Sam Haves
c060385505
PiperOrigin-RevId: 822602481 Change-Id: Icf2b3accd323ee9506ca1838c9a2bfc0f0d159c7
108 lines
4.3 KiB
ReStructuredText
108 lines
4.3 KiB
ReStructuredText
mjcPhysics
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==========
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.. WARNING:: OpenUSD support is currently experimental and subject to frequent change.
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The ``mjcPhysics`` `schema <https://openusd.org/release/api/_usd__page__generating_schemas.html>`__ allows for detailed
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specification of a MuJoCo simulation environment directly within a USD file. The aim is not to replace `UsdPhysics
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<https://openusd.org/release/api/usd_physics_page_front.html>`__, but to extend existing concepts and create new types
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only where is necessary.
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The schema can be use `codeless <https://openusd.org/dev/api/_usd__page__generating_schemas.html#Codeless_Schemas>`__,
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or can be built with its C++ bindings. We've pre-generated `the code
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<https://github.com/google-deepmind/mujoco/tree/main/src/experimental/usd/mjcPhysics>`__ via `usdGenSchema
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<https://openusd.org/dev/api/_usd__page__generating_schemas.html>`_ for internal MuJoCo usage, but it should also work
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outside of MuJoCo.
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API Schemas
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-----------
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MjcSceneAPI
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^^^^^^^^^^^
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This API schema provides global options for the MuJoCo simulation. It is an
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amalgamation of the ``<option>``, ``<option/flag>`` and ``<compiler>`` elements in
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MJCF. Users should apply this to an existing
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`UsdPhysicsScene <https://openusd.org/dev/api/class_usd_physics_scene.html>`__
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prim.
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Key attributes include:
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- **mjc:option**: Attributes in this namespace map to the ``<option>`` element.
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- **mjc:flag**: Attributes in this namespace map to the ``<option/flag>`` element.
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- **mjc:compiler**: Attributes in this namespace map to the ``<compiler>`` element.
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MjcSiteAPI
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^^^^^^^^^^
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This API class is used to define a MuJoCo site, it can be applied to
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`UsdGeomSphere <https://openusd.org/dev/api/class_usd_geom_sphere.html>`__,
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`UsdGeomCapsule <https://openusd.org/dev/api/class_usd_geom_capsule.html>`__,
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`UsdGeomCylinder <https://openusd.org/dev/api/class_usd_geom_cylinder.html>`__, and
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`UsdGeomCube <https://openusd.org/dev/api/class_usd_geom_cube.html>`__.
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MjcImageableAPI
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^^^^^^^^^^^^^^^
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This API class provides attributes for strictly visual entities in MuJoCo, in
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MuJoCo terms we would quantify these has having ``contype = conaffinity = 0``.
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MjcCollisionAPI
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^^^^^^^^^^^^^^^
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This API class is applied to prims that represent collision geometry and should
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be applied alongside
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`UsdPhysicsCollisionAPI <https://openusd.org/dev/api/class_usd_physics_collision_a_p_i.html>`__.
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MjcMeshCollisionAPI
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^^^^^^^^^^^^^^^^^^^
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This API class is applied to prims that represent mesh collision geometry and
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should be applied alongside
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`UsdPhysicsMeshCollisionAPI <https://openusd.org/dev/api/class_usd_physics_mesh_collision_a_p_i.html>`__.
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MjcJointAPI
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^^^^^^^^^^^
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This API class is applied to `UsdPhysicsJoint <https://openusd.org/dev/api/class_usd_physics_joint.html>`__ prims,
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adding extra attributes to fully describe MuJoCo joints.
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MjcMaterialAPI
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^^^^^^^^^^^^^^
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This API class provides attributes for physical materials and is an extension of `UsdPhysicsMaterialAPI
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<https://openusd.org/dev/api/class_usd_physics_material_a_p_i.html>`__
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Type Schemas
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------------
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MjcActuator
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^^^^^^^^^^^
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This class represents a MuJoCo actuator, which is responsible for applying force to a transmission target joint, body,
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or site specific via a `relationship <https://openusd.org/dev/api/class_usd_relationship.html>`__.
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We do not use the existing `UsdPhysicsDriveAPI <https://openusd.org/dev/api/class_usd_physics_drive_a_p_i.html>`__ as it
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is closer to a runtime construct and the concepts do not map very closely.
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MjcKeyframe
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^^^^^^^^^^^
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This type holds tensor values representing simulator state at specific time values.
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In MJCF this is the ``<keyframe>`` element and has a ``time`` attribute. In USD we map the time attribute to
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`timeSamples <https://openusd.org/release/tut_xforms.html>`__ instead.
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The order of the values in the keyframes should map to the depth first ordered traversal of rigidbodies in the composed
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stage.
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MjcTendon
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^^^^^^^^^
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This type represents both fixed and spatial tendons.
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In MJCF this is the ``<tendon>`` element. The tendon path is represented by the ordered list of targets in the
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``mjc:path`` relationship attribute. In MJCF we can specify attributes such as ``sidesite`` and ``divisor`` on
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path targets; but in USD we cannot attach data to relationship attributes as elegantly, so these become indexed
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array attributes such as ``mjc:sideSites`` and ``mjs:path:divisors``.
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