mjcPhysics

.. WARNING:: OpenUSD support is currently experimental and subject to frequent change.

The mjcPhysics schema <https://openusd.org/release/api/_usd__page__generating_schemas.html>__ allows for detailed specification of a MuJoCo simulation environment directly within a USD file. The aim is not to replace UsdPhysics <https://openusd.org/release/api/usd_physics_page_front.html>__, but to extend existing concepts and create new types only where is necessary.

The schema can be use codeless <https://openusd.org/dev/api/_usd__page__generating_schemas.html#Codeless_Schemas>, or can be built with its C++ bindings. We've pre-generated the code <https://github.com/google-deepmind/mujoco/tree/main/src/experimental/usd/mjcPhysics> via usdGenSchema <https://openusd.org/dev/api/_usd__page__generating_schemas.html>_ for internal MuJoCo usage, but it should also work outside of MuJoCo.

API Schemas

MjcSceneAPI ^^^^^^^^^^^

This API schema provides global options for the MuJoCo simulation. It is an amalgamation of the <option>, <option/flag> and <compiler> elements in MJCF. Users should apply this to an existing UsdPhysicsScene <https://openusd.org/dev/api/class_usd_physics_scene.html>__ prim.

Key attributes include:

mjc:option: Attributes in this namespace map to the <option> element.
mjc:flag: Attributes in this namespace map to the <option/flag> element.
mjc:compiler: Attributes in this namespace map to the <compiler> element.

MjcSiteAPI ^^^^^^^^^^

This API class is used to define a MuJoCo site, it can be applied to UsdGeomSphere <https://openusd.org/dev/api/class_usd_geom_sphere.html>, UsdGeomCapsule <https://openusd.org/dev/api/class_usd_geom_capsule.html>, UsdGeomCylinder <https://openusd.org/dev/api/class_usd_geom_cylinder.html>, and UsdGeomCube <https://openusd.org/dev/api/class_usd_geom_cube.html>.

MjcImageableAPI ^^^^^^^^^^^^^^^

This API class provides attributes for strictly visual entities in MuJoCo, in MuJoCo terms we would quantify these has having contype = conaffinity = 0.

MjcCollisionAPI ^^^^^^^^^^^^^^^

This API class is applied to prims that represent collision geometry and should be applied alongside UsdPhysicsCollisionAPI <https://openusd.org/dev/api/class_usd_physics_collision_a_p_i.html>__.

MjcMeshCollisionAPI ^^^^^^^^^^^^^^^^^^^

This API class is applied to prims that represent mesh collision geometry and should be applied alongside UsdPhysicsMeshCollisionAPI <https://openusd.org/dev/api/class_usd_physics_mesh_collision_a_p_i.html>__.

MjcJointAPI ^^^^^^^^^^^

This API class is applied to UsdPhysicsJoint <https://openusd.org/dev/api/class_usd_physics_joint.html>__ prims, adding extra attributes to fully describe MuJoCo joints.

MjcMaterialAPI ^^^^^^^^^^^^^^

This API class provides attributes for physical materials and is an extension of UsdPhysicsMaterialAPI <https://openusd.org/dev/api/class_usd_physics_material_a_p_i.html>__

Type Schemas

MjcActuator ^^^^^^^^^^^

This class represents a MuJoCo actuator, which is responsible for applying force to a transmission target joint, body, or site specific via a relationship <https://openusd.org/dev/api/class_usd_relationship.html>__.

We do not use the existing UsdPhysicsDriveAPI <https://openusd.org/dev/api/class_usd_physics_drive_a_p_i.html>__ as it is closer to a runtime construct and the concepts do not map very closely.

MjcKeyframe ^^^^^^^^^^^

This type holds tensor values representing simulator state at specific time values.

In MJCF this is the <keyframe> element and has a time attribute. In USD we map the time attribute to timeSamples <https://openusd.org/release/tut_xforms.html>__ instead.

The order of the values in the keyframes should map to the depth first ordered traversal of rigidbodies in the composed stage.

MjcTendon ^^^^^^^^^

This type represents both fixed and spatial tendons.

In MJCF this is the <tendon> element. The tendon path is represented by the ordered list of targets in the mjc:path relationship attribute. In MJCF we can specify attributes such as sidesite and divisor on path targets; but in USD we cannot attach data to relationship attributes as elegantly, so these become indexed array attributes such as mjc:sideSites and mjs:path:divisors.