RealityKit Physics Speed

Physics speed can be controlled with the clock property of PhysicsSimulationComponent using CMTimebase. Physics speed does not change the delta time between physics ticks. RealityKit physics timestep is fixed at 1/60s. What changes is how frequently those 60Hz ticks happen in real-world time (wall clock).

Below is the basic code setup:

/// The parent entity of the simulation
let anchor = AnchorEntity()
scene.addAnchor(anchor)

/// Get the system's clock
let sourceClock = CMClock.hostTimeClock

/// Create a controllable timebase from source clock
/// A timebase can be paused, sped up, or slowed down
var timebase: CMTimebase?
let status = CMTimebaseCreateWithSourceClock(
    allocator: kCFAllocatorDefault, // Use default memory allocator
    sourceClock: sourceClock, // Base it on the host clock
    timebaseOut: &timebase // Output the created timebase
)

guard status == noErr, let physicsTimebase = timebase else {
    return print("\(self) - Physics Timebase creation failed")
}

/// Set the playback rate
/// - rate = 1.0: Real-time (default)
/// - rate = 0.5: Half speed (slow motion)
/// - rate = 2.0: Double speed (fast forward)
/// - rate = 0.0: Paused
///
/// Example with rate = 2.0:
/// In 1 real-world second, physics executes 120 steps (2× the normal 60)
/// Each step still simulates 1/60s of physics time
/// Result: 2 seconds of physics simulation in 1 real-world second
CMTimebaseSetRate(physicsTimebase, rate: 2.0)

/// Apply the custom timebase to physics simulation
var physicsSimulation = PhysicsSimulationComponent()
physicsSimulation.clock = physicsTimebase
anchor.components.set(physicsSimulation)

/// Now all physics bodies in this simulation will run at the specified speed

This doesn't mean we can simulate 100s worth of physics in one second of CPU time. But lower values such as 0.5× or 2× are usable. Note that slowing down physics produces stuttering motion, not a smooth slow-motion effect, because the minimal time resolution is still 1/60s.

Links

• Apple Documentation, PhysicsSimulationComponent.clock.

• Apple Documentation, CMClockOrTimebase.