PhysicsFixedJoint#

Welds two bodies (or a body and the world) with no degrees of freedom. Owns no columns beyond the PhysicsJoint base set. Standalone it is a PxD6Joint with all six axes locked; inside an articulation it is an eFIX reduced-coordinate joint and, when one side is the world, the usual fixed-base anchor.

Type chain: Imageable, Typed, PhysicsJoint
Extends: PhysicsJoint; its columns, derived values, interactions and findings are inherited and listed here.
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#

As PhysicsJoint. The type declares no properties of its own.

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` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	bool8	none	false	resolved USD value (authored, else raw fallback)	false	Collision between the two jointed bodies is disabled.
`physics:breakForce` (inherited from `PhysicsJoint`)	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` (inherited from `PhysicsJoint`)	float32	mass*length^2/s^2	`inf`	resolved USD value (authored, else raw fallback)	`FLT_MAX`	Unbreakable; see physics:breakForce.
`physics:excludeFromArticulation` (inherited from `PhysicsJoint`)	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.
PhysxJointAPI: physxJoint:jointFriction is still read and applied on an articulation fixed joint; armature and maxJointVelocity have no axis to land on.
PhysicsArticulationRootAPI: A fixed joint from a link to the world (absent body relationship, or a static or kinematic body) is the canonical fixed-base anchor; the elected root is then the joint prim and fixBase is set.

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#

Instance matrix for this type: none. No PhysicsLimitAPI, PhysicsDriveAPI, PhysxLimitAPI, PhysxJointAxisAPI, PhysxDrivePerformanceEnvelopeAPI or PhysicsJointStateAPI instance is read.
The parser builds a FixedPhysxJointDesc from the base fields alone (copyBaseFields). A Newton mimic-joint relationship may additionally be read for this type; it belongs to the Newton schema and is not part of this contract.