data/doc/programming/index.rst

===========
Programming
===========

.. _inIntro:

Introduction
~~~~~~~~~~~~

This chapter is the MuJoCo programming guide. A separate chapter contains the :doc:`../APIreference/index`
documentation. MuJoCo is a dynamic library compatible with Windows, Linux and macOS, which requires a processor with AVX
instructions. The library exposes the full functionality of the simulator through a compiler-independent shared-memory C
API. It can also be used in C++ programs.

The MuJoCo codebase is organized into subdirectories corresponding to different major areas of functionality:

Engine
   The simulator (or physics engine) is written in C. It is responsible for all runtime computations.
Parser
   The XML parser is written in C++. It can parse MJCF models and URDF models, converting them into an internal mjCModel
   C++ object which is exposed to the user via mjSpec.
Compiler
   The compiler is written in C++. It takes an mjCModel C++ object constructed by the parser, and converts it into an
   mjModel C structure used at runtime.
Abstract visualizer
   The abstract visualizer is written in C. It generates a list of abstract geometric entities representing the
   simulation state, with all information needed for actual rendering. It also provides abstract mouse hooks for camera
   and perturbation control.
OpenGL renderer
   The renderer is written in C and is based on fixed-function OpenGL. It does not have all the features of
   state-of-the-art rendering engines (and can be replaced with such an engine if desired) but nevertheless it provides
   efficient and informative 3D rendering.
Thread
   The Threading framework (new in MuJoCo 3.0) is written in C++ and exposed in C. It provides a ThreadPool interface
   to process Tasks asynchronously. To enable use in MuJoCo, create a ThreadPool and assign it to the thread_pool field
   in mjData.
UI framework
   The UI framework is written in C. UI elements are rendered in OpenGL. It has its own event
   mechanism and abstract hooks for keyboard and mouse input. The code samples use it with GLFW, but it can also be used
   with other window libraries.

.. _inStart:

Getting started
~~~~~~~~~~~~~~~

MuJoCo is an open-source project. Pre-built dynamic libraries are available for x86_64 and arm64 machines running
Windows, Linux, and macOS. These can be downloaded from the `GitHub Releases page
<https://github.com/google-deepmind/mujoco/releases>`_. Users who do not intend to develop or modify core MuJoCo code
are encouraged to use our pre-built libraries, as these come bundled with the same versions of dependencies that we
regularly test against, and benefit from build flags that have been tuned for performance. Our pre-built libraries are
almost entirely self-contained and do not require any other library to be present, outside the standard C runtime. We
also hide all symbols apart from those that form MuJoCo's public API, thus ensuring that it can coexist with any other
libraries that may be loaded into the process (including other versions of libraries that MuJoCo depends on).

The pre-built distribution is a single .zip on Windows, .dmg on macOS, and .tar.gz on Linux. There is no installer.
On Windows and Linux, simply extract the archive in a directory of your choice. From the ``bin`` subdirectory, you can
now run the precompiled code samples, for example:

.. code-block:: Text

     Windows:           simulate ..\model\humanoid\humanoid.xml
     Linux and macOS:   ./simulate ../model/humanoid/humanoid.xml

The directory structure is shown below. Users can re-organize it if needed, as well as install the dynamic libraries in
other directories and set the path accordingly. The only file created automatically is MUJOCO_LOG.TXT in the executable
directory; it contains error and warning messages, and can be deleted at any time.

.. code-block:: Text

     bin     - dynamic libraries, executables, MUJOCO_LOG.TXT
     doc     - README.txt and REFERENCE.txt
     include - header files needed to develop with MuJoCo
     model   - model collection
     sample  - code samples and CMakeLists.txt needed to build them

After verifying that the simulator works, you may also want to re-compile the code samples to ensure that you have a
working development environment. We provide a cross-platform `CMake
<https://github.com/google-deepmind/mujoco/blob/main/sample/CMakeLists.txt>`__ setup that can be used to build sample
applications independently of the MuJoCo library itself.

On macOS, the DMG disk image contains ``MuJoCo.app``, which you can double-click to launch the ``simulate`` GUI. You can
also drag ``MuJoCo.app`` into the ``/Application`` on your system, as you would to install any other app. As well as the
``MuJoCo.app`` `Application Bundle <https://developer.apple.com/go/?id=bundle-
structure>`__, the DMG includes the ``mujoco.framework`` subdirectory containing the MuJoCo dynamic library and all of
its public headers. If you are using Xcode, you can import it as a framework dependency on your project. (This also
works for Swift projects without any modification). If you are building manually, you can use ``-F`` and
``-framework mujoco`` to specify the header search path and the library search path respectively.

.. _inBuild:

Building from source
~~~~~~~~~~~~~~~~~~~~

To build MuJoCo from source, you will need CMake and a working C++17 compiler installed. The steps are:

#. Clone the ``mujoco`` repository: ``git clone https://github.com/deepmind/mujoco.git``
#. Create a new build directory and ``cd`` into it.
#. Run :shell:`cmake $PATH_TO_CLONED_REPO` to configure the build.
#. Run ``cmake --build .`` to build.

MuJoCo's build system automatically fetches dependencies from upstream repositories over the Internet using CMake's
`FetchContent <https://cmake.org/cmake/help/latest/module/FetchContent.html>`_ module.

The main CMake setup will build the MuJoCo library itself along with all sample applications, but the Python
bindings are not built. Those come with their own build instructions, which can be found in the :doc:`../python`
section of the documentation.

Additionally, the CMake setup also implements an installation phase which will copy and organize the output files to a
target directory.

5. Select the directory: :shell:`cmake $PATH_TO_CLONED_REPO -DCMAKE_INSTALL_PREFIX=<my_install_dir>`
#. After building, install with ``cmake --install .``

**Notes:**

- When building on Windows, use Visual Studio 2019 or later and make sure Windows SDK version 10.0.22000 or later is
  installed (see :github:issue:`862` for more details).
- To optimize runtime performance build with ``-DCMAKE_BUILD_TYPE=Release``

.. tip::
   As a reference, a working build configuration can be found in MuJoCo's
   `continuous integration setup <https://github.com/google-deepmind/mujoco/blob/main/.github/workflows/build.yml>`_ on
   GitHub.

.. _inBuildDocs:

Building the docs
~~~~~~~~~~~~~~~~~

If you wish to build the documentation locally, for example to test pull-requests that improve it, do:

1. Clone the ``mujoco`` repository: ``git clone https://github.com/deepmind/mujoco.git``
2. Go to the ``doc/`` directory: ``cd mujoco/doc``
3. Install the dependencies: ``pip install -r requirements.txt``
4. Build the HTML: ``make html``
5. Open ``_build/html/index.html`` in your browser of choice.

.. _inHeader:

Header files
~~~~~~~~~~~~

The distribution contains several header files which are identical on all platforms. They are also available from the
links below, to make this documentation self-contained.

`mujoco.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mujoco.h>`__
   This is the main header file and must be included in all programs using MuJoCo. It defines all API functions and
   global variables, and includes all other header files except mjxmacro.h.
`mjmodel.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjmodel.h>`__
   Defines the C structure :ref:`mjModel` which is the runtime representation of the
   model being simulated. It also defines a number of primitive types and other structures needed to define mjModel.
`mjdata.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjdata.h>`__
   Defines the C structure :ref:`mjData` which is the workspace where all computations
   read their inputs and write their outputs. It also defines primitive types and other structures needed to define
   mjData.
`mjvisualize.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjvisualize.h>`__
   Defines the primitive types and structures needed by the abstract visualizer.
`mjrender.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjrender.h>`__
   Defines the primitive types and structures needed by the OpenGL renderer.
`mjui.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjui.h>`__
   Defines the primitive types and structures needed by the UI framework.
`mjtnum.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjtnum.h>`__
   Defines MuJoCo's ``mjtNum`` floating-point type to be either ``double`` or ``float``. See :ref:`mjtNum`.
`mjspec.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjspec.h>`__
   Defines enums and structs used for :doc:`procedural model editing <modeledit>`.
`mjplugin.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjplugin.h>`__
   Defines data structures required by :ref:`engine plugins<exPlugin>`.
`mjthread.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjthread.h>`__
   Defines data structures and functions required by :ref:`thread<Thread>`.
`mjmacro.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjmacro.h>`__
   Defines C macros that are useful in user code.
`mjxmacro.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjxmacro.h>`__
   This file is optional and is not included by mujoco.h. It defines :ref:`X Macros <tyXMacro>` that can
   automate the mapping of mjModel and mjData into scripting languages, as well as other operations that require
   accessing all fields of mjModel and mjData.
`mjexport.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjexport.h>`__
   Macros used for exporting public symbols from the MuJoCo library. This header should not be used directly by client
   code.
`mjsan.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjsan.h>`__
   Definitions required when building with sanitizer instrumentation.

.. _inVersion:

Versions and compatibility
~~~~~~~~~~~~~~~~~~~~~~~~~~

MuJoCo has been used extensively since 2010 and is quite mature (even though our version numbering scheme is quite
conservative). Nevertheless it remains under active development, and we have many exciting ideas for new features and
are also making changes based on user feedback. This leads to unavoidable changes in both the modeling language in the
API. While we encourage users to upgrade to the latest version, we recognize that this is not always feasible,
especially when other developers release software that relies on MuJoCo. Therefore we have introduced simple
mechanisms to help avoid version conflicts, as follows.

The situation is more subtle if existing code was developed with a certain version of MuJoCo, and is now being
compiled and linked with a different version. If the definitions of the API functions used in that code have changed,
either the compiler or the linker will generate errors. But even if the function definitions have not changed, it may
still be a good idea to assert that the software version is the same. To this end, the main header (mujoco.h) defines
the symbol :ref:`mjVERSION_HEADER <glNumeric>` and the library provides the function
:ref:`mj_version`. Thus the header and library versions can be compared with:

.. code-block:: C

   // recommended version check
   if (mjVERSION_HEADER!=mj_version())
     complain();

Note that only the main header defines this symbol. We assume that the collection of headers released with each software
version will stay together and will not be mixed between versions. To avoid complications with floating-point
comparisons, the above symbol and function use integers that are 100x the version number, so for example in software
version 2.1 the symbol mjVERSION_HEADER is defined as 210.

.. _inNaming:

Naming convention
~~~~~~~~~~~~~~~~~

All symbols defined in the API start with the prefix "mj". The character after "mj" in the prefix determines the family
to which the symbol belongs. First we list the prefixes corresponding to type definitions.

``mj``
   Core simulation data structure (C struct), for example :ref:`mjModel`. If all characters
   after the prefix are capital, for example :ref:`mjMIN`, this is a macro or a symbol (#define).
``mjt``
   Primitive type, for example :ref:`mjtGeom`. Except for mjtByte and mjtNum, all other
   definitions in this family are enums.
``mjf``
   Callback function type, for example :ref:`mjfGeneric`.
``mjv``
   Data structure related to abstract visualization, for example :ref:`mjvCamera`.
``mjr``
   Data structure related to OpenGL rendering, for example :ref:`mjrContext`.
``mjui``
   Data structure related to UI framework, for example :ref:`mjuiSection`.
``mjs``
   Data structure related :doc:`procedural model editing <modeledit>`, for example :ref:`mjsJoint`.

Next we list the prefixes corresponding to function definitions. Note that function prefixes always end with underscore.

``mj_``
   Core simulation function, for example :ref:`mj_step`. Almost all such functions have
   pointers to mjModel and mjData as their first two arguments, possibly followed by other arguments. They usually write
   their outputs to mjData.
``mju_``
   Utility function, for example :ref:`mju_mulMatVec`. These functions are self-contained
   in the sense that they do not have mjModel and mjData pointers as their arguments.
``mjv_``
   Function related to abstract visualization, for example :ref:`mjv_updateScene`.
``mjr_``
   Function related to OpenGL rendering, for example :ref:`mjr_render`.
``mjui_``
   Function related to UI framework, for example :ref:`mjui_update`.
``mjcb_``
   Global callback function pointer, for example :ref:`mjcb_control`. The user can install
   custom callbacks by setting these global pointers to user-defined functions.
``mjd_``
   Functions for computing derivatives, for example :ref:`mjd_transitionFD`.
``mjs_``
   Functions for :doc:`procedural model editing <modeledit>`, for example :ref:`mjs_addJoint`.

.. _inOpenGL:

Using OpenGL
~~~~~~~~~~~~

The use of MuJoCo's native OpenGL renderer will be explained in :ref:`Rendering`. For rendering, MuJoCo uses OpenGL 1.5
in the compatibility profile with the ``ARB_framebuffer_object`` and ``ARB_vertex_buffer_object`` extensions. OpenGL
symbols are loaded via `GLAD <https://github.com/Dav1dde/glad>`_ the first time the :ref:`mjr_makeContext` function
is called. This means that the MuJoCo library itself does not have an explicit dependency on OpenGL and can be used
on systems without OpenGL support, as long as ``mjr_`` functions are not called.

Applications that use MuJoCo's built-in rendering functionalities are responsible for linking against an appropriate
OpenGL context creation library and for ensuring that there is an OpenGL context that is made current on the running
thread. On Windows and macOS, there is a canonical OpenGL library provided by the operating system. On Linux, MuJoCo
currently supports GLX for rendering to an X11 window, OSMesa for headless software rendering, and EGL for hardware
accelerated headless rendering.

Before version 2.1.4, MuJoCo used GLEW rather than GLAD to manage OpenGL symbols, which required linking against
different GLEW libraries at build time depending on the GL implementation used. In order to avoid having manage OpenGL
dependency when no rendering was required, "nogl" builds of the library was made available. Since OpenGL symbols are
now lazily resolved at runtime after the switch to GLAD, the "nogl" libraries are no longer provided.

.. toctree::
    :hidden:

    simulation
    visualization
    ui
    modeledit
    samples
    extension