Scripting with Blueprints & C++

Every MuJoCo component in Unreal Robotics Lab is a regular Unreal component. Articulations are actors. You interact with them the same way you would with any other actor or component — get references, call functions, bind events. No need to touch MuJoCo directly unless you want to.

For the full class-by-class API listing, see the auto-generated API Reference.

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Getting References

An AMjArticulation is just an actor. Get a reference any way you normally would:

Blueprint: Get All Actors of Class, cast from a hit result, or store a reference variable.

C++:

AAMjManager* Manager = AAMjManager::GetManager();
AMjArticulation* Robot = Manager->GetArticulation("MyRobot");

From the manager you can look up articulations by name or get all of them:

Blueprint: Get Manager → Get Articulation (name) / Get All Articulations

C++:

TArray<AMjArticulation*> All = Manager->GetAllArticulations();

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Working with Components

Once you have an articulation, its MuJoCo components are just child components. Access them by name or as arrays:

Blueprint: Get Actuator (name), Get Joint (name), Get Sensor (name) — or Get Actuators, Get Joints, Get Sensors, Get Bodies for arrays. You can also drag directly from the component tree in your Blueprint.

C++:

UMjActuator* Act = Robot->GetActuator("shoulder");
TArray<UMjJoint*> Joints = Robot->GetJoints();
UMjBody* Body = Robot->GetBodyByMjId(3);  // by compiled ID

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Controlling Actuators

Blueprint: Set Actuator Control (Name, Value) on the articulation. Wire Get Game Time in Seconds → Sin → Set Actuator Control into Event Tick for a simple sine wave.

Use Get Actuator Range to get a Vector2D (min, max) for clamping.

C++:

Robot->SetActuatorControl("shoulder", 1.57f);
FVector2D Range = Robot->GetActuatorRange("shoulder");

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Reading Sensors

Blueprint:

• Get Sensor Scalar (name) → Float — for 1D sensors (touch, joint pos, clock)
• Get Sensor Reading (name) → Array of Float — for vector sensors (force, accelerometer)
• Get Joint Angle (name) → Float — shortcut for joint position

C++:

float Touch = Robot->GetSensorScalar("fingertip_touch");
TArray<float> Force = Robot->GetSensorReading("wrist_force");
float Angle = Robot->GetJointAngle("elbow");

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Reacting to Collisions

AMjArticulation has an On Collision event dispatcher. Fires with: SelfGeom (UMjGeom), OtherGeom (UMjGeom), ContactPos (FVector).

Blueprint: Select the articulation reference → Assign On Collision → wire into your logic.

C++:

Robot->OnCollision.AddDynamic(this, &AMyActor::HandleCollision);

void AMyActor::HandleCollision(UMjGeom* SelfGeom, UMjGeom* OtherGeom, FVector ContactPos)
{
    // Stop gripper, play effect, etc.
}

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Simulation Lifecycle

All on the manager:

Node	What it does
Set Paused (bool)	Pause/resume the physics thread
Reset Simulation	Zero positions, velocities, time
Step Sync (N)	Advance N steps synchronously (RL-style loops)
Get Sim Time	Current simulation clock

C++:

Manager->SetPaused(true);
Manager->StepSync(10);
Manager->ResetSimulation();
float Time = Manager->GetSimTime();

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Snapshots

Save and restore the full simulation state:

Blueprint: Capture Snapshot → store in a variable → Restore Snapshot later.

C++:

UMjSimulationState* Snap = Manager->CaptureSnapshot();
// ... try something ...
Manager->RestoreSnapshot(Snap);

Hold multiple snapshots for A/B testing, checkpointing, or undo.

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Keyframe API

On AMjArticulation:

Node	What it does
ResetToKeyframe(Name)	Teleports to a named keyframe (sets qpos/qvel/ctrl)
HoldKeyframe(Name)	Continuously maintains a keyframe pose
StopHoldKeyframe()	Releases the held keyframe
GetKeyframeNames()	Returns names of all keyframes on this articulation
IsHoldingKeyframe()	Returns true if currently holding a pose

The MjSimulate widget provides a keyframe dropdown and Reset/Hold/Stop buttons for interactive use.

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Recording and Replay

With an AMjReplayManager in the level:

Blueprint: Start Recording / Stop Recording / Start Replay / Stop Replay / Save Recording to File / Load Recording from File

C++:

Replay->StartRecording();
// ... simulation runs ...
Replay->StopRecording();
Replay->SaveRecordingToFile("C:/data/experiment.dat");
Replay->StartReplay();

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Switching Control Source

Toggle between dashboard and external ZMQ control:

Blueprint: Get Manager → Set Control Source → UI or ZMQ

C++:

Manager->SetControlSource(EControlSource::ZMQ);

Per-articulation override available on AMjArticulation::ControlSource.

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MjKeyframeController

Cycles through named poses on an articulation with ease-in-out blending.

Node	Description
LoadPreset (name)	Load a built-in pose sequence
Play / Stop	Start or pause playback
GoToKeyframe (index)	Jump to a specific keyframe
GetPresetNames	Returns all available preset names

For full details on the keyframe controller, presets, and FMjKeyframePose struct, see Controller Framework.

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MjKeyframeCameraActor

AMjKeyframeCameraActor is a cinematic camera that smoothly interpolates through a list of waypoints (FMjCameraWaypoint: Position, Rotation, Time). It uses a UCineCameraComponent and displays a spline preview of the path in the editor.

Key functions:

Node	Description
Play	Start waypoint playback
Pause	Freeze at current position
TogglePlayPause	Toggle (also bound to O key)
Reset	Return to first waypoint
CaptureCurrentView	(Editor only) Snapshot the viewport camera as a new waypoint

Properties: bAutoPlay, bAutoActivate (sets as player view target), bLoop, StartDelay, bSmoothInterp (cubic vs linear).

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MjImpulseLauncher

AMjImpulseLauncher applies a velocity-based impulse to an MjBody. Two modes:

• Direct — launches along the actor's forward vector (or DirectionOverride).
• Targeted — set LaunchTarget to an actor and it computes a ballistic arc toward it. ArcHeight controls the lob.

Node	Description
FireImpulse	Apply the impulse once
ResetAndFire	Teleport projectile back to launcher position and fire (also bound to F key)

Properties: TargetActor, TargetBodyName (optional, defaults to first MjBody), LaunchSpeed (m/s), LaunchTarget, ArcHeight, bAutoFire, AutoFireDelay.

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Hotkeys

Handled by AAMjManager::Tick. Active during PIE:

Key	Action
1	Toggle debug contacts
2	Toggle visual meshes
3	Toggle articulation collision wireframes
4	Toggle debug joints
5	Toggle quick-convert collision wireframes
P	Pause / resume simulation
R	Reset simulation
O	Toggle orbit camera orbit + keyframe camera play/pause
F	Reset and fire all impulse launchers

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Per-Articulation Control Source

Each AMjArticulation has a ControlSource field (0 = ZMQ, 1 = UI) that overrides the manager-level EControlSource setting. This lets you run some robots from the dashboard sliders while others receive external ZMQ commands in the same scene.

Blueprint: Set ControlSource on the articulation reference in Details or via Set node.

C++:

Robot->ControlSource = 1; // UI control for this robot

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Advanced: Direct MuJoCo Access

For users who need low-level access, every UMjComponent exposes Get Mj ID (compiled integer ID), Get Mj Name (prefixed name), and Is Bound (compilation status). Use these to index directly into mjData from C++. For most workflows, the API above is all you need.