformat13 from the nightly repo.

Install the sdformat-mjcf package

pip install sdformat-mjcf

Running tests

Simple run of test can be done by using:

python -m unittest

Tools for converting SDFormat to MJCF

Use the commnad line tool sdf2mjcf:

usage: sdf2mjcf [-h] input_file output_file

positional arguments:
  input_file   Path to input SDFormat file (World or Model)
  output_file  Desired path for the output MJCF file

optional arguments:
  -h, --help   show this help message and exit

To run the MJCF file in Mujoco, download a Mujoco release from https://github.com/deepmind/mujoco/releases, extract the contents and run

<path/to/mujoco>/bin/simulate output_file.xml

Suported features

• Models/Worlds
• Links
• Sensors
  • Altimeter
  • Camera
  • Force torque
  • IMU
• Joints
  • Ball
  • Continuous
  • Fixed
  • Prismatic
  • Revolute
• Materials

Unsuported features

• Nested models
• Links with multiple parents and kinematic loops
• Revolute2 and Universal joints
• <scene> element
• <physics> element
• Models from Fuel
• Models contain URIs with schemes such as model:// or package://.

Other limitations:

• Collada (.dae) meshes are not supported by Mujoco. Therefore, the user has to first convert each .dae file to .obj or .stl file using available tools such as blender or meshlab. The SDFormat file has to then be updated to point to the converted mesh files instead of the .dae files.
• Mujoco does not support composite .obj files. However, users may process the output .obj files with obj2mjcf to split them into individual .obj files.
• Only the diffuse texture from a PBR material is converted to MJCF. Other textures are not supported.

Tools for converting MJCF to SDFormat

Use the commnad line tool mjcf2sdf:

usage: mjcf2sdf [-h] [--export_world_plugins] input_file output_file

positional arguments:
  input_file            Path to input MJCF file
  output_file           Desired path for the output SDFormat file

optional arguments:
  -h, --help            show this help message and exit
  --export_world_plugins
                        Export world plugins

If you are going to use the converted file in Gazebo Sim you should use the flag --export_world_plugins to export some of the plugins that are required to make the new world work properly in Gazebo Sim.

To run the SDFormat file in GazeboSim, follow these instructions to install Gazebo Sim

Suported features

• Bodies
• Geoms
• Sensors
  • Camera
  • Force torque
  • IMU
• Joints
  • Fixed
  • Free
  • Hinge
  • Slide
• Materials

Unsuported features

• Tendon
• Generation of procedural textures
• Actuators
• Equality Constraints
• Collision filters
• hfields
• skins
• option.timestep
• Fitting of meshes with primitives, convex hulls or AABBs
• Settings that affect the constraint solver
• Lights that track or target objects

Other limitations:

• Only the values xyz and XYZ are supported for compiler.eulerseq
• local coordinates are assumed for compiler.coordinate
• Each kinematic tree in <worldbody> is placed inside a <model> when converted to SDFormat. The <self_collide> element is always set to false for <model>s to avoid collisions between links connected by mulitple joints in series.