Author Lyceum
24 Stars
Updated Last
1 Year Ago
Started In
December 2019


A Julia wrapper for the MuJoCo physics simulator, a physics simulator designed for multi-joint dynamics with contacts that has become a standard in robotics, reinforcement learning, and trajectory optimization, both in both academia and industry.

MuJoCo.jl is part of the Lyceum, a software suite for machine learning, control, and trajectory optimization. For an example of how to use MuJoCo.jl, check out LyceumMuJoCo.jl.

Obtaining a License

To use MuJoCo, you'll need a valid license which you can obtain from here. Up to three, thirty-day trials can be obtained for free from MuJoCo's webiste and students are eligible for a free personal license. Once you have obtained the license file, set the environment variable MUJOCO_KEY_PATH to point to its location. On Linux machines this would look like:

$ export MUJOCO_KEY_PATH=/path/to/mjkey.txt


MuJoCo.jl provides the following:

  1. MuJoCo.MJCore: A minimal wrapper for the MuJoCo physics simulator, providing a nearly one-to-one mapping to MuJoCo's C interface. MuJoCo features that Julia already provides are not wrapped (e.g. mju_add3 is just a normal array operation). Additionally, MuJoCo's C interface may be altered in cases where it improves ease of use or safety (e.g. MuJoCo.jl never requires pre-allocated error string buffers to be passed in).
  2. MuJoCo: Zero-cost abstractions over the types contained in MuJoCo.MJCore that provide a more convenient interface for interacting with MuJoCo (e.g. by wrapping a raw Ptr{Float64} with a Array{Float64}). These types are compatible with all the functions defined in MuJoCo.MJCore. We also provide extra utility functions that you may find useful. These functions and types are prefixed with jl_ and jl, respectively.

Functions from each module are exported by MuJoCo.jl, while only the types in MuJoCo are exported (e.g. jlModel). We recommend using these types over the raw MJCore types wherever possible.

All functions are documented for convenience:

help?> mj_step

For more complete about the MuJoCo physics simulator, see MuJoCo's documentation.


jlData and jlModel

MuJoCo provides jlData and jlModel which differ from MJCore.mjData and MJCore.mjModel as follows:

  1. Fields of type SArray{S,T,N} become Array{T, N} (see #7)
  2. Field of type Ptr{T<:Number} become Array{T, N} (e.g. size(jlData.qpos) == (nq, )).
  3. For fields that change size dynamically (e.g., we provide dynamically-sized view's into the underlying array (e.g. size( == (ncon, ) not (nconmax, )).
  4. Fields that are not useful in Julia are not exposed (e.g. mjData.buffer)
  5. mj_deleteModel/mj_deleteData are called by Julia's garbage collector automatically.

jlData and jlModel (as well as mjData and mjModel) are mutable structs, while the fields of these types are not. To assist in mutating nested structs, we recommend using the @set!! macro provided by BangBang.jl:

using MuJoCo, BangBang
m = jlModel("humanoid.xml")
@assert m.opt.timestep == 0.002
@set!! m.opt.timestep = 0.001
@assert m.opt.timestep == 0.001


All MuJoCo globals are available under in the MJCore module (e.g. MJCore.mjVISSTRING). const global primitives like mjMAXIMP and mjVISSTRING are defined directly in Julia, while global callbacks are stored in a Ref-like object called a CRef and can be used in the same manner as RefValue. Under the hood, calls to getindex/setindex! perform the appropriate load/store from the corresponding Ptr{Cvoid}.

@assert MJCore.mjMAXIMP == 0.9999
@assert MJCore.mjDISABLESTRING[1] == "Constraint"
# Set the `mjcb_user_warning_cb` callback to generate Julia warnings (note that MuJoCo.jl
# already installs proper warning/error handlers, which you most likely don't want to override)
my_warning_cb() = (@warn unsafe_string(msg); nothing)
warncb = @cfunction(my_warning_cb, Cvoid, (Cstring,))
MJCore.mju_user_warning[] = warncb

See this blog post for more information on how to set C callbacks from Julia.

Getting Started

MuJoCo.jl is currently registered in Lyceum's package registry. Until it is moved to General, you will need to add Lyceum/LyceumRegistry.

From the Julia REPL, type ] to enter Pkg mode:

julia> ]
(v1.3) pkg> registry add
(v1.3) pkg> add MuJoCo

Below we simulate passive dynamics and print out joint positions at each timestep:

using MuJoCo
# alternatively: set the environment variable `MUJOCO_KEY_PATH`
#                before `using MuJoCo`.

m = jlModel("humanoid.xml")
d = jlData(m)
for i=1:100
    mj_step(m, d);

and compared to C:

#include "mujoco.h"
#include <stdio.h>
char error[1000] = "Could not load xml model";

m = mj_loadXML("humanoid.xml", 0, error, 1000);
d = mj_makeData(m);
for( int step=0; step<100; step++ )
    mj_step(m, d);
    for(i=0; i < m->nq; i++)
        printf("%d ", d->qpos[i]);

Big thanks to @klowrey for providing the original MuJoCo wrapper.