Optimizing Game Loops: Fixed Vs. Variable Time Steps

The game loop is the central component that drives the timing and frame rate of a game. This infinite loop runs continuously during gameplay, updating game states, processing user input, rendering graphics, and handling other tasks required for a smooth gaming experience.

At the core of the game loop is the time step – the amount of real time that passes during each iteration of the loop. The time step directly impacts the overall timing and consistency of the games’s visuals and simulation. Game developers have two main options when implementing the game loop time step:

Fixed time steps – Using a constant time step that does not change.
Variable time steps – Allowing the time step to vary dynamically based on real-time performance.

Both fixed and variable game loop time steps have their own unique tradeoffs and implications when it comes to optimization and overall gameplay quality. Identifying the right approach is crucial for achieving high and consistent frame rates matched to the game’s style.

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Example Code of a Basic Game Loop

Here is C++ code for a game loop using a fixed time step. The key aspects are:

Target timestep constant – E.g. 16.6ms for 60 FPS cap
Timing and interpolation via accumulators
Game state updates inside fixed time step loop
Rendering and buffer swaps after updates

const float TARGET_TIMESTEP = 0.0166f; // 60 FPS 

float lastFrameTime = 0.0f;
float accumulator = 0.0;

while (gameIsRunning) 
{
  float currTime = getSystemTime();
  float deltaTime = currTime - lastFrameTime;
  lastFrameTime = currTime;

  accumulator += deltaTime;

  while(accumulator >= TARGET_TIMESTEP) 
  {
    // Fixed timestep update logic
    UpdateGameState(TARGET_TIMESTEP); 
    
    accumulator -= TARGET_TIMESTEP;		
  }

  // Interpolate between updated states
  LerpGameStates(accumulator / TARGET_TIMESTEP); 
  
  // Render, swap buffers, etc
  RenderAndPresentScene();
}

This example shows how fixed timesteps combined with interpolation and accumulation help maintain consistent game timing.

Choosing Between Fixed and Variable Time Steps

So which approach to game loop timing should you choose – fixed or variable? There are worthwhile tradeoffs to understand for both options before deciding.

The Pros and Cons of Fixed Time Steps

Using a predetermined and constant time step value has some notable advantages:

Consistent Gameplay

The most significant benefit of fixed time steps is consistent overall gameplay behavior. With an unwavering time step driving state updates, the game logic and responses remain reliable and steady during runs. This prevents strange variability in mechanics tied to frame timings.

Possible Wasted CPU Cycles

The downside of fixed time steps manifests when frame rates surpass the target FPS cap. Some CPU cycles inevitably get wasted waiting to synchronize again at the next time step. So the gameplay remains steady, but hardware utilization decreases past a point.

The Pros and Cons of Variable Time Steps

In contrast, variable game loop time steps that change dynamically have their own set of tradeoffs:

Efficient CPU Usage

By allowing the time step to scale fluidly in line with real-time CPU speeds, no wasted cycles occur. Hardware resources get maximized by adapting the timing instead of waiting. This efficiency can translate into higher peak frame rates.

Inconsistent Gameplay

The main pitfall of variable time steps emerges in gameplay consistency at different frame rates. State updates tied to unsteady timings can cause overt differences in mechanics between low, medium, and high FPS scenarios.

These pros and cons highlight why choosing between fixed and variable time steps involves balancing tradeoffs around consistency and performance.

Hybrid Approaches for Optimization

Rather than viewing fixed and variable time steps as mutually exclusive options, hybrid game loop designs can optimize further. Some examples include:

Using Both Fixed and Variable Time Steps

Certain game states and systems can retain fixed time steps for consistency, while others use variable timesteps for better performance. For example, a physics engine may update at 60 FPS while other systems scale up.

Adaptive Time Steps Based on Gameplay Conditions

Time steps could start fixed but then switch to variable steps during intense gameplay moments where peak FPS matters more. This balances steady behavior with dynamic performance.

Benchmarking Different Approaches

Testing gameplay using a range of time step constants and targets helps determine optimal settings. Lower time steps increase consistency at the cost of performance.

Blending fixed and variable timesteps alongside other adaptations allows game developers to have their cake and eat it too.

Finding the Right Balance for Your Game

Identifying ideal game loop time step behavior requires analyzing gameplay needs, target platforms, and testing.

Factors to Consider Like Gameplay Style and Target Hardware

Game genres with precise mechanics like fighters and rhythm games demand fixed time steps throughout. Slower platformers can use more variable timings. Target console and computers also determine optimal FPS caps and timestep choices.

Tuning Time Steps During Development and Testing

Iteratively tweaking time step values and FPS caps while observing gameplay helps pinpoint configurations that balance optimization targets. Playtest analytics guides tuning.

Tools for Analyzing Frame Timing Consistency

In-engine tools like caution buffers visualize frame pacing issues, helping validate time step settings. Targeting 99th percentile frame times ensures smooth overall experiences.

Considering these factors throughout development leads to game loops with time steps tailored precisely for gameplay needs and hardware capabilities, crucial for both fun and performance.