Skip to content

Physics

CAT separates the authoritative Assembly State from its runtime physics representation. The Assembly Component owns structure and revisions. CAT Physics Execution builds the Chaos projection, binds actuators and simulation features, and selects the execution path for the current network role.

Runtime physics flow

Part Model and Assembly
          ↓
     Assembly State
          ↓
 CAT Physics Execution
          ↓
    Chaos simulation

Structural changes update the Assembly State, then rebuild its physics representation. Resolve cached physics references and Controller actuator handles again when the revision changes.

Control input and physics

Controllers do not simulate lift, buoyancy, or collision. They convert Input Actions into a complete Control Frame containing normalized Motor and Fan signals.

Input Action
      ↓
Control Session Controller
      ↓
Control Frame
      ↓
CAT Physics Execution
      ↓
Motor / Fan output

Depending on the execution mode, CAT applies the Control Frame through the normal component path or stages it for Network Physics prediction and resimulation.

Authored physics data

  • Body: Mesh, Dynamic or Kinematic mode, mass, collision profile, physical material, and local transform.
  • Connector and Relation: Placement frame and optional Constraint Profile. These determine which projected bodies are joined and how they may move.
  • Fan: Applies linear force along its authored axis at its force application point.
  • Motor: Applies axial torque around its authored axis.
  • Aerodynamic Feature: Calculates lift and drag for authored surfaces and applies the resulting force and moment. See Aerodynamic Feature.
  • Buoyancy Feature: Samples supported Unreal Water surfaces for authored pontoons, then applies buoyancy, heave damping, linear/angular drag, and slide resistance. See Buoyancy Feature.

Simulation features and Controllers

These layers work together but have different responsibilities:

System Responsibility
Aerodynamics Produces lift and drag from the current airflow and surface definitions.
Buoyancy Produces flotation and water resistance from pontoons and the sampled water surface.
Ground Drive Controller Converts movement intent into Motor output, including differential steering.
Aircraft Controller Converts throttle and rotation intent into Fan and Motor output; Aerodynamics produces the actual lift and drag.
Watercraft Controller Converts movement intent into Fan and Motor output and can stabilize against the sampled water orientation; Buoyancy produces flotation and water resistance.

Physics execution modes

CAT resolves the effective mode from authority, ownership, and network settings:

  • Authority Simulation: Simulates the authoritative Assembly.
  • Owner Prediction: Runs Network Physics input prediction and resimulation for the controlling owner.
  • Remote Dynamic Simulation: Keeps remote bodies dynamic and corrects them toward authoritative snapshots.
  • Remote Snapshot Playback: Plays remote snapshots through a passive representation instead of independently simulating control forces.

See Networking and Network Settings before changing remote simulation behavior.

CAT Physics settings

Configure project-wide safeguards in Project Settings > CAT Physics:

  • Velocity Limit: Optional soft and emergency limits for excessive linear or angular velocity.
  • Actuator Output: Optional Fan and Motor output reduction near configured speed limits.
  • Post-Attach Collision Recovery: Teleport correction and bounded smooth separation for overlap introduced by runtime attachment.
  • Diagnostics: Physics Assist debug drawing and reporting controls.

These settings protect numerical stability and recover from exceptional states. They should not compensate for incorrect mass, collision, Connector frames, actuator axes, or Controller tuning.

Diagnostics

Use Physics Assist debug drawing, Assembly execution reports, console commands, and NetworkPerf tools to investigate development builds. They are diagnostic tools, not shipping gameplay APIs.

Start with the Assembly revision, body transforms and velocities, active execution mode, and actuator output. Change Set Physics Sync Tier only when a tested network or performance policy requires a different snapshot tier; see Network Settings.

Tuning order

  1. Verify body mesh, collision, physics mode, mass, and transform.
  2. Verify Connector frames and Constraint Profiles.
  3. Tune the Aerodynamic or Buoyancy Feature without Controller correction hiding the result.
  4. Verify Motor and Fan axes, application points, force, and torque.
  5. Tune the Ground Drive, Aircraft, Watercraft, or custom Controller.
  6. Enable or tighten CAT Physics safeguards only after the authored setup behaves correctly.