FixedStep vs Update

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Understanding the difference between fixed-step simulation and variable-timestep updates in ZenECS.

Overview

ZenECS uses a 3-phase frame structure:

  1. BeginFrame (Update): Variable-timestep input and sync
  2. FixedStep (FixedUpdate): Deterministic simulation
  3. LateFrame (LateUpdate): Variable-timestep presentation

Frame Structure

BeginFrame (Update)

Runs every frame with variable delta time:

  • FrameInput Systems: Device input, window events
  • FrameSync Systems: Camera, client prediction

Use for:

  • Input sampling
  • Camera movement
  • UI updates
  • Non-deterministic operations

FixedStep (FixedUpdate)

Runs with fixed timestep (deterministic):

  • FixedInput Systems: Player input sampling
  • FixedDecision Systems: AI, pathfinding
  • FixedSimulation Systems: Physics, gameplay
  • FixedPost Systems: Cleanup, events

Use for:

  • Gameplay logic
  • Physics simulation
  • Network synchronization
  • Deterministic operations

LateFrame (LateUpdate)

Runs every frame with variable delta time:

  • FrameView Systems: Interpolation, transforms
  • FrameUI Systems: UI, HUD updates
  • Binding Application: View bindings applied

Use for:

  • Visual presentation
  • UI updates
  • Interpolation
  • View synchronization

System Groups

FixedGroup

For deterministic simulation:

[FixedGroup]
public class MovementSystem : ISystem
{
    public void Run(IWorld world, float deltaTime)
    {
        // deltaTime is fixed (e.g., 1/60)
    }
}

FrameGroup

For variable-timestep updates:

[FrameGroup]
public class CameraSystem : ISystem
{
    public void Run(IWorld world, float deltaTime)
    {
        // deltaTime is variable (actual frame time)
    }
}

When to Use Which

Use FixedGroup For

  • ✅ Physics simulation
  • ✅ Gameplay logic
  • ✅ AI systems
  • ✅ Network synchronization
  • ✅ Deterministic operations

Use FrameGroup For

  • ✅ Input handling
  • ✅ Camera movement
  • ✅ UI updates
  • ✅ Visual effects
  • ✅ Non-deterministic operations

Examples

FixedStep System

[FixedGroup]
public class PhysicsSystem : ISystem
{
    public void Run(IWorld world, float deltaTime)
    {
        // deltaTime is always 1/60 (fixed)
        // Deterministic physics simulation
        foreach (var (entity, pos, vel) in world.Query<Position, Velocity>())
        {
            // Physics calculations
        }
    }
}

Variable-Timestep System

[FrameGroup]
public class CameraSystem : ISystem
{
    public void Run(IWorld world, float deltaTime)
    {
        // deltaTime is variable (actual frame time)
        // Smooth camera movement
        foreach (var (entity, pos) in world.Query<Position>())
        {
            // Camera interpolation
        }
    }
}

Best Practices

✅ Do

  • Use FixedGroup for gameplay: Deterministic simulation
  • Use FrameGroup for presentation: Smooth visuals
  • Separate concerns: Don't mix fixed and variable timestep
  • Test determinism: Verify fixed-step systems are reproducible

❌ Don't

  • Don't mix timesteps: Keep fixed and variable separate
  • Don't use variable time for gameplay: Breaks determinism
  • Don't use fixed time for visuals: Causes stuttering

See Also

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