UW EXTENSION

CERTIFICATE PROGRAM INGAME DEVELOPMENT

2ND QUARTER:ADVANCED GRAPHICSGame program main loop

Goals

1. Understand the structure of the game’s main loop

2. Know how the other components affect rendering

The game’s main loop Three things to do, periodically:

Gather inputs Mouse, keyboard, game controller,

network Run the game simulation

Physics, AI, scripting, logic Return (render) results

Graphics, audio, force-feedback, network

Gather inputs

Game simulatio

n

Render results

Exe

cuti

on

Data

The game simulation The key, central part that makes gameplay work

If it doesn’t run, there’s no game Inputs and results are as loosely tied to it as possible

Multiple kinds: Constant vs. variable rate Deterministic vs. non-deterministic

Main graphics-related concern: Ensure none of the rendering affects the simulation Isolate… by all means necessary

Non-gameplay simulation tasks For instance, physics is part of the simulation But not all the physics!

Grenade bouncing off walls belongs in the simulation Sparks bouncing off walls don’t belong in the simulation

Eye candy only Also, sometimes rendering uses some simulation code

Local simulation, meant to predict or extrapolate Used to reduce perceived latency

The input Some input is used for the simulation

Shooting or jumping buttons Some input is used only for rendering

Mouse cursor or 1st person camera orientation Latency and granularity are two key concepts

here Low sampling rate: low granularity and higher latency Latency also affected by the structure of the code

Latency Time it takes for the player to see the

reaction to her actions It is a factor of many sequential events Some can be budgeted

Time it takes for the CPU or the GPU to render

Some can’t be controlled Video frame rate

Human beings can perceive latencies of 100 ms or more Some can detect much smaller latencies

Player presses button

Game reads button press

Game simulation finishes

Game sends frame to GPU

GPU renders frame

Half the input sample rate

Frame becomes visible

8.33 ms

Simulation CPU budget

8.33 ms

Render CPU budget8.33 ms

Half the max GPU queue

33.33 ms

Half the video frame rate

8.33 ms

The rendering Must aim for a constant, guaranteed framerate

At the screen’s refresh rate Budgeted for both: CPU and GPU Must be able to throttle back

If frames take too long to render or simulate Keep all animation running at the intended speed Less framerate: more choppiness, not slower

animation

Main loop – Constant-step simulation

startTime = GetTime();while (true){ Process Windows messages (keep it happy) Read simulation inputs time = GetTime(); while (time – startTime > frameTime) { startTime += frameTime; Run simulation } Render frame((time – startTime) / frameTime)}

Main loop – Variable-step simulation

startTime = GetTime();while (true){ Process Windows messages (keep it happy) Read simulation inputs time = GetTime(); Run simulation(time – startTime) startTime = time; Render frame}