PhysicsJoint

Base joint between two rigid bodies (or one body and the world). Owns the columns every joint subtype reads: body relationships, local frames, enable flags, break limits and the articulation exclusion flag. A prim typed exactly PhysicsJoint is ALSO the D6 joint: it reads its per-axis limits, drives and states from the multi-apply PhysicsLimitAPI, PhysicsDriveAPI and PhysicsJointStateAPI instances and its rotational axis properties from PhysxJointAxisAPI:rotX/rotY/rotZ. Subtypes extend this component and add only their own columns.

• Type chain: Imageable, Typed

• Applicable APIs: PhysicsArticulationRootAPI, PhysicsDriveAPI, PhysicsJointStateAPI, PhysicsLimitAPI, PhysicsMaterialAPI, PhysxArticulationAPI, PhysxDrivePerformanceEnvelopeAPI, PhysxJointAPI, PhysxJointAxisAPI, PhysxLimitAPI, PhysxMaterialAPI, PhysxPhysicsDistanceJointAPI, PhysxSceneAPI

• Discovery: by prim_type, probe physics:localPos0

• Transform columns: none, not Xformable

• Uses blocks: prim

What USD population writes

Published whenever the prim type is one of the eight joint types the consumer enumerates (PhysicsJoint, PhysicsFixedJoint, PhysicsRevoluteJoint, PhysicsPrismaticJoint, PhysicsSphericalJoint, PhysicsDistanceJoint, PhysxPhysicsGearJoint, PhysxPhysicsRackAndPinionJoint) or a consumer-registered custom joint token. Every declared attribute of the type and of its applied APIs is published at its resolved value; body0/body1 and the other relationships only when authored. The producer additionally creates physxJoint:maxJointVelocity = FLT_MAX on every joint-typed prim when unauthored, whether or not PhysxJointAPI is applied (see PhysxJointAPI). purpose, visibility and proxyPrim are never published.

Columns

Raw fallback is the USD schema value. Producer writes is what ovstage population puts in the column for a USD stage. Parser default applies on the consumer when the column is absent, in stage units. diverges means writing the raw fallback literally parses differently from omitting the column (the parser default): USD population may well publish that very value for an unauthored attribute, so the marker is about the column being present, not about its value.

Column	Type	Units	Raw fallback	Producer writes	Parser default	If absent
physics:body0	uint64 array, RELATIONSHIP_PATH_ID			not written		Side 0 anchors to the world. The consumer keeps localPos0/localRot0 verbatim as a world-space frame (only the quaternion is normalised, no body scale is baked) and, for articulations, treats the joint as a fixed-base anchor candidate. Only the first target is read; it is resolved to the nearest enclosing enabled rigid body, which may differ from the target itself.
physics:body1	uint64 array, RELATIONSHIP_PATH_ID			not written		Side 1 anchors to the world; see physics:body0. A joint with both relationships absent is still enumerated and parsed but no PhysX joint is created (no actor on either side).
physics:localPos0	float32[3], POINT	length	(0, 0, 0)	resolved USD value (authored, else raw fallback)	(0, 0, 0)	Frame origin at the body-0 origin. Stored in stage length units; there is no metersPerUnit scaling, but the resolved body’s world scale is multiplied component-wise into the authored value because PhysX actors carry no scale (localPos0 (1,1,1) under scale (2,3,4) reaches PhysX as (2,3,4)). When the relationship target is not the resolved body the frame is lifted to world through the target and pulled back into the body frame with scale and shear removed.
physics:localRot0	float32[4], QUATERNION	none	(0, 0, 0, 1)	resolved USD value (authored, else raw fallback)	(0, 0, 0, 1)	Identity rotation. The consumer normalises the quaternion unconditionally, even when the body relationship is absent.
physics:localPos1	float32[3], POINT	length	(0, 0, 0)	resolved USD value (authored, else raw fallback)	(0, 0, 0)	Frame origin at the body-1 origin. Same scale and re-anchoring rules as physics:localPos0.
physics:localRot1	float32[4], QUATERNION	none	(0, 0, 0, 1)	resolved USD value (authored, else raw fallback)	(0, 0, 0, 1)	Identity rotation; see physics:localRot0.
physics:jointEnabled	bool8	none	true	resolved USD value (authored, else raw fallback)	true	Joint is created and simulated. false creates no PhysX joint and removes the joint from the articulation graph entirely.
physics:collisionEnabled	bool8	none	false	resolved USD value (authored, else raw fallback)	false	Collision between the two jointed bodies is disabled.
physics:breakForce	float32	mass*length/s^2	inf	resolved USD value (authored, else raw fallback)	FLT_MAX	Unbreakable. The consumer seeds FLT_MAX before reading; the USD schema fallback is inf. Either value reaches PhysX as FLT_MAX because the runtime maps every non-finite break force to FLT_MAX.
physics:breakTorque	float32	mass*length^2/s^2	inf	resolved USD value (authored, else raw fallback)	FLT_MAX	Unbreakable; see physics:breakForce.
physics:excludeFromArticulation	bool8	none	false	resolved USD value (authored, else raw fallback)	false	The joint is a candidate articulation joint: inside an articulation subtree it becomes a PxArticulationJointReducedCoordinate and its body-to-world side (if any) elects a fixed base. true keeps it a standalone PxJoint (graph weight +1000 instead of +100000/+100, no recursion through it).

Interactions

• prim: When a body relationship is absent (or its target resolves to no enabled dynamic body) that side of the joint is anchored to the world: the authored local frame is used verbatim in world space and no body scale is baked into it. When a body resolves, its world scale, decomposed from the composed transform, is multiplied component-wise into localPos on that side.

• PhysicsRigidBodyAPI: body0/body1 are resolved to the nearest enclosing prim with an enabling rigid body. Without an actor on at least one side no PhysX joint is created.

• PhysicsArticulationRootAPI: A joint with one side anchored to the world (absent relationship, or a static, disabled or kinematic body) and excludeFromArticulation false becomes the fixed-base anchor of an articulation that contains its other body; the joint prim itself is elected root and fixBase is set.

• PhysxJointAPI: The producer creates physxJoint:maxJointVelocity = FLT_MAX on every joint-typed prim when unauthored, even without PhysxJointAPI in usd-schemas; the consumer ignores it unless the API is listed.

• PhysicsLimitAPI: Only a prim typed exactly PhysicsJoint (the D6) reads PhysicsLimitAPI:; typed joints read their own limit columns.

• PhysicsDriveAPI: The D6 reads PhysicsDriveAPI:transX..rotZ and :distance; revolute reads :angular, prismatic reads :linear; the other subtypes read no drive.

Known divergences

• note (consumer) physics:breakForce: The ovstage walker seeds breakForce/breakTorque with FLT_MAX while the USD walker leaves the schema fallback inf, so the descriptor differs bit-for-bit between backends when the columns are unauthored. No simulation difference: the runtime maps any non-finite value to FLT_MAX before calling PxJoint::setBreakForce.

• fragile (consumer) physics:localRot0: The PhysxJointDesc constructor default for localPose0Orientation/localPose1Orientation is {1, 0, 0, 0} in the (x, y, z, w) layout, a 180 degree rotation about X rather than identity. Masked today because copyBaseFields always overwrites it from JointInfo, whose default {0, 0, 0, 1} is identity. Any future path that builds a joint descriptor without copyBaseFields inherits a flipped frame.

• note (producer) physics:localRot0: Quaternion lanes are published (x, y, z, w) although USD authors (w, x, y, z); the consumer reads the lanes verbatim. A hand populator that copies the USD tuple order produces a wrong frame with no diagnostic. Also applies to physics:localRot1.

Notes

• Enumeration is one exact prim-type query per joint type, so subtypes derived from PhysicsJoint by IsA are not found; the type list is closed apart from consumer-registered custom joint tokens. physics:localPos0 is only the fallback probe column: normally the query’s discovered-attribute set materialises the prim list, so a joint with no authored localPos0 is still found.

• A prim typed exactly PhysicsJoint is the D6 joint (ObjectType eJointD6). Its multi-apply instance names are: PhysicsLimitAPI and PhysicsDriveAPI: transX, transY, transZ, rotX, rotY, rotZ, distance; PhysicsJointStateAPI: transX, transY, transZ, rotX, rotY, rotZ (no distance); PhysxLimitAPI: the same seven as PhysicsLimitAPI; PhysxJointAxisAPI and PhysxDrivePerformanceEnvelopeAPI: rotX, rotY, rotZ only.

• Minimal column set for a working joint between two bodies: the prim type token, physics:body0 and physics:body1. Everything else falls to the parser defaults above; localPos0/1 default to the body origins.

• Standalone (non-articulation) fixed, revolute, prismatic, spherical and D6 joints are all created as PxD6Joint by the runtime; only distance, gear and rack-and-pinion joints use their own PhysX joint classes. Verify expressions therefore go through harness helpers that dispatch on PxJoint::getConcreteType().

• PhysicsJointStateAPI, PhysxJointAxisAPI and PhysxDrivePerformanceEnvelopeAPI are read for standalone joints but the runtime ignores them with a warning; they only take effect on articulation joints.

• The consumer never falls back to USD for joints on ovstage: an unpublished relationship reads as absent and hasAuthoredAttribute is always true, so every resolved value the producer publishes wins over the parser default.