Prof. Roger CrawfisComputer Science & Engineering

The Ohio State University

Advanced Game Design

Course OverviewProject-based / Team-based

Little lecturingFocus on programming for games

Systems integration – graphics, sound, AI, networking, user-interfaces, physics, scripting

Utilize higher-level toolkits, allowing for more advanced progress while still developing programming skills.

Course StructureWe will lecture for about the first two

weeks.Student game project groups will provide

several presentations on their game ideas and progress.

Student technology teams will provide an intermediate and an advanced lecture on their findings and analysis about their area.

Project GoalsLarge-scale software developmentTeam-based (synergistic development)Toolkit-based (fast-start development)Learn and utilize the many non-graphical

elements needed for games.Leverage and extend your graphics and AI

expertise.

ElementsGaming Engine

Responsible for providing primitivesHardware abstractionHandle different areas of the game

Physics, AI, etc.

GameDefined by a genreDefines the gameplay

Requirements of a gaming engineStunning VisualsArtificial IntelligenceImmersive sound stageSimulationAnimationNetworking

Requirements of a gameScriptingVaried input/output devicesSupporting Tools

Optimizing game contentDeveloping game contentExtending game contentDebugging / Tuning of game performance

Stunning VisualsAdding realismSmarter ModelsClutterUse hardware

Bump-mappingDynamic water or other liquidsRich textures (Billboards, gloss-maps, light-

maps, etc.)ShadowsParticle systems

Artificial IntelligenceGames need specialized AI

StrategyPath findingModeling behaviorLearning

Immersive sound stageMulti-track sound supportPositional sound effects (3D immersion)Dynamic sounds / movement (doppler

effects)

Input devicesCommonly available devices are

Keyboard, mouse, gamepads and joysticks

Force feedback (haptic) devices are gaining popularitySteering wheelsJoysticksMotion tracking

Output devicesMultiple monitorsHead mounted displays

ScalabilityMultiple hardware capabilitiesMulti-resolution modelsMulti-user supportLOD

Multiple model definitionsMulti-res modelsSubdivision surfaces

ScalabilityMultiple hardware capabilitiesMulti-resolution modelsMulti-user supportLOD (Level-Of-Detail)

Control – when to switchConstruction – what levels to have

AnimationLinear transformationsModeled animations

Articulated motionLip syncingFacial Expressions

Blending animations

Networking

Multi-player support essentialCommon problems

LatencySynchronizationScalabilityConsistent game stateSecurity

ScriptingStrict coding is tedious Support for scripting is essential for RADScripting has added a whole new fun factor

for many games.

Artificial IntelligenceGames need specialized AI

StrategyPath findingModeling behaviorLearning

Non-perfect!Fast!

ToolsCreating varied content

models, video, images, soundIntegrating content

Common file format supportSupporting existing popular tools via plug-

ins3DS Max, Lightwave, Maya etc.Adobe premier, Adobe Photoshop

Interactive ProgramsGames are interactive systems - they must

respond to the userHow?

Interactive Program Structure

Event driven programmingEverything happens in

response to an eventEvents come from

two sources:The userThe system

Events are also called messagesAn event causes a

message to be sent…

Initialize

User Does Somethingor

Timer Goes Off

System Updates

User EventsThe OS manages user input

Interrupts at the hardware level …Get converted into events in queues at the

windowing level …Are made available to your program

It is generally up to the application to make use of the event stream

Windowing system / Game Framework may abstract the events for you

Polling for Events

Most windowing systems provide a non-blocking event functionDoes not wait for an event, just returns NULL if one

is not readyWhat type of games might use this structure?Why wouldn’t you always use it?

while ( true )if ( e = checkEvent() )

switch ( e.type )…

do more work

Waiting for Events

Most windowing systems provide a blocking event functionWaits (blocks) until an event is availableUsually used with timer events. Why?

On what systems is this better than the previous method?

What types of games is it useful for?

e = nextEvent();switch ( e.type )

…

The Callback AbstractionA common event abstraction is the callback

mechanismApplications register functions they wish to

have called in response to particular eventsTranslation table says which callbacks go with

which eventsGenerally found in GUI (graphical user

interface) toolkits“When the button is pressed, invoke the callback”Many systems mix methods, or have a catch-all

callback for unclaimed eventsWhy are callbacks good? Why are they bad?

Upon Receiving an Event …Event responses fall into two classes:

Task events: The event sparks a specific task or results in some change of state within the current modeeg Load, Save, Pick up a weapon, turn on the lights, …Call a function to do the job

Mode switches: The event causes the game to shift to some other mode of operationeg Start game, quit, go to menu, …Switch event loops, because events now have different

meanings

Software structure reflects this - menu system is separate from run-time game system, for example

Real-Time Loop

At the core of interactive games is a real-time loop:

What else might you need to do?The number of times this loop executes

per second is the frame rate# frames per second (fps)

while ( true )process eventsupdate animation / scenerender

LagLag is the time between when a user does

something and when they see the result - also called latencyToo much lag and causality is distortedWith tight visual/motion coupling, too much lag makes

people motion sickBig problem with head-mounted displays for virtual reality

Too much lag makes it hard to target objects (and track them, and do all sorts of other perceptual tasks)

High variance in lag also makes interaction difficultUsers can adjust to constant lag, but not variable lag

From a psychological perspective, lag is the important variable

Computing LagLag is NOT the time

it takes to compute 1 frame!

What is the formula for maximum lag as a function of frame rate, fr?

What is the formula for average lag?

Process input

Update state

Render

Process input

Update state

Render

Process input

Frame time

Lag

frlag

frlag

average

5.1

2max

Event time

Frame Rate QuestionsWhat is an acceptable frame rate for twitch

games? Why?What is the maximum useful frame rate?

Why?What is the frame rate for NTSC television?What is the minimum frame rate required

for a sense of presence? How do we know?How can we manipulate the frame rate?

Frame Rate Answers (I)

Twitch games demand at least 30fs, but the higher the better (lower lag)Users see enemy’s motions soonerHigher frame rates make targeting easier

The maximum useful frame rate is the monitor refresh rateTime taken for the monitor to draw one screenSynchronization issues

Buffer swap in graphics is timed with vertical sweep, so ideal frame rate is monitor refresh rate

Can turn of synchronization, but get nasty artifacts on screen

Frame Rate Answers (II)NTSC television draws all the odd lines of the

screen, then all the even ones (interlace format)Full screen takes 1/30th of a secondUse 60fps to improve visuals, but only half of each

frame actually gets drawn by the screenDo consoles only render 1/2 screen each time?

It was once argued that 10fps was required for a sense of presence (being there)Head mounted displays require 20fps or higher to

avoid illnessMany factors influence the sense of presencePerceptual studies indicate what frame rates are

acceptable

Reducing Lag

Faster algorithms and hardware is the obvious answerDesigners choose a frame rate and put as much into the

game as they can without going below the thresholdPart of design documents presented to the publisherThreshold assumes fastest hardware and all game features

turned onOptions given to players to reduce game features and improve

their frame rateThere is a resource budget: How much of the loop is

dedicated to each aspect of the game (graphics, AI, sound, …)

Some other techniques allow for more features and less lag

Decoupling ComputationIt is most important to minimize lag between

the user actions and their direct consequencesSo the input/rendering loop must have low latency

Lag between actions and other consequences may be less severeTime between input and the reaction of enemy can be

greaterTime to switch animations can be greater

Technique: Update different parts of the game at different rates, which requires decoupling themFor example, run graphics at 60fps, AI at 10fpsDone in Unreal engine, for instance

Animation and SoundAnimation and sound need not be

changed at high frequency, but they must be updated at high frequencyFor example, switching from walk to run can

happen at low frequency, but joint angles for walking must be updated at every frame

Solution is to package multiple frames of animation and submit them all at once to the rendererGood idea anyway, makes animation

independent of frame rateSound is offloaded to the sound card