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Arcade Game Animation PDF

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Chapter 9 Arcade Game Animation In this chapter, you’ll learn about: (cid:1) Animation (cid:1) Animation properties and fundamentals (cid:1) Sprites and grid squares (cid:1) Core arcade game animation techniques (cid:1) Creating animation sequences for arcade games (cid:1) General animation tips 293 294 Chapter 9 / Arcade Game Animation What is Animation? Animationistheprocessthatproducestheillusionofmovement.Itworksbydis- playingtwoormoreimagefragmentscalledframes(alsocommonlyreferredtoas cells).Whentheseframesaredisplayedinrapidsuccessionwithsubtlechanges madetotheircontent,oureyesregisterthesechangesasmovement. Animationisnotamysticalart.Rather,it’sawell-establishedprocessthatcom- binestheaestheticsofdesignwithreal-worldphysicsinordertobreathelifeinto whatareotherwisestaticobjectsandscenes.Thischapterwillintroducethefun- damentalconceptsbehindanimationtoyousothatyoucancreateandimplement animationinyourownarcadegameprojects. Animation Properties and Fundamentals Tobeabletocreateeffectiveanimation,youmustlearnhowtodividetheele- mentsofmotionintotheirbasiccomponents.Thismeansbreakingthemdown intoasequenceofeasy-to-followframes.However,beforeyoucandothis,you mustfirstlearnandmastertwothings:theartofobservationandthecharacteris- ticsofbasicmotion. Thesecretbehindcreatinggreatanimationliesinkeenobservationandtheability tofocusonthesubtledetailsofhowdifferentobjectsmove.Everyobjectexhibits certainpeculiaritiesasitmoves.Someoftheseidiosyncrasiesareslightwhile othersaremorepronounced.Assuch,thereareseveralcharacteristicsofmotion thatyoushouldbeawareofbeforeattemptingtoanimateanobject.Thesecharac- teristicsincludesuchthingsas: (cid:2) Motionlines (cid:2) Motionangles (cid:2) Key-framesandin-betweens (cid:2) Weightandgravity (cid:2) Flexibility (cid:2) Secondaryactions (cid:2) Cyclesandloops (cid:2) Tempo Motion Lines Amotionline(sometimesreferredtoasanaturalpath)isaninvisiblelinecreated byanobjectasitperformsaseriesofsequentialmovements. 295 Chapter 9 / Arcade Game Animation Motion Line FIGURE9-1:MotionLineExample Motionlinesareessentialtocreatingeffectiveanimations,andmanipulatingthe motionlinecanaddrealisticemphasistoanimatedobjects.Forexample,youcan createverysmoothanimationsbymakingsmallalterationstothemotionline. Conversely,youcanproduceverydramaticanimationsbymakinglargeorexag- geratedchangestothemotionline. Evenmoreinterestingtotheanimatorishowobjectsproducedifferentshaped motionlinesdependingonhowtheymove.Forexample,abullethasamotionline thatisstraightandevenwhileabouncingballhasamotionlinethatiswavyand uneven.Thisbeingsaid,motionlinesmustbeconsistentwithanobject’sreal- worldbehaviorinordertoproducerealistic-lookinganimation.Otherwise,the qualityoftheanimationwillsuffer.Therefore,youcanuseanobject’smotionline asameansofdeterminingwhetherornotitisbeinganimatedcorrectlyand convincingly. Thebestwaytofollowanobject’smotionlineistolocateitscenterofgravity.The locationofthecenterofgravityvariesaccordingtothetypeofobjectinvolved. Tohelpyouaccomplishthis,Table9-1providessomeexamplesofwherethecen- terofgravityisforanumberofcommonobjects.Usingthisinformation,you shouldthenbeabletoidentifythecenterofgravityforothertypesofobjects. TABLE 9-1: Location of the Center of Gravity in Different Objects ObjectType EstimatedCenterofGravity 2-legged animals Head 4-legged animals Chest Flying animals Torso Humans Head Insects Torso Spaceships or airplanes Hull or fuselage Tracked or wheeled vehicles Turret or body 296 Chapter 9 / Arcade Game Animation Motion Angles Motionanglesareoneofthemostobviouscluesastoanobject’sdirectionasit moves.It’simportanttopointoutthatthereisadirectrelationshipbetweenan object’sdirectionanditsmotionangle.Almostanychangeinanobject’sspeedor directionwillrequireasimilaradjustmenttoitsmotionangle.Therefore,the sharperthemotionangle,thefasterorthemoreextremethechangeinthe object’smotionordirectionwillbe. Motionanglesareparticularlyusefulforconveyingasenseofrealisminanimated objects.Forexample,ajetfightermakingasteepbankwillhaveamotionangle thatissharperthanajetfighterthatisflyingstraightandlevelasshowninFigure 9-2.Inthiscase,youcanuseitsmotionangletovisuallydiscernthatitistravel- ingatahighspeed,whichultimatelyhelpstoreinforcetheillusionofrealism. Althoughtheactuallocationofmotionanglesvaries,mostmotionanglesare locatedalongthespineofanobject,i.e.,thebackboneofahumanbeingorthehull ofaspaceship. StraightMotionAngle SharpMotionAngle FIGURE9-2:MotionAngleExample Key-frames Mostpeopleareawareoftheextremesthatoccurduringmovement,i.e.,such noticeablethingsastheflappingofwingsorkickingoflegs.Inanimation,werefer totheseactionsaskey-frames. Beingabletodeterminewhichframesinananimatedsequencearethe key-framesisanextremelyimportantpartoftheanimationprocess.Thisis becausekey-framesserveastheframeworkfortheentireanimationsequence. Inaddition,thereisadirectrelationshipbetweenthenumberofkey-framesused andhowsmoothaparticularanimationappears.Thepresenceofmorekey-frames inananimatedsequencemeanssmallerincrementalchangesintheanimationand 297 Chapter 9 / Arcade Game Animation resultsinsmootheroverallmovement.Havingfewerkey-framespresent,onthe otherhand,resultsincoarserandjerkieranimation.Pleasekeepthisveryimpor- tantrelationshipinmindasithasadirecteffectonthequalityofanyanimation youcreate. Key-framesaremosteffectivewhentheyincorporateveryexaggeratedordra- maticelements,sincetheseactionscanbeusedtoemphasizethemostcritical movementsinananimatedsequence.Inaddition,exaggerationcanhelpyouto betterdeterminethemosteffectivestarting,middle,andending pointsforananimatedsequence.Forexample,Figure9-3shows thetwokey-framesforabirdflying.Noticehowtheymirrorthe twoextremestatesoftheaction,i.e.,thewingmovingupand thewingmovingdown.Thesetwoframesareextremelyexag- FIGURE9-3: gerated,whichmakesthemideallysuitedaskey-framesbecause Key-frame theirdifferencesareclearlydistinguishabletotheobserver. Example It’simportanttorealizethatthemorekey-framesaparticularanimationhas,the morecomplexitisandthelongeritwilltaketodesign.Thisiscertainlysome- thingtoconsiderwhendesigningtheartworkforanarcade-stylegame,especially whenworkingunderatightdeadline.Inadditiontotakinglongertocreate,com- plexanimationsmakeiteasiertointroduceerrorsandmistakesintotheanimation sequence,whichcanhaveanegativeimpactontheanimation’soverallqualityand effectiveness. Inreality,however,theactualsituationdictateswhichapproachtotakeandhow manykey-framestouseforagivenanimation. Therewillbeinstanceswhereyoucangetawaywithusingfewerkey-framesthan inothers.Inmostcases,youcanuseasfewastwoorthreekey-framesperobject inarcade-stylegames,withlittleornodetrimentalimpact.However,youshould lookateachgameonacase-by-casebasisbeforedecidingonaparticularnumber ofkey-framestouse.Ifyoudon’t,youruntheriskofreducingthequalityofyour game’sanimationsand,ultimately,thequalityofthegame. PleaserefertoTable9-2forgeneralsuggestionsonkey-frameusageindifferent arcadegamegenres. TABLE 9-2: Object Key-frame Quantity Suggestion Chart ArcadeGameType Key-frameUsageSuggestions Pong games 2 per animated object Maze/chase games 2-3 per animated object Puzzlers 2-3 per animated object Shooters 2-4 per animated object Platformers 2-6 per object 298 Chapter 9 / Arcade Game Animation NOTE: These are subjective estimates only. Each game situation will be differ- ent and will depend on the design time, the game’s target platform, and the overall level of animation quality you want to achieve. It’simportanttonotethatkey-framescanoccuratanypointwithinananimated sequence.However,certainfactorssuchasthetypeofanimationinvolvedandits relativecomplexitycaninfluencewhereinthesequencetheymightactually appear.Forexample,non-repetitivemotionssuchasexplosionshavemany key-framesandtendtobelocatedatseveralpointswithintheanimationsequence orwhereverthereisamajorchange.Incomparison,repetitivemotionssuchas walkingorflyinghaveonlyafewkey-frames(i.e.,twoorthree).Thesetendtobe distributedatthestart,middle,orendoftheanimationsequence. Afterthekey-framesoftheanimationareidentifiedandestablished,the in-betweenframesmustthenbeadded.In-betweensareframesofanimationthat areusedtosmoothoutthetransitionbetweenindividualkey-frames. Inanimation,key-framesareimportantfordefiningtheobject’sprimarymove- ments,whilein-betweensareresponsibleformakingtheentireanimationlook smoothandconvincing.Introducingslightorsubtlechangestoeachframe betweenkey-framescreatesin-betweens.Thisprocessisalsoknownastweening, andgreatcaremustbetakentoensurethatthesechangesaresmallinorderto maintaintheillusionofcontinuousandrealisticmovement. Figure9-4showsanexampleofhowin-betweensco-existwithkey-frames.This exampleshowsananimationofamanswingingastick.Thestartofthesequence showsthemanwiththestickupattheshoulderandreadytoswing,whilethe finalframeshowstheendoftheswingingprocesswiththestickfullyextended. Thesearethekey-framesoftheanimationwhiletheframesthatoccurin-between themarethein-betweensoftheanimation. Key-frame Key-frame Key-frame In-Between In-Between FIGURE9-4:Key-frameandIn-BetweenExample 299 Chapter 9 / Arcade Game Animation Creatingin-betweensisoneofthemosttediousandtime-consumingaspectsof designingarcadegameanimation.Fortunately,mostpaintingprogramsallowyou toeasilyduplicateexistingframessothateachin-betweendoesn’thavetobe re-createdentirelyfromscratch.Intraditionalanimation,theprocessofduplicat- inganexistingframeofanimationtocreateanewoneiscalledonionskinning. Determiningthenumberofframesnecessaryforcreatingaparticulartypeofani- matedmovementtakestimeandexperiencetofigureout.Tohelpyouonyour way,Table9-3liststhenumberofframesrequiredtoproduceavarietyofdifferent animationeffects.Usetheseasageneralguideiftheissueiseverinquestionin yourowngameprojects. TABLE 9-3: Common Frame Animation Frame Requirements Object Minimum#ofFrames Maximum#ofFrames 4-legged animal running 4 16 Animal biting 2 5 Crawling 2 12 Explosions 5 16 Falling 3 5 Flying 2 12 Jumping 2 10 Kicking 2 6 Punching 2 6 Rotating/spinning 4 16 Running 2 12 Swinging (an object) 2 8 Throwing (an object) 2 6 Vehicle flying 2 4 Vehicle moving 2 8 Walking 2 12 Weight and Gravity Ifyouintendtocreaterealistic-lookinggameanimations,youmustconsiderhow theelementsofweightandgravitycanaffectyourwork.Whendesigninganima- tion,youmustrememberthatreal-worldphysicsshouldalwaysapplytothe sequencesthatyoucreate.Oneofthemostimportantoftheseinfluencesisthatof weight.Weightaffectsspeed.So,forexample,thelargertheobject,theheavierit isandthesloweritislikelytomove. 300 Chapter 9 / Arcade Game Animation Inadditiontoinfluencingthespeedatwhichobjectstravel,weightcanalsoaffect howeasilyanobjectcanmove.Forexample,thinkabouthowfastaracecarmoves incomparisontoabattletank.Thetankwillplodacrossthegroundwhereasthe racecarwillquicklyskimacrossit.Thisisbecausetheracecarweighsless. Thenthere’stheissueofgravity.Gravityalsoinfluencesthemotionofobjects.In therealworld,gravitywillexertmoreforce(resistance)againstheavierordenser objectsthanitwillagainstlighterones.Toseehowthisworks,considerhow quicklyabombfallsfromtheskywhencomparedtoafeather,orhowarock bouncesoffthegroundincomparisontoarubberball.Rememberthatpeople aren’talwayseasilyfooled.Believeme,theyarewellawareofsuchthingsand failingtoaccountforthesebehaviorsinyouranimationscanhaveanegative impactonhowtheyareperceivedinagame. Incidentally,theprinciplesofweightandgravitycanbeappliedtovirtuallyany animatedobject.Therefore,themoreattentionyoupaytothem,themorerealistic youranimationscanbe. Flexibility Flexibilityisessentialtoproducingconvincingandfluidanimation,particularly whendepictingcomplex,jointedobjectssuchasanimals,insects,andhuman beings.Animationswithoutproperflexibilitycanappearstiffandrigid,whichfor thesetypesofobjectsisalessthandesirableeffect. Thekeytoaddingflexibilitytoyouranimationsiscarefulplanningcoupledwith carefulobservationofhowdifferentobjectsbehavewhentheymove.Whenever youanimateajointedobject,youmustalwaysdeterminewhichpartsoftheobject (i.e.,arms,legs,etc.)leadthemovementandwhichonesfollowit.It’sveryimpor- tanttorealizethatnotallbodypartsactuallymoveatthesametimeonallobjects. Forexample,considerhowaswordsmanmightslashwithhissword.Theswords- manleadswithhislegsfirsttogainasolidfootingandthenfollowsthroughwith hisarmtocompletethecuttingmotion.Inanimation,thisflowofmovementis calledtherangeofmotion. Whenattemptingtoincorporateflexibilityintoananimationyoumusttakecareto accountforthelimitsofanatomy.Thatistosay,neverattempttounrealistically extendtheavailablerangeofmotionthatagivenobjecthas.Don’tdepictobjects movinginwaysthataren’tpossibleintherealworld.Examplesofthismight include,butaren’tlimitedto,suchthingsasbendingajointbackwardorinaposi- tionthatisnotnormallypossibleforonetomake. 301 Chapter 9 / Arcade Game Animation Secondary Actions Asmentionedintheprevioussectiononflexibility,notallpartsofanobjectmove simultaneouslywhenanimated.Therearepartsthatleadtheflowofmovement andpartsthatfollowit.Thepartsthatfollowthemovementarecalledsecondary actions. Secondaryactionsareextremelyimportanttotheanimationprocessbecausethey addanextradimensionofrealismtoanimations.Essentially,anythingthatisfree movingcanqualifyasasecondaryaction.Thisincludeseverythingfromhairand clothingtoeyesandlips.So,forexample,ifacharacterinagameiswearinga capeandwalking,thesecondaryactionofthisactionwouldoccurwhenthechar- acter’scapebouncesandswaysasthecharactermoves.Figure9-5showsthisin frames1and2. Secondary Action 1 2 3 4 FIGURE9-5:SecondaryActionExample Secondaryactionsarenotlimitedtosmalldetailssuchasclothingorhair.It’s importantthatyouunderstandthattheycanbevirtuallyanythinginananimation sequencethatisn’tthemainfocus. Cycles and Loops Inanimation,cyclesaretherepetitiousmovementsthatmanyanimatedobjects makewhendisplayedinasequence.Asyourabilitytoobservehowobjectsmove innatureimproves,youwillsoondiscoverthatmanyobjectsincludecyclesin theirmovements. Cyclescanbeconsideredthetimesaversoftheanimationprocess.Theyhelpus avoidthetediumofconstantlyhavingtore-createframestoconstructbasic actions.Forexample,withoutcyclesyoumighthavetodrawhundredsofframes ofanimationtoshowabirdflyingacrossthescreen.However,byusingcycles, yousimplyhavetoidentifytheobject’srepeatedmotionsanddisplaythem instead.So,ifyourbirdanimationused30framestodisplaythecompleteanima- tionsequenceyouwouldonlyhavetodrawthethreeorfourframesthatbest 302 Chapter 9 / Arcade Game Animation representthemajorpointsofmovement.Althoughkey-framesusuallyfallinto thiscategory,it’simportanttorealizethatthereisn’talwaysadirectone-to-one relationshipbetweenkey-framesandcycles. Whilecyclesfocusonrepeatingtheoccurrenceofspecificframeswithinananima- tionsequence,loopsemphasizetherepetitionoftheentiresequenceasawhole. Forexample,ananimatedsequenceofamanwalkinggoesthroughanumberof framestoproducetheillusionofmovement.Byloopingthesequence,youcan simulatetheeffectofconstantmotionsothatthewalkingsequenceappearstobe continuous.Therefore,itcanbesaidthatloopshelpustokeepanimatedmove- mentconstant. However,thisbeingsaid,it’simportanttounderstandthatnotallobjectsrequire constantmotionwhilebeinganimated.Loopsareadevicetohelpusachievethis buttheyshouldn’tbeusedinallanimations.Infact,quiteafewtypesofanimation don’trelyonloopsatall. Inadditiontoaddingrealismandcontinuitytoyouranimations,loopingcanalso saveyoutimebypreventingyoufromhavingtomanuallyrepeattheindividual framesoftheentireanimationsequence. Figure9-6showsanexampleofhowcyclesandloopsworktogetherinanima- tions.Here,frames1and5arecyclesbecausetheyarerepeatingthesameframe ofanimation.Whenthesequencereachesframe5,itwouldloopbacktoframe1to createtheillusionofcontinuousmovement. FIGURE9-6:CycleandLoopExample Tempo Everyanimatedobjectcandisplayataspecificedtempo,orspeed.Tempocanbe usedtocontroltherateatwhichentireanimationsequencesareshownaswellas thespeedoftheindividualframesthatcomprisethesequence. Tempoisimportantbecauseitallowsustocreatemorerealistic-lookinganimation sequences.Itsusefulnessbecomesimmediatelyapparentwhenyouconsiderthat mostreal-worldobjectsmoveatdifferentratesduringthecourseoftheirmove- ments.Forexample,considertheaveragemarathonrunner.Whenrunning,a marathoner’slegsmovefasterduringmid-stridethantheydowhentheyarefully extended.Youcanaccountforthisfactinyourownanimationsbyusingthedis- tanceoftheobjectbetweensuccessiveframesasameansofdepictinganimations

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w Creating animation sequences for arcade games w General . ent and will depend on the design time, the game's target platform, and the overall level of .. sprites can be created using most of the painting programs mentioned in.
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