UMjPhysicsEngine¶

Core physics engine component that owns the MuJoCo simulation lifecycle.

Manages mjSpec compilation, mjModel/mjData ownership, the async physics loop, and callback registration for pre/post step hooks.

Attribute	Value
Kind	`Class`
UE Macro	`UCLASS`
Inherits	`UActorComponent`
Blueprint Spawnable	✅ Yes

Properties Functions

Properties¶

Public Properties¶

Property	Type	Description
`m_spec`	`mjSpec*`	Pointer to the MuJoCo specification structure.
`m_vfs`	`mjVFS`	MuJoCo Virtual File System instance.
`m_model`	`mjModel*`	Pointer to the compiled MuJoCo model.
`m_data`	`mjData*`	Pointer to the active MuJoCo simulation data.
`AsyncPhysicsFuture`	`TFuture<void>`	Future for the async physics thread, used to wait for exit in EndPlay.
`FMujocoStepCallback`	`using`	Delegate for custom physics stepping.
`FMujocoPostStepCallback`	`using`	Delegate called immediately after mj_step (or custom step).
`PendingStateVector`	`TArray<double>`	Plain-data copy of the state to restore.
`Options`	FMuJoCoOptions	Simulation Options (Timestep, Gravity, etc.).
`SimSpeedPercent`	`float`	Simulation speed percentage (0-100). 100 = realtime, 50 = half speed.
`bIsPaused`	`bool`	If true, the simulation loop skips stepping the physics.
`bSaveDebugXml`	`bool`	If true, saves compiled scene XML and MJB to Saved/URLab/ on each compile.
`ControlSource`	EControlSource	Global default control source (ZMQ or UI).
`m_MujocoComponents`	TArray<UMjQuickConvertComponent*>	List of custom physics components registered with this manager.
`m_articulations`	TArray<AMjArticulation*>	List of articulations (multi-body structures) registered with this manager.
`m_heightfieldActors`	TArray<AMjHeightfieldActor*>	List of heightfield actors registered with this manager.
`m_ArticulationMap`	TMapAMjArticulation\*>	O(1) articulation lookup. Key = actor name.
`m_LastCompileError`	`FString`	Error string from the most recent Compile() call. Empty on success.

m_spec¶

Pointer to the MuJoCo specification structure.

Type: mjSpec*

m_vfs¶

MuJoCo Virtual File System instance.

Type: mjVFS

m_model¶

Pointer to the compiled MuJoCo model.

Type: mjModel*

m_data¶

Pointer to the active MuJoCo simulation data.

Type: mjData*

AsyncPhysicsFuture¶

Future for the async physics thread, used to wait for exit in EndPlay.

Type: TFuture

FMujocoStepCallback¶

Delegate for custom physics stepping.

Type: using

FMujocoPostStepCallback¶

Delegate called immediately after mj_step (or custom step).

Type: using

PendingStateVector¶

Plain-data copy of the state to restore.

Type: TArray

Options¶

Simulation Options (Timestep, Gravity, etc.).

Type: FMuJoCoOptions
Editor: ✏️ EditAnywhere
Blueprint: 🔵 ReadWrite
Category: MuJoCo|Options

SimSpeedPercent¶

Simulation speed percentage (0-100). 100 = realtime, 50 = half speed.

Type: float
Editor: ✏️ EditAnywhere
Blueprint: 🔵 ReadWrite
Category: MuJoCo|Options

bIsPaused¶

If true, the simulation loop skips stepping the physics.

Type: bool
Editor: ✏️ EditAnywhere
Blueprint: 🔵 ReadWrite
Category: MuJoCo|Status

bSaveDebugXml¶

If true, saves compiled scene XML and MJB to Saved/URLab/ on each compile.

Type: bool
Editor: ✏️ EditAnywhere
Blueprint: 🔵 ReadWrite
Category: MuJoCo|Debug

ControlSource¶

Global default control source (ZMQ or UI).

Type: EControlSource
Editor: ✏️ EditAnywhere
Blueprint: 🔵 ReadWrite
Category: MuJoCo|Runtime

m_MujocoComponents¶

List of custom physics components registered with this manager.

Type: TArray<UMjQuickConvertComponent*>

m_articulations¶

List of articulations (multi-body structures) registered with this manager.

Type: TArray<AMjArticulation*>

m_heightfieldActors¶

List of heightfield actors registered with this manager.

Type: TArray<AMjHeightfieldActor*>

m_ArticulationMap¶

O(1) articulation lookup. Key = actor name.

Type: TMapAMjArticulation\*>

m_LastCompileError¶

Error string from the most recent Compile() call. Empty on success.

Type: FString

Functions¶

Public Functions¶

General¶

Function	Returns	Description
`Compile()`	void	Compiles the aggregated mjSpec into an mjModel and initializes mjData.
`PreCompile()`	void	Scans the scene for MuJoCo components and articulations to populate m_spec.
`PostCompile()`	void	Finalizes setup after compilation (actuator maps, PostSetup calls).
`ApplyOptions()`	void	Applies the settings from Options to m_model->opt.
`RunMujocoAsync()`	void	Starts the asynchronous thread for stepping the MuJoCo simulation.
`SetPaused()`	void	Pauses or Resumes the physics simulation.
`IsRunning()`	`bool`	Checks if the simulation is currently running (initialized and not paused).
`IsInitialized()`	`bool`	Checks if the MuJoCo model is compiled and data is allocated.
`GetSimTime()`	`float`	Gets the current MuJoCo simulation time in seconds.
`GetTimestep()`	`float`	Gets the simulation timestep from Options (dt).
`ResetSimulation()`	void	Resets the MuJoCo simulation data to initial state.
`StepSync()`	void	Steps the simulation synchronously on the calling thread.
`CompileModel()`	`bool`	Re-runs Compile() and restarts the async simulation loop.
`CaptureSnapshot()`	UMjSimulationState*	Captures a full state snapshot of the current simulation.
`RestoreSnapshot()`	void	Schedules a simulation state restore for the next physics step.
`SetControlSource()`	void	Sets the global default control source.
`GetControlSource()`	EControlSource	Gets the current control source.
`GetArticulation()`	AMjArticulation*	Gets a registered articulation by its Actor name.
`GetAllArticulations()`	TArray<AMjArticulation*>	Gets all registered articulations.
`GetAllQuickComponents()`	TArray<UMjQuickConvertComponent*>	Gets all registered QuickConvert components.
`GetAllHeightfields()`	TArray<AMjHeightfieldActor*>	Gets all registered Heightfield actors.
`GetLastCompileError()`	`FString`	Returns the error string from the most recent failed compile.
`RegisterPreStepCallback()`	void	Register a callback to be invoked before each physics step.
`RegisterPostStepCallback()`	void	Register a callback to be invoked after each physics step.
`ClearCallbacks()`	void	Clear all registered pre/post step callbacks.

Compile¶

Compiles the aggregated mjSpec into an mjModel and initializes mjData.

Signature: Compile()

PreCompile¶

Scans the scene for MuJoCo components and articulations to populate m_spec.

Signature: PreCompile()

PostCompile¶

Finalizes setup after compilation (actuator maps, PostSetup calls).

Signature: PostCompile()

ApplyOptions¶

Applies the settings from Options to m_model->opt.

Signature: ApplyOptions()

RunMujocoAsync¶

Starts the asynchronous thread for stepping the MuJoCo simulation.

Signature: RunMujocoAsync()

SetPaused¶

Pauses or Resumes the physics simulation.

Signature: SetPaused(bool bPaused)

IsRunning¶

Checks if the simulation is currently running (initialized and not paused).

Signature: IsRunning()
Returns: bool

IsInitialized¶

Checks if the MuJoCo model is compiled and data is allocated.

Signature: IsInitialized()
Returns: bool

GetSimTime¶

Gets the current MuJoCo simulation time in seconds.

Signature: GetSimTime()
Returns: float

GetTimestep¶

Gets the simulation timestep from Options (dt).

Signature: GetTimestep()
Returns: float

ResetSimulation¶

Resets the MuJoCo simulation data to initial state.

Signature: ResetSimulation()

StepSync¶

Steps the simulation synchronously on the calling thread.

Signature: StepSync(int32 NumSteps)

CompileModel¶

Re-runs Compile() and restarts the async simulation loop.

Signature: CompileModel()
Returns: bool

CaptureSnapshot¶

Captures a full state snapshot of the current simulation.

Signature: CaptureSnapshot()
Returns: UMjSimulationState*

RestoreSnapshot¶

Schedules a simulation state restore for the next physics step.

Signature: RestoreSnapshot(UMjSimulationState* Snapshot)

SetControlSource¶

Sets the global default control source.

Signature: SetControlSource(EControlSource NewSource)

GetControlSource¶

Gets the current control source.

Signature: GetControlSource()
Returns: EControlSource

GetArticulation¶

Gets a registered articulation by its Actor name.

Signature: GetArticulation(const FString& ActorName)
Returns: AMjArticulation*

GetAllArticulations¶

Gets all registered articulations.

Signature: GetAllArticulations()
Returns: TArray<AMjArticulation*>

GetAllQuickComponents¶

Gets all registered QuickConvert components.

Signature: GetAllQuickComponents()
Returns: TArray<UMjQuickConvertComponent*>

GetAllHeightfields¶

Gets all registered Heightfield actors.

Signature: GetAllHeightfields()
Returns: TArray<AMjHeightfieldActor*>

GetLastCompileError¶

Returns the error string from the most recent failed compile.

Signature: GetLastCompileError()
Returns: FString

RegisterPreStepCallback¶

Register a callback to be invoked before each physics step.

Signature: RegisterPreStepCallback(FPhysicsCallback Callback)

RegisterPostStepCallback¶

Register a callback to be invoked after each physics step.

Signature: RegisterPostStepCallback(FPhysicsCallback Callback)

ClearCallbacks¶

Clear all registered pre/post step callbacks.

Signature: ClearCallbacks()