mjcPhysics

Warning

OpenUSD support is currently experimental and subject to frequent change.

The mjcPhysics schema allows for detailed specification of a MuJoCo simulation environment directly within a USD file. The aim is not to replace UsdPhysics, but to extend existing concepts and create new types only where is necessary.

The schema can be use codeless, or can be built with its C++ bindings. We’ve pre-generated the code via usdGenSchema 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 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, UsdGeomCapsule, UsdGeomCylinder, and UsdGeomCube.

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.

MjcMeshCollisionAPI

This API class is applied to prims that represent mesh collision geometry and should be applied alongside UsdPhysicsMeshCollisionAPI.

MjcJointAPI

This API class is applied to UsdPhysicsJoint prims, adding extra attributes to fully describe MuJoCo joints.

MjcMaterialAPI

This API class provides attributes for physical materials and is an extension of UsdPhysicsMaterialAPI

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.

We do not use the existing UsdPhysicsDriveAPI 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 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.