.. _MJW:

====================
MuJoCo Warp (MJWarp)
====================

.. toctree::
    :hidden:

    API <api.rst>

MuJoCo Warp (MJWarp) is an implementation of MuJoCo written in `Warp <https://nvidia.github.io/warp/>`__ and optimized
for `NVIDIA <https://nvidia.com>`__ hardware and parallel simulation. MJWarp lives in the
`google-deepmind/mujoco_warp <https://github.com/google-deepmind/mujoco_warp>`__ GitHub repository and is currently in
beta.

MJWarp is developed and maintained as a joint effort by `NVIDIA <https://nvidia.com>`__ and
`Google DeepMind <https://deepmind.google/>`__.

.. TODO: remove after release

.. admonition:: Beta software
   :class: warning

   - MJWarp is beta software and is under active development.
   - MJWarp developers will triage and respond to
     `bug reports and feature requests <https://github.com/google-deepmind/mujoco_warp/issues>`__.
   - MJWarp is mostly feature complete but requires performance optimization, documentation, and testing.
   - The intended audience during Beta are physics engine enthusiasts and learning framework integrators.

.. _MJW_tutorial:

Tutorial notebook
=================

The MJWarp basics are covered in a
`tutorial
notebook <https://colab.research.google.com/github/google-deepmind/mujoco_warp/blob/main/notebooks/tutorial.ipynb>`__.

.. _MJW_install:

Installation
============

The beta version of MuJoCo Warp is installed from GitHub. Please note that the beta version of MuJoCo Warp does not
support all versions of MuJoCo, Warp, CUDA, NVIDIA drivers, etc.

.. code-block:: shell

    git clone https://github.com/google-deepmind/mujoco_warp.git
    cd mujoco_warp
    python3 -m venv env
    source env/bin/activate
    pip install --upgrade pip
    pip install uv
    uv pip install -e .[dev,cuda]

Test the Installation

.. code-block:: shell

    pytest

.. _MJW_Usage:

Basic Usage
===========

Once installed, the package can be imported via ``import mujoco_warp as mjw``. Structs, functions, and enums are
available directly from the top-level :mod:`mjw <mujoco_warp>` module.

Structs
-------
Before running MJWarp functions on an NVIDIA GPU, structs must be copied onto the device via
:func:`mjw.put_model <mujoco_warp.put_model>` and :func:`mjw.make_data <mujoco_warp.make_data>` or
:func:`mjw.put_data <mujoco_warp.put_data>` functions. Placing an :ref:`mjModel` on device yields an
:class:`mjw.Model <mujoco_warp.Model>`. Placing an :ref:`mjData` on device yields an
:class:`mjw.Data <mujoco_warp.Data>`.

.. code-block:: python

  mjm = mujoco.MjModel.from_xml_string("...")
  mjd = mujoco.MjData(mjm)
  m = mjw.put_model(mjm)
  d = mjw.put_data(mjm, mjd)

These MJWarp variants mirror their MuJoCo counterparts but have a few key differences:

#. :class:`mjw.Model <mujoco_warp.Model>` and :class:`mjw.Data <mujoco_warp.Data>` contain Warp arrays that are copied
   onto device.
#. Some fields are missing from :class:`mjw.Model <mujoco_warp.Model>` and :class:`mjw.Data <mujoco_warp.Data>` for
   features that are unsupported.

Batch sizes
-----------

MJWarp is optimized for parallel simulation. A batch of simulations can be specified with three parameters:

- :attr:`nworld <mujoco_warp.Data.nworld>`: Number of worlds to simulate.
- _`nconmax`: Expected number of contacts per world. The maximum number of contacts for all worlds is
  ``nconmax * nworld``.
- _`naconmax`: Alternative to `nconmax`_, maximum number of contacts over all worlds. If `nconmax`_ and `naconmax`_ are
  both set then `nconmax`_ is ignored.
- _`njmax`: Maximum number of constraints per world.

.. admonition:: Semantic difference for `nconmax`_ and `njmax`_.
  :class: note

  It is possible for the number of contacts per world to exceed `nconmax`_ if the total number of contacts for all
  worlds does not exceed ``nworld x nconmax``. However, the number of constraints per world is strictly limited by
  `njmax`_.

.. admonition:: XML parsing
  :class: note

  Values for `nconmax`_ and `njmax`_ are not parsed from :ref:`size/nconmax <size-nconmax>` and
  :ref:`size/njmax <size-njmax>` (these parameters are deprecated). Values for these parameters must be provided to
  :func:`mjw.make_data <mujoco_warp.make_data>` or :func:`mjw.put_data <mujoco_warp.put_data>`.

Functions
---------

MuJoCo functions are exposed as MJWarp functions of the same name, but following
`PEP 8 <https://peps.python.org/pep-0008/>`__-compliant names. Most of the :ref:`main simulation <Mainsimulation>` and
some of the :ref:`sub-components <Subcomponents>` for forward simulation are available from the top-level
:mod:`mjw <mujoco_warp>` module.

Minimal example
---------------

.. code-block:: python

   # Throw a ball at 100 different velocities.

   import mujoco
   import mujoco_warp as mjw
   import warp as wp

   _MJCF=r"""
   <mujoco>
     <worldbody>
       <body>
         <freejoint/>
         <geom size=".15" mass="1" type="sphere"/>
       </body>
     </worldbody>
   </mujoco>
   """

   mjm = mujoco.MjModel.from_xml_string(_MJCF)
   m = mjw.put_model(mjm)
   d = mjw.make_data(mjm, nworld=100)

   # initialize velocities
   wp.copy(d.qvel, wp.array([[float(i) / 100, 0, 0, 0, 0, 0] for i in range(100)], dtype=float))

   # simulate physics
   mjw.step(m, d)

   print(f'qpos:\n{d.qpos.numpy()}')

.. _mjwCLI:

Command line scripts
--------------------

Benchmark an environment with testspeed

.. code-block:: shell

    mjwarp-testspeed benchmark/humanoid/humanoid.xml

Interactive environment simulation with MJWarp

.. code-block:: shell

    mjwarp-viewer benchmark/humanoid/humanoid.xml

Feature Parity
==============

MJWarp supports most of the main simulation features of MuJoCo, with a few exceptions. MJWarp will raise an exception if
asked to copy to device an :ref:`mjModel` with field values referencing unsupported features.

The following features are **not supported** in MJWarp:

.. list-table::
   :width: 90%
   :align: left
   :widths: 2 5
   :header-rows: 1

   * - Category
     - Feature
   * - :ref:`Integrator <mjtIntegrator>`
     - ``IMPLICIT``, ``IMPLICITFAST`` not supported with fluid drag
   * - :ref:`Solver <mjtSolver>`
     - ``PGS``, ``noslip``, :ref:`islands <soIsland>`
   * - Fluid Model
     - :ref:`flEllipsoid`
   * - :ref:`Sensors <mjtSensor>`
     - ``GEOMDIST``, ``GEOMNORMAL``, ``GEOMFROMTO``
   * - Flex
     - ``VERTCOLLIDE=false``, ``INTERNAL=true``
   * - Jacobian format
     - ``SPARSE``
   * - Option
     - :ref:`contact override <COverride>`
   * - Plugins
     - ``All`` except ``SDF``
   * - :ref:`User parameters <CUser>`
     - ``All``

.. _mjwPerf:

Performance Tuning
==================

The following are considerations for optimizing the performance of MJWarp.

.. _mjwGC:

Graph capture
-------------

MJWarp functions, for example :func:`mjw.step <mujoco_warp.step>`, often comprise a collection of kernel launches. Warp
will launch these kernels individually if the function is called directly. To improve performance, especially if the
function will be called multiple times, it is recommended to capture the operations that comprise the function as a CUDA
graph

.. code-block:: python

  with wp.ScopedCapture() as capture:
    mjw.step(m, d)

The graph can then be launched or re-launched

.. code-block:: python

  wp.capture_launch(capture.graph)

and will typically be significantly faster compared to calling the function directly. Please see the
`Warp Graph API reference <https://nvidia.github.io/warp/modules/runtime.html#graph-api-reference>`__ for details.

Batch sizes
-----------

The maximum numbers of contacts and constraints, `nconmax`_ / `naconmax`_ and `njmax`_ respectively, are specified when
creating :class:`mjw.Data <mujoco_warp.Data>` with :func:`mjw.make_data <mujoco_warp.make_data>` or
:func:`mjw.put_data <mujoco_warp.put_data>`. Memory and computation scales with the values of these parameters. For best
performance, the values of these parameters should be set as small as possible while ensuring the simulation does not
exceed these limits.

It is expected that good values for these limits will be environment specific. In practice, selecting good values
typically involves trial-and-error. :func:`mjwarp-testspeed <mujoco_warp.testspeed>` with the flag `--measure_alloc` for
printing the number of contacts and constraints at each simulation step and interacting with the simulation via
:func:`mjwarp-viewer <mujoco_warp.viewer>` and checking for overflow errors can both be useful techniques for
iteratively testing values for these parameters.

Solver iterations
-----------------

MuJoCo's default solver settings for the maximum numbers of :ref:`solver iterations<option-iterations>` and
:ref:`linesearch iterations<option-ls_iterations>` are expected to provide reasonable performance. Reducing MJWarp's
settings :attr:`Option.iterations <mujoco_warp.Option.iterations>` and/or
:attr:`Option.ls_iterations <mujoco_warp.Option.ls_iterations>` limits may improve performance and should be secondary
considerations after tuning `nconmax`_ / `naconmax`_ and `njmax`_.

Reducing these limits too much may prevent the constraint solver from converging and can lead to inaccurate or unstable
simulation.

.. admonition:: Impact on Performance: MJX (JAX) and MJWarp
  :class: note

  In :ref:`MJX<mjx>` these solver parameters are key for controlling simulation performance. With MJWarp, in contrast,
  once all worlds have converged the solver can early exit and avoid unnecessary computation. As a result, the values
  of these settings have comparatively less impact on performance.

Contact sensor matching
-----------------------

Scenes that include :ref:`contact sensors<sensor-contact>` have a parameter that specifies the maximum number of matched
contacts per sensor :attr:`Option.contact_sensor_max_match <mujoco_warp.Option.contact_sensor_max_match>`. For best
performance, the value of this parameter should be as small as possible while ensuring the simulation does not exceed
the limit. Matched contacts that exceed this limit will be ignored.

The value of this parameter can be set directly, for example ``model.opt.contact_sensor_maxmatch = 16``, or via an XML
custom numeric field

.. code-block:: xml

   <custom>
     <numeric name="contact_sensor_maxmatch" data="16"/>
   </custom>

Similar to the maximum numbers of contacts and constraints, a good value for this setting is expected to be environment
specific. :func:`mjwarp-testspeed <mujoco_warp.testspeed>` and :func:`mjwarp-viewer <mujoco_warp.viewer>` may be useful
for tuning the value of this parameter.

Parallel linesearch
-------------------

In addition to the constraint solver's iterative linesearch, MJWarp provides a parallel linesearch routine that
evaluates a set of step sizes in parallel and selects the best one. The step sizes are spaced logarithmically from
:attr:`Model.opt.ls_parallel_min_step <mujoco_warp.Option.ls_parallel_min_step>` to 1 and the number of step sizes to
evaluate is set via :attr:`Model.opt.ls_iterations <mujoco_warp.Option.ls_iterations>`.

In some cases the parallel routine may provide improved performance compared to the constraint solver's default
iterative linesearch.

To enable this routine set ``Model.opt.ls_parallel=True`` or add a custom numeric field to the XML

.. code-block:: xml

   <custom>
     <numeric name="ls_parallel" data="1"/>
   </custom>

.. admonition:: Experimental feature
  :class: note

  The parallel linesearch is currently an experimental feature.

.. _mjwBatch:

Batched :class:`Model <mujoco_warp.Model>` Fields
=================================================

To enable batched simulation with different model parameter values, many :class:`mjw.Model <mujoco_warp.Model>` fields
have a leading batch dimension. By default, the leading dimension is 1 (i.e., ``field.shape[0] == 1``) and the same
value(s) will be applied to all worlds. It is possible to override one of these fields with a ``wp.array`` that has a
leading dimension greater than one. This field will be indexed with a modulo operation of the world id and batch
dimension: ``field[worldid % field.shape[0]]``.

.. admonition:: Graph capture
  :class: warning

  The field array should be overridden prior to
  :ref:`graph capture <mjwGC>` (i.e., ``wp.ScopedCapture``)
  since the update will not be applied to an existing graph.

.. code-block:: python

   # override shape and values
   m.dof_damping = wp.array([[0.1], [0.2]], dtype=float)

   with wp.ScopedCapture() as capture:
     mjw.step(m, d)

It is possible to override the field shape and set the field values after graph capture

.. code-block:: python

   # override shape
   m.dof_damping = wp.empty((2, 1), dtype=float)

   with wp.ScopedCapture() as capture:
     mjw.step(m, d)

   # set batched values
   dof_damping = wp.array([[0.1], [0.2]], dtype=float)
   wp.copy(m.dof_damping, dof_damping)  # m.dof = dof_damping will not update the captured graph

Modifying fields
----------------

The recommended workflow for modifying an :ref:`mjModel` field is to first modify the corresponding :ref:`mjSpec` and
then compile to create a new :ref:`mjModel` with the updated field. However, compilation currently requires a host call:
1 call per new field instance, i.e., ``nworld`` host calls for ``nworld`` instances.

Certain fields are safe to modify directly without compilation, enabling on-device updates. Please see
:ref:`mjModel changes<sichange>` for details about specific fields. Additionally,
`GitHub issue 893 <https://github.com/google-deepmind/mujoco_warp/issues/893>`__ tracks adding on-device updates for a
subset of fields.

.. admonition:: Heterogeneous worlds
   :class: note

   Heterogeneous worlds, for example: per-world meshes or number of degrees of freedom, are not currently available.

.. _mjwFAQ:

Frequently Asked Questions
==========================

Learning frameworks
-------------------

**Does MJWarp work with JAX?**

Yes. MJWarp is interoperable with `JAX <https://jax.readthedocs.io/>`__. Please see the
`Warp Interoperability <https://nvidia.github.io/warp/modules/interoperability.html#jax>`__ documentation for details.

Additionally, :ref:`MJX <mjx>` provides a JAX API for a subset of MJWarp's :doc:`API <api>`. The backend is
specified with ``impl='warp'``.

**Does MJWarp work with PyTorch?**

Yes. MJWarp is interoperable with `PyTorch <https://pytorch.org>`__. Please see the
`Warp Interoperability <https://nvidia.github.io/warp/modules/interoperability.html#pytorch>`__ documentation for
details.

**How to train policies with MJWarp physics?**

For examples that train policies with MJWarp physics, please see:

- `Isaac Lab <https://github.com/isaac-sim/IsaacLab/tree/feature/newton>`__: Train via
  `Newton API <https://github.com/newton-physics/newton>`__.
- `mjlab <https://github.com/mujocolab/mjlab>`__: Train directly with MJWarp using PyTorch.
- `MuJoCo Playground <https://github.com/google-deepmind/mujoco_playground>`__: Train via :ref:`MJX API <mjx>`.

Features
--------

**Is MJWarp differentiable?**

No. MJWarp is not currently differentiable via
Warp's `automatic differentiation <https://nvidia.github.io/warp/modules/differentiability.html#differentiability>`__
functionality. Updates from the team related to enabling automatic differentiation for MJWarp are tracked in this
`GitHub issue <https://github.com/google-deepmind/mujoco_warp/issues/500>`__.

**Does MJWarp work with multiple GPUs?**

Yes. Warp's ``wp.ScopedDevice`` enables multi-GPU computation

.. code-block:: python

   # create a graph for each device
   graph = {}
   for device in wp.get_cuda_devices():
     with wp.ScopedDevice(device):
       m = mjw.put_model(mjm)
       d = mjw.make_data(mjm)
       with wp.ScopedCapture(device) as capture:
         mjw.step(m, d)
       graph[device] = capture.graph

   # launch a graph on each device
   for device in wp.get_cuda_devices():
     wp.capture_launch(graph[device])

Please see the
`Warp documentation <https://nvidia.github.io/modules/devices.html#example-using-wp-scopeddevice-with-multiple-gpus>`__
for details and
`mjlab distributed training <https://github.com/mujocolab/mjlab/tree/main/docs/api/distributed_training.md>`__ for a
reinforcement learning example.

**Is MJWarp on GPU deterministic?**

No. There may be ordering or *small* numerical differences between results computed by different executions of the same
code. This is characteristic of non-deterministic atomic operations on GPU. Set device to CPU with
``wp.set_device("cpu")`` for deterministic results.

Developments for deterministic results on GPU are tracked in this
`GitHub issue <https://github.com/google-deepmind/mujoco_warp/issues/562>`__.

**How are orientations represented?**

Orientations are represented as unit quaternions and follow :ref:`MuJoCo's conventions<siLayout>`:
``w, x, y, z`` or ``scalar, vector``.

.. admonition:: ``wp.quaternion``
  :class: note

  MJWarp utilizes Warp's `built-in type <https://nvidia.github.io/warp/modules/functions.html#warp.quaternion>`__
  ``wp.quaternion``. Importantly however, MJWarp does not utilize Warp's ``x, y, z, w`` quaternion convention or
  operations and instead implements quaternion routines that follow MuJoCo's conventions. Please see
  `math.py <https://github.com/google-deepmind/mujoco_warp/blob/main/mujoco_warp/_src/math.py>`__ for the
  implementations.

**Does MJWarp have a named access API / bind?**

No. Updates for this feature are tracked in this
`GitHub issue <https://github.com/google-deepmind/mujoco_warp/issues/884>`__.

**Why are contacts reported when there are no collisions?**

1 contact will be reported for each unique geom pair that contributes to any collision sensor, even if this geom pair is
not in collision. Unlike MuJoCo or MJX where :ref:`collision sensors<collision-sensors>` make separate calls to
collision routines while computing sensor data, MJWarp computes and stores the data for these sensors in contacts while
running its main collision pipeline.

:ref:`Contact sensors<sensor-contact>` will report the correct information for contacts affecting the physics.

**Why are Jacobians always dense?**

Sparse Jacobians are not currently implemented and ``Data`` fields: ``ten_J``, ``actuator_moment``, ``flexedge_J``, and
``efc.J`` are always represented as dense matrices. Support for sparse Jacobians is tracked in GitHub issue
`#88 <https://github.com/google-deepmind/mujoco_warp/issues/88>`__.

**Why do some arrays have different shapes compared to mjModel or mjData?**

By default for batched simulation, many :class:`mjw.Data <mujoco_warp.Data>` fields having a leading batch dimension of
size ``Data.nworld``. Some :class:`mjw.Model <mujoco_warp.Model>` fields having a leading batch dimension with size
``1``, indicating that this
:ref:`field can be overridden with an array of batched parameters for domain randomization <mjwBatch>`.

Additionally, certain fields including ``Model.qM``, ``Data.efc.J``, and ``Data.efc.D`` are padded to enable fast
loading on GPU.

Compilation
-----------

**How can compilation time be improved?**

Limit the number of unique colliders that require the general convex collision pipeline. These colliders are listed as
``_CONVEX_COLLISION_PAIRS`` in
`collision_convex.py <https://github.com/google-deepmind/mujoco_warp/blob/main/mujoco_warp/_src/collision_convex.py>`__.
Improvements to the compilation time for the pipeline are tracked in this
`GitHub issue <https://github.com/google-deepmind/mujoco_warp/issues/813>`__.

**Why are the physics not working as expected after upgrading MJWarp?**

The Warp cache may be incompatible with the current code and should be cleared as part of the debugging process. This
can be accomplished by deleting the directory ``~/.cache/warp`` or via Python

.. code-block:: python

   import warp as wp
   wp.clear_kernel_cache()

**Is it possible to compile MJWarp ahead of time instead of at runtime?**

Yes. Please see Warp's
`Ahead-of-Time Compilation Workflows <https://nvidia.github.io/warp/codegen.html#ahead-of-time-compilation-workflows>`__
documentation for details.