Multiple Physics Solvers#

Goal#

Author and validate one SimReady prop so it can run under PhysX, Newton, or MuJoCo without a separate USD file per solver.

After this guide you can:

Explain how runtime physics variants isolate solvers on one asset
Map Core / Rigid Body / Multibody features to each solver
Point validators and samples at the correct runtime selection
Find the requirement, profile, and isolation references

Why multiple solvers on one asset#

Robotics pipelines rarely share one physics backend. PhysX is the reference path for Isaac Sim, Newton is the target runtime for Isaac Lab, and MuJoCo remains common for research and interchange. Separate USD files per solver break shared packaging, thumbnails, materials, and semantics.

SimReady Foundation uses runtime physics variants: a neutral OpenUSD base plus optional payload layers. Each solver opts in through a USD variant set. With every physics variant Disabled, the base stays valid. Enabling one runtime composes only that runtime’s schemas on top of neutral physics data.

For prop assets, use the samples linked below as the reference layout. Robot body multiphysics support may differ by profile version — check the profile you target.

How it works#

        flowchart TB
    Base["Neutral SimReady base<br/>UsdPhysics + visuals + materials"]
    Base --> VS["Variant sets on defaultPrim<br/>PhysX / Newton / MuJoCo"]
    VS -->|Enabled| PX["runnables/physics/physx.usd"]
    VS -->|Enabled| NW["runnables/physics/newton.usd"]
    VS -->|Enabled| MJ["runnables/physics/mujoco.usd"]
    VS -->|Disabled| Empty["Empty section<br/>no foreign schemas"]

Three layers enforce the contract:

Layer	What it defines	Where to read
Runtime Variants (RV)	Variant set names, payload paths, metadata, purity, isolation	Runtime Variants
Runtime features	Solver rigid-body and multibody rules (`FET_003_`, `FET_004_`)	Feature pages below
Profiles	Which optional runtime features a prop profile accepts	Robotics-Prop, Prop-Robotics-Physx

Feature expansion pattern#

Neutral features describe OpenUSD physics. Each solver expands them with a runtime-tagged feature that the validator enables before checking:

Concern	Neutral	PhysX	Newton	MuJoCo
Core scaffolding	`FET_000_STANDARD`	`FET_000_PHYSX`	`FET_000_NEWTON`	`FET_000_MUJOCO`
Rigid body	`FET_003_STANDARD`	`FET_003_PHYSX`	`FET_003_NEWTON`	`FET_003_MUJOCO`
Multibody	`FET_004_STANDARD`	`FET_004_PHYSX`	`FET_004_NEWTON`	`FET_004_MUJOCO`

Physics runtime features set a runtime field (PhysX, Newton, or MuJoCo) so simready-validate enables that variant set before those rules run. See Runtime Variant Field.

Isolation rules#

When PhysX = Enabled and the other physics variants stay Disabled, the composed stage may contain only:

Neutral UsdPhysics / physics: data
PhysX schemas and physx*: attributes

Newton or MuJoCo schemas in that composition fail RV.011. The same rule applies when Newton or MuJoCo is selected. With all physics variants Disabled, no solver-specific schemas may remain.

Authoritative policy: Runtime Physics Isolation Matrix.

Common mistakes:

Authoring physics:approximation = "sdf" on the neutral base (PhysX-only)
Merging PhysX and Newton schemas into one payload
Leaving a second runtime variant Enabled while validating another

Author a multiphysics prop#

Prerequisites#

A prop that already meets the neutral robotics contract (FET_000_STANDARD, FET_001_STANDARD, plus the physics features you need)
Familiarity with USD variant sets and payloads
simready-validate for the target Foundation release

1. Keep solver data out of the neutral base#

Author shared visuals, materials, mass, and OpenUSD physics on the root asset. Do not apply Physx*, Newton*, or Mjc* schemas on the base layer.

2. Add one variant set per runtime on `defaultPrim`#

Name each set exactly PhysX, Newton, or MuJoCo. Expose Disabled / Enabled, and default to Disabled:

variantSet "PhysX" = {
    "Disabled" {
    }
    "Enabled" {
        prepend payload = @../runnables/physics/physx.usd@
    }
}

Repeat for Newton → runnables/physics/newton.usd and MuJoCo → runnables/physics/mujoco.usd. The Enabled section must contain only that anchored payload (RV.010).

3. Record variant metadata#

Under root-layer customLayerData.SimReady_Metadata.Variants.Physics, record prim, variantSetName, and activateOption for each runtime (RV.003, RV.006, RV.009). Profile authoring guides show the expected shape.

4. Author each payload for one solver only#

Payload	Feature contracts	Typical content
`runnables/physics/physx.usd`	`FET_003_PHYSX`, `FET_004_PHYSX`	PhysX collider APIs; SDF approximation when required
`runnables/physics/newton.usd`	`FET_003_NEWTON`, `FET_004_NEWTON`	`NewtonCollisionAPI` plus mesh or SDF collision API
`runnables/physics/mujoco.usd`	`FET_003_MUJOCO`, `FET_004_MUJOCO`	`MjcCollisionAPI` / `MjcMeshCollisionAPI`; convex hull

5. Opt into features on a profile that allows them#

Robotics-Prop@3.1.0 and Prop-Robotics-Physx@2.2.0 (and later) list the FET_000_* / FET_003_* / FET_004_* runtime features as optional. A missing runtime does not fail the profile; a present runtime must pass its rules.

Stamp validation metadata with the profile name and version you target. See Robotics-Prop runtime physics variants.

6. Validate per runtime#

simready-validate groups profile features by the runtime field and checks each group with only that variant enabled. To inspect a payload manually, enable one variant (Composer or USD APIs), keep the others Disabled, then validate against the matching feature set.

Reference samples#

Sample	Role
`obs_orange_a02`	Unibody prop with PhysX, Newton, and MuJoCo payloads under `runnables/physics/`
`obs_electricians_large_tool_box_a01`	Multibody prop with the same runtime-variant layout

Feature adapters#

Feature adapters mutate neutral physics into a runtime-specific contract (Neutral → PhysX, Neutral → Newton, and so on). They author payload content; Core FET_000_* features own the variant scaffolding around those payloads. See Feature Adapters.

Related specification pages