Balancing Update Frequency, Rendering, And Input Latency In Game Loops

The Core Issue: Frame Rate, Input, and Rendering Conflicts

A game loop needs to balance several conflicting requirements – maintaining a high, consistent frame rate, minimizing input latency so controls feel responsive, and ensuring complex game worlds render efficiently. If the game loop fails at any one of these, the gameplay experience suffers.

For example, a complex 3D scene may require substantial CPU work to determine what is visible and needs rendering. This computational load can force frame rates down. Lower FPS makes games feel sluggish and unresponsive. Meanwhile, delayed reading of player inputs can make control laggy. If the game only samples input at the start of rendering, this lag gets worse.

Ideally, a game needs to sustain high FPS to feel smooth, respond to inputs quickly so controls are snappy, and render efficiently to free up CPU/GPU for better effects. But these goals often conflict – more game logic, physics, AI, and rendering load tend to increase the workload per frame. The core challenge is balancing these systems so no single aspect bogs down overall responsiveness.

Key Metrics: FPS, Input Latency, Draw Calls

There are 3 key metrics that indicate how responsive a game feels:

• Frames Per Second (FPS) – Higher and more consistent FPS makes games feel smoother and allows faster response to input.
• Input Latency – The delay between a player input and its effect on screen. Lower latency feels more responsive.
• Draw Calls – Lower draw calls allow more efficient CPU/GPU usage. Too many can bog down frame rates.

If FPS drops too low, games feel choppy. If input latency rises too high, controls feel laggy and unresponsive. Too many draw calls can overburden system resources, limiting frame rates. A performant game loop needs to balance these factors – maintaining responsive input handling without excessive rendering overhead dragging FPS down.

Managing Rendering Load

Complex game worlds with large meshes, textures, shaders and effects can quickly overwhelm rendering capacity. Several optimizations help manage this load:

Examples: Frustum Culling, Occlusion Culling, LODs

• Frustum Culling – Only render what is within the camera’s field of view
• Occlusion Culling – Skip rendering objects hidden behind terrain, buildings, etc
• Level of Detail (LODs) – Use lower detail models when objects are distant

These techniques ensure only essential visible objects get rendered. This in turn reduces draw calls and speeds up the rendering pipeline, freeing up system resources for better performance.

Optimizing Materials and Draw Calls

Several other strategies help optimize rendering:

• Reduce number of textures and materials exposing less variety to the GPU.
• Batch objects sharing textures/shaders to reduce state changes.
• Use texture atlases packing sprites into a single texture rather than many small ones.
• Simplify overly complex meshes to require less vertex transformations.

With careful planning, scenes can render efficiently while still achieving visual variety through smart reuse of textures and materials balanced against draw call overheads.

Efficient Update Frequencies

In addition to graphics rendering, the game logic also needs updating every frame. This includes:

• Gameplay systems – enemy AI, user controls, cameras, etc.
• Physics simulation – collisions, rag dolls, vehicles, etc.

Updating these subsystems too frequently wastes computation vs visual changes on screen. But updating them too slowly causes visual hitches and lags in response. Variable update rates help balance this.

Physics vs. Gameplay vs. Rendering Updates

Ideally physics, gameplay logic and rendering updates run at different frequencies:

• Gameplay at 30-60 Hz for responsive controls and cameras
• Physics at 30-120 Hz for smooth visuals but not overkill
• Rendering at 30-90 Hz to sustain good FPS

This allows finding an optimal balance. Gameplay always needs high responsiveness. Physics can run slower than rendering without visual distraction. While rendering targets the maximum useful FPS to feel smooth without wasting unused frames.

Variable Time Steps

Rather than fixed updates each frame, variable time steps help balance load. This means:

• Logic updates using a variable DT each frame depending on actual frame time
• Long frame times increase DT for that frame to catch up
• Short frames reduce DT to avoid visual glitches

By smoothing updates across frames, variable steps reduce visual glitches while allowing gracefully backing off workload in tough scenarios – e.g. complex physics enabling simpler collisions to sustain FPS.

Reading Input Late in the Frame

When player input is only sampled at the start of a frame, this introduces an entire frame of latency before seeing the result on screen. Reading input as late as possible reduces this:

Input Buffering

Input buffering accumulates control data as it arrives, applying it just before rendering. This ensuresLatest buttons pressed affect the current frame where possible.

Avoiding Feedback Loops

Sometimes input affects systems which then further modify input later – causing feedback loops. For example, auto aim asssists rotating view direction which then impacts controls.

Keeping input buffering simple avoids these complex interactions polluting responsiveness. Save any expensive input processing for after rendering so visuals remain snappy.

Putting it All Together

Combining all the above concepts allows building a responsive game loop:

Update Order for Responsiveness

Careful ordering ensures user input latency stays low:

1. Gamepad/keyboard input polling
2. Gameplay critical systems – player, camera, weapons, etc
3. Physics simulation
4. Remaining gameplay systems – AI, vehicles, etc
5. Apply final input accumulation buffer
6. Rendering latest game state

This progression ensures input gets applied as late as possible while background tasks get updated early on.

Budgeting to Target Platform

Understanding the CPU/GPU constraints of target devices helps set sensible budgets per frame for:

• Draw calls
• Texture sampling
• Physics/AI updates

Keeping gameplay responsive with headroom for future additions. Avoiding overburdening vulnerable systems. And taking advantage of power in high end machines.

By following these principles – managing workload, variable updates, late input sampling, and sharp focus on key metrics – games can deliver super responsive play balanced against rich presentation and deep mechanics.