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Possession & Twist Control

URLab articulations are Unreal Pawns — they can be possessed by a PlayerController to enable direct input control. The twist controller captures WASD/gamepad input and broadcasts velocity commands over ZMQ.

Possessing an Articulation

From the MjSimulate widget: Click the Possess button next to the articulation selector. The button toggles to Release while possessed.

From Blueprint/C++: 

APlayerController* PC = GetWorld()->GetFirstPlayerController();
PC->Possess(MyArticulation);
// Later:
PC->UnPossess();

When possessed: - Enhanced Input mapping context (IMC_TwistControl) is added - A spring arm + camera attaches to the root MjBody for follow-cam - WASD/QE input feeds the twist controller

When released: - Mapping context removed, twist state zeroed - Camera components cleaned up - Original pawn re-possessed

UMjTwistController

Auto-created on every AMjArticulation at BeginPlay. Captures input and stores thread-safe twist state.

Properties (editable in Details panel or via locomotion sliders):

Property Default Description
MaxVx 0.8 m/s Max forward speed
MaxVy 0.5 m/s Max strafe speed
MaxYawRate 1.57 rad/s Max turn rate

Input mapping:

Key Action
W/S Forward/backward
A/D Strafe left/right
Q/E Turn left/right
1-0 Action keys (10-slot bitmask)

ZMQ broadcast: The sensor broadcaster publishes twist state every physics step:

Topic Payload
<prefix>/twist 3 x float32: vx (m/s), vy (m/s), yaw_rate (rad/s)
<prefix>/actions int32 bitmask of pressed action keys

Follow Camera

On possession, a spring arm + camera component attaches to the root MjBody:

  • Camera lag: speed 8, rotation lag enabled for smooth tracking
  • Arm length: 300 units behind, 100 units above
  • Tagged PossessCamera for cleanup on release

The camera follows the actual physics body position, not the static actor root.

Connecting to Policies

The Python policy bridge reads twist commands via the UnrealTwistCtrl controller class:

# In policy_gui.py — twist controller masquerades as JoystickCtrl
# so the Unitree policy's _get_commands() finds the axes data
ctrl_list = [UnrealTwistCtrlCfg()]

Twist values map to policy commands: - vx → forward velocity command - vy → lateral velocity command - yaw_rate → turn command

The policy interprets these as walking direction/speed.

Locomotion Sliders

When an articulation with a TwistController is selected in the MjSimulate widget, a LOCOMOTION section appears with three sliders:

  • Max Forward Speed (0.0 – 2.0 m/s)
  • Max Strafe Speed (0.0 – 1.0 m/s)
  • Max Turn Rate (0.0 – 3.14 rad/s)

These control how much velocity a full keypress produces.

Global Simulation Hotkeys

See Hotkeys for all keyboard shortcuts.

MjKeyframeCameraActor

For scripted camera work (e.g., filming demos), use MjKeyframeCameraActor instead of the possession follow-cam. It plays back predefined camera keyframes with interpolation, independent of any possessed pawn.

Input Assets

Located in Content/Input/: - IA_TwistMove — Axis2D input action (WASD) - IA_TwistTurn — Axis1D input action (Q/E) - IMC_TwistControl — Input mapping context binding both actions

Auto-loaded from plugin content when the TwistController is created.