Enemy NPC Design Patterns In Shooter Games

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Enemy NPC Design Patterns in Shooter GamesGabriel Rivera, Kenneth Hullett, Jim WhiteheadAugmented Design LabUniversity of California, Santa CruzSanta Cruz, CA 9506gaariver@ucsc.edu, {khullett, ejw} @soe.ucsc.eduABSTRACTGameplay in single-player shooter games primarily consists ofcombat with Non-player Characters (NPCs). However, there hasbeen little research done to study how enemy NPCs affect playerbehavior in shooter games. As a consequence, there is no formallanguage for designers to discuss how NPCs are used in shootergames. This paper presents design patterns for NPCs in shootergames and explores their effects on gameplay. These patterns helpdesigners communicate about and explore new ideas for enemyNPCs and further our understanding about how NPC types can beimplemented, enabling more engaging experiences.Categories and Subject DescriptorsK.8 [Personal Computing]: Games.General TermsDesign, Human FactorsKeywordsgame design, design patterns.1. INTRODUCTIONThe analysis presented here is based on commercially-availablegames in the shooter genre. Shooter games allow players totraverse expansive, 3-dimensional spaces where they interact withthe environment and other entities primarily through combat.This research is focused on the enemy NPCs that populate a givenlevel. Enemy NPCs are controlled by the computer and are themain source of conflict during gameplay. NPCs are one of theelements a designer can use to craft the pacing, challenge, andtension of a level. Designers can control not only where the NPCis placed but also the NPC’s scripted behavior, how they areequipped, their level of health, their level of armor, and othervariables.Designers consider all of these factors when creating a level.There may be a terminology that many designers can use in orderto talk about this; for-instance people can reference boss battles orother encounters. However, there is currently no cataloguedlanguage with a consistent terminology that can be used fordescribing these concepts. Design patterns help solve this problemPermission to make digital or hard copies of all or part of thiswork for personal or classroom use is granted without feeprovided that copies are not made or distributed for profit orcommercial advantage and that copies bear this notice and thefull citation on the first page. To copy otherwise, or republish, topost on servers or to redistribute to lists, requires prior specificpermission and/or a fee.DPG 2012, May 29, 2012, Raleigh, North Carolina, USA.Copyright 2012 ACM 978-1-4503-1854-9/12/05 15.00.by creating a consistent terminology that can be referenced bypeople in the field. This results in people being able tocommunicate high level, overarching concepts faster to each otherwhen design challenges come up. These concepts should beproven patterns that people know will work and that can bepassed down to people newer to the field. By having definedpatterns, it creates a lower barrier to entry for more complicatedconcepts to newer people in the field because there is a now aterminology that can be referenced and studied. Furthermore,patterns can help to explore new ideas when viewing a designchallenge through all the different patterns. Overall, theintroduction of design patterns to enemy NPC types would help tofoster a more creative environment by allowing for clearercommunication across designers of all levels.For this research we explored elements that pertain to all NPCswithin the shooter genre and then analyzed various games to see ifNPCs consistently fell into patterns. Patterns were identified byobserving NPC behavior and discerning which elements werecombined in the same way within a number of games. Eachpattern is accompanied by our observations about how it’s usedby designers to create gameplay, as well as a list of elements thatdefine the pattern.The primary contribution of this work is to create a commonvocabulary for the different types of NPCs found in shooter gamesin order to label the different relationships that these NPCs havewith the game play of the level. Through these patterns, a basicframework should be set for designers to collaborate and work offof in order to create better experiences for the player.2. RELATED WORKSThere are two main fields of study that pertains to this research:design patterns and level design. The concept of creating aformalized language to describe a field was pioneered byChristopher Alexander in his book “A Pattern Language” [1]. Thebook revolved around architecture and how you could create aformalized language for how to approach architectural structuresby classifying them as different patterns that were organized bytheir function, detailing how to solve each one. This was thenexpanded into the field of computer science in order to approachproblems when organizing programs, as detailed in DesignPatterns: Elements of Reusable Object - Oriented Software [3].The concept of design patterns and how they relate to games hasbeen explored by other sources. The first instance of this wasBjörk and Holopainen’s Patterns in Game Design, which was thefirst foray into investigating how patterns were applied to themedium. As games research has expanded other people havebegun looking at more specific patterns in game design. Hullettand Whitehead’s Design Patterns in FPS Levels [5] investigatespatterns in level design and how it affects a player’s experience ofthe game. The patterns represented in this paper do affect NPCs

and are mentioned briefly in section 6. However, due to timeconstraints it wasn’t feasible to extensively examine how theseLevel Design Patterns (LDPs) affected the NPC patterns presentedin this paper.Other people have examined how NPCs affect a player’sexperience during combat. Mike Birkhead investigates NPCswithin combat in Tips from a combat designer: The art of bossdesign, which examines the idea of boss encounters and how todesign a NPC to fit one. Benson Russell’s A Deeper Look IntoThe Combat Design of Uncharted 2 also investigates designingcombat in the shooter genre with a small emphasis on NPCinteraction. However these articles do not apply patterns to NPCdesign or combat within shooters. They examine the overalldesign and dissect what happens in games but don’t try to developa large language that can be used to communicate concepts toother designers with.3. ELEMENTS OF A NPCBelow is a list of elements that make up a NPC as well as a briefdescription of how they can be used by a designer to creategameplay during combat. These will be used in the patterncollection to categorize the specific patterns.Movement Type – This describes the way the NPC will typicallymove in a combat situation. Many NPCs employ multipleMovement Types and can switch between them depending on thesituation. Flanking Intensive – The NPC will move to attack fromunexpected directions, i.e. the NPC tries to approach theplayer from a different side than where the player’s attentionis directed. Passive – The NPC will not move when attacking. Neverstraying too far from that location and available cover. Slow Push – The NPC will slowly advance on the position ofthe opposing force, usually in a straight line. This can bewithout the need for cover, but it is possible for the NPC toutilize cover while making its way forward. This maindifference between this and Cautious is that it will constantlytry to close the distance from its target and not try to stayaway. Rush – The NPC will make a dash at a specific target withoutany regard for their safety, typically in a straight line.However, the main aspect of this movement type is that theywill attack very fast and often try to close the distancebetween themselves and their target as fast as possible. Cautious – When used, it means that the NPC is opting tomove around the battlefield but tries to maintain a distancefrom its target. Often trying to utilize cover when possibleand not closing the distance when possible. This is differentfrom a slow push because this NPC tries to maintain aspecific radius around its target, without advancing.Movement Range - This is how far the NPC will move during anengagement. This can be Low, Medium, or High.Movement Frequency - This is how often the NPC will changetheir position during an engagement. This can be Low, Medium,or High.Attack Frequency – This describes how often the NPC willinitiate an attack. This can be Low, Medium, or High.Weapon Type – This general category utilizes the weaponpatterns from Weapon Design Patterns in First-Person ShooterGames [4] and can be referenced for further information on them.The patterns include the following: Sniping Weapon Close Blast Assault Weapon Projectile Power Weapon Melee WeaponWeapon Damage – A general indicator on how much damage theNPC will do to the player’s Health, Shields, or Armor. This canbe Low, Medium, or High.Armor/Health – This denotes how much damage the NPC cantake before being killed. This will typically be linked to how hardthe NPC is to defeat. This can be Low, Medium, or High.Motive – This is an indicator of what type of combat encounterthe NPC would create and shows its purpose to the designer. Thishinges on three main factors that an NPC can affect: Challenge – The degree of difficulty within a combatencounter. Tension – The degree of mental stress the playerexperiences during a combat encounter. Pacing – The degree of movement that the player willengage in during a combat encounter.A pattern can affect each of these three factors by creating asituation where they can be at Low, Medium, or High.4. STRUCTUREOFNPCDESIGNPATTERNS: Name - The name of the pattern should describe whatthe pattern does essentially.Description - A brief description of how the pattern istypically used and the effect it has on gameplay. Thiscan be seen as the primary role of the enemy NPC.Affordances – This section lists the elements of theNPC and what those elements can be broken down into.NPC Relationships – Description of any specialrelationships with other patterns. This includes anEnemy NPC switching to a different pattern.Examples - Some examples from well-known games.5. PATTERN COLLECTIONBelow is a list of all the patterns that we have collected during ourresearch. Each base pattern specifies the primary function of thatgeneral type, while each sub pattern denotes how that function iscarried out. Soldier – Grunt, Elite, Grenadier, SniperAggressor – Suicide, Swarm, BerserkerCarrier – Sacrifice, SummonerTank – Turret, ShieldThe following sections detail all of the base patterns and at leastone of their sub patterns. Not all patterns mentioned above arepresented here due to the constraints on the length of the paper.For a more detailed description of these patterns and all associatedsub patterns, please visit the Level Design Patterns website atldp.soe.ucsc.edu.

5.1 SoldierSoldier is a NPC that will pressure the player from long range. Itsmain strategy is to control the available space in the encounter.NPCs of this type make up the majority of units during anencounter. They are primarily used to control pacing by forcingthe player to take particular paths through the environment. TheseNPCs will have a weapon type that is an Assault, Close Blast,Sniping, or Projectile.5.1.1 GruntDescription:The Grunt is a weak NPC that will try to maintain a mediumdistance away when attacking. The main function this serves is todraw the player to forward through the level and increase theplayer's confidence. This pattern is distinguished by alwayshaving medium movement range, medium movement frequency,and light armor. The motive of the Grunt pattern is to create asituation with low tension and low challenge.Half-Life 2 - The Metro Police Officer utilizes a Slow Push orCautious Movement Type and primarily is equipped with anassault weapon, typically a sidearm. They will shift between themovement types in an effort to move a player forward. Typicallythis means that they will begin in a cautious movement type and,if they player doesn’t pursue them, will move toward the player inorder to get the player to move. This doesn’t occur in anyparticular instance but can be seen where there are Metro PoliceOfficers in levels such as Route Kanal or Water Hazard. In thegame, they basically act as bait to simply pull the player forward.They are primarily seen as the main enemy in the early game andare increasingly used as bait in the latter half of the game.Affordances: Movement type can be Slow Push, Flanking Intensive,or Cautious. Attack frequency can be either Low or Medium. Weapon damage can be either Low or Medium.NPC Relationships:The grunt has a special relationship with the Suicide pattern,because sometimes a grunt may change to the suicide pattern inthe middle of an encounter.Examples:Halo: Combat Evolved - The Grunt is a small unit that appears inevery game within the Halo franchise. It has a low amount ofArmor and is usually to be equipped with an assault weapon thatdoes a low (Plasma Pistol) or medium (Needler) amount ofdamage. They exhibit the special relationship with the Suicidepattern in that they will self-destruct in times of desperation. Therange it keeps is either short or medium but tries to pester theplayer by implementing the Cautious movement type.During the campaign they primarily occur within encounters tocreate a lower challenge but increase the pace of the encounter. Asa consequence, the player feels more empowered and will pursuea route that contains a higher ratio of grunts compared to anyother path. This occurs in the level The Pillar of Autumn; oftenthe designers put grunts down a particular corridor to encouragethe player to move in that direction. This signals to the player thatit is the correct route to follow while lowering challenge,increasing the pace, and lowering player tension.Figure 2: Two Metro police officers in Half-Life 25.1.2 GrenadierDescription:The Grenadier NPC pattern distinguishes itself by being a weakerenemy that tries to stay a decent distance away in order to fireprojectile weapons at the player. The main function that this NPCpattern is designed to do is to encourage the player to movethrough pressure made by the explosives. This serves as theantithesis of the Grunt NPC Pattern because this pattern increaseschallenge and tension to encourage the player to move as opposedto lowering it. The main traits that are required to distinguish thisclass are that they have a medium movement range, lowmovement frequency, a medium attack frequency, and a projectileweapon type. The Motive of this pattern is to create a situationwith high tension and high challenge.Affordances: The Range can be either medium or long. The Movement Type can be Passive, Slow Push, orCautious. The Armor can be either medium or low.NPC Relationships:None.Examples:Gears of War – The Boomer is a large unit that has a long rangeexplosive weapon and medium grade body armor. It utilizes aslow push movement type and appears as a larger version of thetypical locust drone soldier. It fires large explosives that force theplayer to evacuate their current cover position, unless they want toFigure 1: A Group of Grunts in Halo: Reach

be hurt due to the splash damage. This increases the pace at whichthe player moves through the level by pressuring them.amount of challenge over a short period of time. The high amountof damage they do, forces the player to deal with themimmediately. All patterns of this type will have a high amount ofweapon damage and since it is a derivative of the Rush NPCpattern it keeps the same requirements to stay within the RushNPC Pattern. All NPCs within this pattern can have armor of anylevel. The motive of the berserker pattern is to create a situationwith High Challenge.Affordances: There are no affordances in this NPC Type.NPC Relationships:None.Figure 3: A Boomer in Gears of WarExamples:Borderlands – The Psycho wears a mask and the lower portion ofan orange jumpsuit. He rushes the player at a high speed andwields a large hatchet that deals a high amount of damage when ithits. The unit will frequently attack the player and will run acrossthe entire battlefield in order to engage them. A couple of this unittype is usually put into larger groups of enemies in order to putmore pressure the player more by having a more aggressiveenemy.Uncharted 2: Among Thieves - A rocket soldier is a unit with alow amount of armor that maintains a passive movement type anddeals with a projectile weapon type. It typically appears as asoldier in camouflage and bandanna that has an RPG with a bandof ammo clips on its shoulder. It represents the atypical Grenadier,staying as far away as possible by forcing the player to move withhigh damage explosives. Thereby increasing the player’s pacethrough the level without directly exposing itself to danger.Figure 3: A Burning Psycho in BorderlandsFigure 4: An allied NPC, Tenzin, is being shot by a rocket soldierfrom Uncharted 2: Among Thieves.5.2 AggressorAggressor is a NPC that will immediately close the distance to theplayer in order to attack them with a Melee or Close Blastweapon. Its main function is to increase challenge in an encounterby pressuring the player. This type of NPC complicates theplayer's situation by forcing them to deal with an immediate threatat close range. Its presence also serves to increase player tension,because there is an increased amount of pressure from the playerto kill it before it starts to move in and attack. Every sub pattern ofthis type has the Rush movement type, high movement range,high movement frequency, high attack frequency, and a Melee orClose Blast weapon type.5.2.1BerserkerDescription:The Berserker is a sub pattern of the Rush NPC pattern anddistinguishes itself by being an enemy that will rush the player todeal a high amount of damage over a prolonged amount of time.The main function of this NPC type is to give the player a highGears of War – The Butcher is a large unit that appearsthroughout the Gears of War franchise, it carries a cleaver that ituses to strike the player with and has medium armor. It has a lowspeed but it will try to engage the player and rush toward himfrom any area on the battlefield. He will repeatedly try to attackthe player as long as he/she is within its sight. The large, imposingpresence serves to make it immediately noticeable on thebattlefield and it will consistently make players prioritize it firstbecause of the high level of damage it does.

amount of smaller enemies increases the player’s tension level,since the player will only release more enemies when they kill thepregnant necromorph.Figure 5: A pregnant necromorph spawning units after beingkilled in Dead Space.Figure 4: A Butcher from Gears of War5.3 CarrierCarrier is a NPC that will spawn more NPCs during an encounter.Their function is to increase the amount of tension that the playerhas by creating more enemies for the player to engage. The playerknows that the number of enemies will keep increasing if theCarrier isn’t dealt with. Therefore, the NPC's presence will be atthe forefront of the player's mind throughout the battle since it is athreat that should be taken out early. Furthermore, the longer itstays alive, the more enemies the player with have to deal with;which increases the level of challenge. NPCs in this category willall have a high movement range, high movement frequency, andlow armor. This is because the NPC doesn't want to get killed sothere is a high movement frequency and range but designers wantthese to be killed fast so it's given a low amount of armor.5.3.1Halo: Combat Evolved – The Carrier Flood form is a carrier thathas a low amount of armor and spawns units through an explosiveweapon type at close range. It typically tries to assault the playerin the middle of a battle when in a larger heterogeneous group. Itincreases the tension of theleve because the player knows it caninflict a high amount of damage if it explodes nearby and onlyserves to create more ene

A pattern can affect each of these three factors by creating a situation where they can be at . Low, Medium, or High. 4. STRUCTURE OF NPC DESIGN PATTERNS: Name - The name of the pattern should describe what the pattern does essentially. Description - A brief description of how the pattern is

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