FixedUpdate Internals: Step Counting, Spiral of Death Prevention, Time.Scope, and ApplyForce vs ApplyImpulse

FixedUpdate Internals: How the Fixed Timestep Works in s&box

The s&box fixed update system uses a step-counting approach rather than accumulating time. This is important to understand when debugging physics or timing issues.

How It Works

The FixedUpdate class tracks a _step counter. Each frame:

1. curStep = floor( time / delta ) — compute the expected step count at the current time
2. _step is clamped to [curStep - maxSteps, curStep] — prevents spiral of death
3. While _step < curStep, increment _step and call fixedUpdate() with the correct time scope

internal void Run( Action fixedUpdate, double time, int maxSteps )
{
    var delta = Delta;
    long curStep = (long)Math.Floor( time / delta );

    // Clamp to prevent spiral of death
    _step = long.Clamp( _step, curStep - maxSteps, curStep );

    while ( _step < curStep )
    {
        _step++;
        using var timeScope = Time.Scope( (_step * delta), delta );
        fixedUpdate();
    }
}

Key Behaviors

• Time.Delta inside OnFixedUpdate() is always exactly 1 / FixedUpdateFrequency — it is set via Time.Scope.
• Time.Now inside OnFixedUpdate() is the simulation time for that step, not the real frame time.
• If the game falls behind (e.g., a frame takes too long), maxSteps caps how many fixed updates run in one frame. Excess steps are dropped.
• The default maxSteps is ProjectSettings.Physics.MaxFixedUpdates.
• The default frequency is ProjectSettings.Physics.FixedUpdateFrequency.

Time.Scope

using var timeScope = Time.Scope( simulationTime, deltaTime );
// Inside this scope, Time.Now == simulationTime, Time.Delta == deltaTime

This is also used by Scene.Push() to restore the previous scene's time when switching between scenes.

Practical Implications

• TimeSince and TimeUntil use Time.NowDouble, so they work correctly inside OnFixedUpdate().
• Physics forces applied with ApplyForce() are scaled by the physics timestep automatically — you don't need to multiply by Time.Delta.
• ApplyImpulse() is NOT scaled by timestep — it's an instantaneous change.