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分析潜行游戏及其内部的种种系统

发布时间:2014-06-27 10:03:05 Tags:,,,,

作者:Youssef Khatib

本文是对于我的毕业论文(于瑞典的乌普沙拉大学的游戏设计专业)的某部分内容,即关于潜行游戏的调查的总结。它讨论了玩家角色与他在这些游戏中所面对的人造敌人之间的关系,并呈现了如何将其维系在一起而构成潜行游戏。(本文将假设角色为女性,守卫或其它对手为男性。)

首先,这些角色是谁,他们拥有怎样的属性能够帮助他们使用潜行?他们是否具有特别的技能或者他们缺少直面威胁的优势和勇气?在游戏将主题呈现给玩家时总是会出现这些问题。游戏的主题描述通常会提及玩家所期待的游戏机制,这并不只是局限于潜行游戏中。

Vendiagram 3(from gamasutra)

Vendiagram 3(from gamasutra)

潜行游戏中的人工智能的设计是围绕着为潜行游戏创造预想的动态元素。然而我们在此谈论的电子游戏的人工智能从传统意义来看其实并不适合电子游戏。在电子游戏中,AI经常用于模仿智能行为而不是它们本身。这意味着游戏要求它们那受对手驱动的AI去遵循有限的行为模式。如果这样的对手太过“智能”便有可能不利于游戏的发展。这指的是遵循玩家所理解的游戏世界的自然规则的电子游戏。

栖居于潜行游戏的玩家角色将拥有更加多样的特征,但是有些特征可能会比其它更加普通。这些视觉和主题特征将致力于向玩家传递可能违背安全的内容。

违背安全是潜行游戏中经常出现的审美元素。在2006年的GDC关于“面向潜行游戏玩法的关卡构建”的演讲中,Randy Smith便提到了潜行游戏的常见审美目标:

创造一个安全守卫区域的幻觉,让玩家可以使用其独特的能力和工具去创造并利用安全漏洞。

最经典的安全便是以守卫,环境公害,报警系统等常受到某种类型的AI驱动的措施呈现出来。本文将主要着眼于有关角色的守卫的AI,即守卫是玩家需要警惕的对手。现在,我们将专注于玩家“利用他们独特的能力和工具去创造并利用安全漏洞。”这些独特的能力通常都带有相同的目的,但在不同的潜行游戏中将以不同的方式呈现出来。

例如隐藏可以以不同方式呈现出来,如待在阻挡物后面或藏匿于人群中。除了这些隐藏方法,我们还是用了阴影作为藏匿处,这将作为对付AI对手的一种有效的单向镜(玩家可以看到他们,但是对方却看不到玩家),或者是用某种形式的伪装让玩家看起来与周边的环境融为一体。这也包含使用适当的伪装走过敌对的一方。

为了反击这些能力,AI对手需要能够通过抽象感接收并组织信息,为了追赶潜在的入侵者而获取足够的信息。在缺少潜行组件的游戏中,AI对手倾向于不断了解游戏的完整状态。所以为了包含潜行系统,游戏要求AI组件只能拥有有限的角色信息。

这些感觉通常是以视觉,听觉,有时候甚至是嗅觉方式呈现出来。这种感觉将作为守卫去感知周边环境,寻找在感知范围内角色所留下的各种线索的一种方法。然后AI将利用获得的信息去决定如何对所发生的事件作出反应。

现代的潜行游戏让玩家能够为了管理角色的检测性而去操控他们在环境中前行的方式。他们将改变移动节奏去拉低声音,这将潜在地告知玩家他们所处的位置是否具有威胁。守卫的听觉将通过搜寻周边而注意到这些声音。每个声音都有其自身的传播距离,一旦声音的排放与接听半径交叉,守卫便能够注意到并作出回应。

Hearing-Sense(from gamasutra)

Hearing-Sense(from gamasutra)

角色的移动方式与其可见性紧密联系在一起,就像移动将自然地改变角色所处的环境状态。例如,当角色从黑暗的地方前往明亮的地方,从空旷的区域来到拥挤的街道时。所以因为周边环境的变化,移动将会影响玩家的可见性。守卫的视觉通常会因为一个视觉角度或多个视觉角度而被分散。在这种情况下,多个视图维将用于呈现外围和直接视野。

Sight-Sense(from gamasutra)

Sight-Sense(from gamasutra)

这些层面可以用于反映角色的可见性的不同方面。例如守卫的周边视觉可以对突入而来的移动更加敏感。它的中央视觉将察觉到移动并决定角色在与环境相比之下是否能够被察觉到。当角色关卡填满了阻挡物(让玩家能够再次躲藏)时,守卫也必须确保他是否能够真正“看到”角色。为了做到这点,守卫将朝着角色的位置发送射线,如果在触及角色前撞击到某些内容,守卫将忽略角色的位置。基于游戏的需求,可能会出现AI需要执行的额外可见性检测,如此才能判断守卫是否检测到了角色。

在某些游戏中,当守卫因为环境因素不能直接检测到角色的位置时,对手甚至能够对突然的移动做出反应。这其实是在模拟人类对于移动的敏感性,这将添加另外一个抽象的检测能力管理层面让玩家进行思考。

例如,如果玩家能够隐藏在阴影里,AI便可能检测到角色隐藏在黑暗中的深度,测量角色从不同距离检测到角色的可能性。伪装的作用与阴影类似,除了角色需要装备而与环境融合在一起。还有一些例子是守卫在面对伪装和阴影时不可能察觉到角色。

当守卫察觉不到他所追寻的入侵者时,他将需要开始朝着正确的方向搜寻。这时候可以提供给守卫嗅觉。角色将在移动的时候留下一些气味轨迹,而守卫将遵循着这些味道去寻找角色。这些气味将随着时间的发展而慢慢变淡。当这种情况发生时,守卫可能便会失去兴趣而回到之前的活动中。

Smelling-Sense(from gamasutra)

Smelling-Sense(from gamasutra)

为了帮助管理角色所遭遇的威胁,这些游戏通常包含额外的物理方式,工具和其它小工具让玩家能够追踪甚至去衡量危险。角色通常会从后面偷偷接近守卫并悄悄地压制他们。他们也可以从远距离做到这点,但有些武器会包含对于发出声音的惩罚,这可能会改变周边的威胁。

就像之前所提到的,守卫将把通过感觉收集到的信息集中在一起去做出决定。当他们收集到关于状态发生改变的足够信息时便能够做决定。这些操作是有限状态机的一个有效的例子。

这些方法将导致守卫被不确定性或临时性所击倒。这意味着他们可能会在之后清醒过来或者被偶然路过的人唤醒。不管怎样,当有人发现守卫躺在地上,神志不清或者自己醒来,他们便会产生疑虑,并去搜寻相关威胁。为了避免这种情况,角色可以将被击倒的守卫的身体隐藏在其他巡逻不可能找到的地方。

State-Machine-2(from gamasutra)

State-Machine-2(from gamasutra)

现在还有些情况是玩家能够从守卫的行为状态改变中获益。当守卫要阻止角色前行时,他可能需要使用某种元素去分散玩家的注意力。守卫可以使用某种类型的小工具让角色进入一种好奇状态,这将导致他们去搜寻这种分心的来源。这些工具将以各种形式呈现出来,可能是将声音制造设备扔向角色,可能是敲打墙壁,也有可能是诱导角色当前所处区域的对手。

当守卫收集到某些特定的信息时,这些不同的状态将呈现出来。主要状态是警报和空闲的巡逻,但游戏通常会包含各种状态。就像之前所提到的,这些系统也需要作用于游戏设计,所以将其过度复杂化只会损害游戏。

守卫倾向于群体搜寻某些区域。这意味着他们能够在靠近时分享彼此所收集到的信息。这将让守卫们更紧密地协作在一起,从而能够更机智地搜寻威胁。这是他们了解角色的位置并主动寻找他们的状态。与此同时,警报系统将被触发,这将导致其他守卫群组冲向角色。

在《合金装备》中,游戏被划分为不同阶段,在这里守卫将呈现出不同的行为。当玩家进入每个阶段时,守卫将通过广播向最高指挥部进行报告,并最终改变该区域的形势。当角色被发现时,游戏将首次进入警惕阶段,守卫的数量也将增加。既然他们察觉到了角色的位置,他们也将主动追击角色。如果角色暂时逃离了守卫的视觉范围,游戏将进入逃避阶段,在此守卫将从角色最后出现的位置开始搜索他们。当这一阶段结束时,游戏会进入警示阶段,守卫将回到搜寻路线,并伴随着更多增援部队。一旦警示阶段结束,增援部队将离开该区域,假设角色已经离开了这里,那么标准阶段将确保守卫回到未察觉状态中。

在意外检测中,角色将使用能够帮助自己逃离的工具。这些工具将能够暂时打昏或迷惑敌人,提供给角色足够的时间离开那里。例如当角色被察觉到时,他便可以扔下一个烟雾弹去阻隔一群敌人的视线。有时候角色也可以通过设下陷阱去阻止追捕者,这能够有效地放慢对方的速度甚至能够杀死他们。

State-Machine(from gamasutra)

State-Machine(from gamasutra)

避开检测要求玩家在执行任何挑战前能够进行适当的计划。在玩家需要花费大量的时间潜行的游戏中,它必须让玩家能够执行一项侦查活动。游戏的关卡设计也必须适应这点,同时游戏还应该包含一些机制去帮助玩家获取视线范围内的区域的更多信息。这将能够有效地提高玩家找到适当解决方法的机遇。

许多游戏为了能够提供给玩家有关即将遇到的挑战的足够信息而围绕着摄像机模式进行设计。第一人称摄像机模式让玩家能够从远处进行眺望,但这让玩家很难察觉到角色周边情况,而第三人称摄像机模式让角色可以清楚地看到周边事物,但是却察觉不到远处发生了什么。这些游戏有时候还包括一个“越肩视角”摄像机去实现更棒的远距离视角。对于许多发生在三维平面的潜行游戏来说,多个摄像机模式总是用于补偿各种不同的缺陷。其它包含环境敏感型摄像机的方法将在角色靠在附近的墙上时发生改变,这是为了让角色更好地侦查周边情况。

《忍者之印》故意限制了提供给玩家的信息。在基于2D侧边卷轴视角的游戏中通常都会让玩家看到巨大的设计背后的内容。但是这款游戏却故意限制玩家只能看到以角色为中心的圆圈内部的东西,这也构成了角色的视线范围。这一视线范围将根据周边建筑的高低发生改变,并有效模糊了不处于该范围内的东西。当角色在攀升而朝不同方向前进或者当他处于能够观察到周边事物的最高点时,角色视线范围的形状将发生变更。在角色视线范围以外的区域中,玩家可以通过环境中的各种实体看到代表不同排放物的视觉线索。

在这些游戏中还有一些迷你地图能够呈现关卡的结构或者守卫的位置。在不同的游戏中,这些迷你地图系统是遵循不同的规则。有些游戏只会在玩家与守卫拥有视觉接触时才将其呈现在迷你地图上,也有些游戏甚至不需要它。其它游戏将这种迷你地图功能作为与军事雷达类似的声呐检测系统。与许多其它元素相似的是,这些功能的运行也是取决于游戏想要传达的主题以及它想鼓励玩家尝试的潜行类型。然而这一功能并非总是必要的,游戏游戏甚至完全排除了这些系统。

除了迷你地图系统外还有一些受角色驱动的机制和工具能够帮助玩家进一步获得视线范围外的信息。在Tom Clancy的《分裂细胞》系列中,这些工具是以玩家能够看穿的外部摄像机形式呈现出来,如门下的蛇形摄像机或能够附着在墙壁上的粘性摄像机。玩家将使用这些方法去获得前方的信息。

因此降低角色的检测能力是玩家在潜行时所面对的主要挑战。到目前为止我们已经提到了角色在敌对环境中潜行时保持不被发现的能力以及守卫通过角色留下的线索发现他们的轨迹的能力。角色必须为了视线目标而离开安全区域,所以玩家需要进行有效的控制从而避免敌人通过任何方式发现角色的踪迹。

与其它行动游戏不同的是,潜行游戏会在玩家彻底败于潜行前提供给他们一个狭窄的失败边缘。一旦玩家因为角色被发现而遭遇失败时,游戏便会切换到不同模式让玩家可以在打斗与离开间做出选择。当然了,关于潜行游戏的审查并不会终止与此。在我的论文中还有更多的主题,包括关卡设计,讨论如何确定潜行关卡的节奏,平衡多条路径解决方法等等。我们需要通过进一步研究才能明确以潜行为中心的游戏范围中到底还存在哪些类型的潜行游戏。

本文为游戏邦/gamerboom.com编译,拒绝任何不保留版权的转载,如需转载请联系:游戏邦

Examining the Essential Building Blocks of Stealth Play

by Youssef Khatib

This blog post is a summary of an investigation into stealth games as part of my graduation thesis in Game Design at Uppsala University, Sweden. It discusses the relationship between the player character (i.e. avatar) and the artificial opponents she faces in these games and shows how tying them together forms stealth play. (For clarity, this post assumes that the avatar is female and guards or other opponents are male.)

First of all who are these characters and what attributes do they possess that makes them ideal for using stealth? Are they particularly skilled or do they lack the strength and courage to confront threats head-on? These questions are usually answered through the apparent themes the game presents to its players. The thematic presentation of the game generally alludes to the kind of mechanics expected of the game, which of course isn’t exclusive to stealth centric games but most games centered around characters in general.

The design of artificial intelligence in stealth centric games is central in creating the desired dynamics for stealth play.We are, however, talking about artificial intelligence for videogames here, as artificial Intelligence (AI) in its traditional sense doesn’t lend itself well to videogames in general. AI in videogames is more used to emulate intelligent behaviors rather than simulate them. What this means is that games require their AI driven opponents to follow limited patterns of behavior. Making these opponents too ‘intelligent’ may even do more harm than good in the game. This refers to videogames that are designed with the intention of adhering to a set of rules that the players understand to be the natural laws of that game world.

The avatars the player inhabits in stealth centric games tend to have very diverse characteristics in general, however some of them are more prevalent than others. These visual and thematic characteristics serve to communicate the available means the player has against the security he’s about to breach.

Breaching through security is a frequent aesthetic of stealth centric games. During his 2006 GDC talk ”Level Building for Stealth Gameplay,” Randy Smith proposed a common aesthetic goal for stealth games as:

[To] create the illusion of a securely guarded area that the player can sneak through by virtue of leveraging their unique abilities and tools to create and exploit security flaws.

The security is typically present in the form of guards, environmental hazards, alarm systems and other measures that are commonly driven by some kind of AI. This article will, mainly look at the guard’s AI relation to the avatar, as guards are the main adversaries the player needs to keep an eye on. For now, we will focus on the player “leveraging their unique abilities and tools to create and exploit security flaws”. These unique abilities usually have the same purpose but manifest themselves differently in different stealth centric games.

Hiding, for example, can be done in different ways, ranging from standing behind vision-obstructing objects to walking among busy crowds and melting into the scenery. In addition to these hiding methods, we have the use of shadows for concealment, which effectively works as a one-way mirror against AI opponents (the player can see them, but they cannot see the player), or using some kind of camouflage to look like the surrounding environment. This can also include using an appropriate disguise to walk among otherwise hostile opponents.

To counter these abilities, the AI-driven opponents need to be able to individually receive and put together information through abstracted ‘senses’, gaining enough knowledge in order to pursue potential intruders. In games that lack a stealth component, the AI opponents tend to be constantly aware of the complete state of the game. So in order to include stealth systems, the game requires AI opponents to have limited knowledge of the avatar’s presence.

These senses usually come in the form of sight, hearing and sometimes even smelling. These senses serve as a way for the guards to be able to perceive their surroundings, picking up on various clues left by the avatar within the range of their perception. The acquired information is then used by the AI in order to make a decision on how to react to the events taking place.

Contemporary stealth games allow their players to manipulate the way their avatar navigates the environment in order to manage the avatar’s detectability. The pace of movement is altered to reduce sound emissions, which potentially alert threats to the avatar’s location. The guard’s hearing sense can pick up on these emissions, through a hearing radius set up around her, from which he can detect sounds emitted by specific entities in the environment. Each sound can be given its own traveling distance and once an emission intersects with the hearing radius it is registered by the guard and the guard reacts accordingly.

The way in which the avatar moves is closely connected to her visibility, as moving naturally changes the environmental conditions that the avatar is present in. For example, this happens when traveling from darker to lighter areas, from empty to crowded streets, etc. So movement affects the avatar’s visibility in general due to the surrounding environment. The guard’s sight is usually abstracted by giving it one or multiple calculated angles (view cones) that represent different layers of visual perception. In this case, multiple view cones can be used to represent peripheral and direct vision.

These layers can be set up to react to different aspects of the avatar’s visibility. The guard’s peripheral vision can, for example, be more sensitive to sudden movements. His direct vision detects movement and also determines whether the avatar can be seen in contrast to her environment. As game levels are populated with vision-obstructing objects for the avatar to hide behind, the guard also has to make sure he is actually ‘seeing’ the avatar. To do this a ray (raycast/raytrace) is sent towards the avatar’s location and if it hits something before reaching the avatar the guard ignores her presence. Depending on what the game demands, there can be additional visibility checks AI has to perform in order to decide whether or not the guard has detected the avatar.

In some games, adversaries are even able to react to sudden movements when the avatar is present in locations where the guards are otherwise are prevented from directly detecting her due to environmental factors. This emulates the human eye’s sensitivity to movement in general, which adds another abstract layer of detectability management for the player to think about.

For example, if a player is able to hide in the shadows the AI will probably check the degree to which the avatar is concealed by darkness, measuring how likely it is for the guard to detect the avatar from various distances. Camouflage works in a similar way to shadows, with the additional condition that the avatar’s equipped camouflage needs to match her environment. There will probably be instances in which it would be nearly impossible for the guards to detect the avatar when he reaches a certain level of concealment in the case of both camouflage and shadows.

When a guard loses sight of an intruder he is persuing, he will need to start searching in the right direction. This can be addressed by giving the guard a sense of smell. The avatar drops a simulated scent trail as he moves, which the guard can follow once he loses visual contact with the avatar. These scent emissions fade over time as the guard actively searches until the trail runs cold. When this happens the guard loses interest and returns to his former activity.

In order to help manage the threats the avatar encounters, these games commonly include additional physical means, tools or other gadgets that allow the player to tackle or even gauge the dangers ahead. The avatar can typically sneak up on the guards from behind and neutralize them silently. This can also be done from a distance, but some weapons include a penalty of emitting sound, possibly alerting nearby threats.

As mentioned before, the information gathered by the guards’ senses are put together in order for the guard to able to make decisions. These decisions are made whenever they have enough information for a change of to state occur, altering the guards’ behavioral patterns. This kind of operation is a good example of a finite state machine.

These methods can cause guards to be either disposed of indefinitely or knocked out temporarily. This means that they might wake up later by themselves or be awoken by a passing comrade who stumbles upon them. Either way, when a guard is found lying on the ground dead, knocked out or even wakes up by himself, suspicion will arise, possibly initiating a search for threats. To avoid this, the avatar can more often than not, carry the bodies of fallen/unconscious guards, to hide them in locations away from other patrol routes.

Now, there are cases where the player benefits from changing the behavioral state of the guard. Having a guard blocking the avatar’s progression sometimes calls for using distractions to divert attention from the player’s intended path. This can be done by stimulating a guard’s various senses with some type of gadget causing them to enter a curious state, which leads them to investigate the source of the distraction. These tools come in a variety of forms, ranging from noisemaking devices that can be thrown to activate them remotely, to knocking on walls and attracting the adversary to the avatar’s current location.

These different states are entered when certain kinds of information are gathered by the guard. The main states are alert and idle patrol, but the game will generally include a variation of states in between. As stated before, these systems also need to serve the game’s design, so complicating them excessively might harm the game.

Guards tend to patrol areas in groups. This means that they are able to share some of the information they’ve acquired with each other when they are near each other. This can be used to make guards work together and that makes them seem more intelligent when they are actively searching for threats while coordinating. Upon detecting the avatar, the guards generally enter an alert state. This is a state in which they are aware of the avatar’s location and actively hunting him. In conjunction with this, an alarm system can be set off, leading to the possibility of additional groups of guards rushing towards the avatar.

In the Metal Gear Solid series the game is split into several phases, during which guards have different sets of behaviors they can take on. Each phase is entered through a guard reporting to his high command by radio, eventually changing the state of affairs in the area. When the avatar is detected, the game first goes into an Alert phase in which the number of guards increases. Now that they are aware of the avatar’s location, they actively chase him. If the avatar manages to break visual contact for a while, the game will move into the Evasion phase, in which the guards start searching for the avatar in her last known location. When this phase runs out a Caution phase is entered in which the guards return to their patrol routes with reinforcements still present. Once the Caution phase ends, the reinforcements leave the area, assuming the avatar has left it for another and the Normal phase ensues returning the guards to a state of obliviousness.

In the case of sudden detection, the avatar can in some instances make use of tools that help her escape. These come in the form of gadgets that temporarily stun or blind enemies, giving the avatar enough time to disappear. For example, when the avatar is detected, she can drop a smoke bomb, confusing a group of adversaries. Sometimes the avatar can even halt her pursuers by leaving traps behind, effectively slowing them down or killing them.

Avoiding detection altogether requires the player to do some measure of planning before taking on any challenge. In a game where players spend a substantial amount of time sneaking around, the game must allow for the player to be able to perform some scouting. While the game’s level design must accommodate this, the game should also include some mechanics to aid the player in acquiring further information beyond her field of view. This is practical for increasing the players’ opportunities for improvisation and finding alternate solutions.

Many of these games are designed around their camera model in order to give their players sufficient information about the challenges they are about to encounter. A first-person camera model allows the player to see far away, but gives the player a weak sense of the avatar’s immediate surroundings, while a third-person camera model gives a good view of the avatar’s vicinity but generally doesn’t provide a very good view far ahead. These games sometimes include an ‘over the shoulder’ camera for a better long-distance view, though. For many stealth games taking place in three dimensions, multiple camera models are frequently used to compensate for their different shortcomings. Other means involve context-sensitive camera shifts that happen when the avatar is leaning against the wall near a corner, in order for the avatar to scout around it.

Mark Of The Ninja is a game that intentionally limits the information available to the player. In a game played from a 2D side-scrolling perspective there generally isn’t much stopping the player from seeing what lies behind a large contraption taking up space in the room which is viewed. This game intentionally limits what the player is able to see beyond circle centered on the avatar, which constitutes her field of view. This field of view can be blocked by the level’s surrounding architecture altering its shape, effectively blurring out the content not uncovered by it. The shape of the avatar’s field of view is altered further when she leans towards different directions while climbing or when she is seated on a perch point observing her surroundings. In areas beyond the avatar’s field of view, the player can, however, see expanding visual cues representing different emissions by various entities in the environment, practically marking locations of potential threats and their movements.

There are also mini-map systems in these games that can display the level’s architecture or the guard’s locations and more often than not, both. In different games, these mini-map’s systems follow different rules. Some games only mark guards on the mini-map when the player has established visual contact with them, while others don’t require it. Other games make these mini-maps function as sonar detection system similar to that of military radar and so on. Like many other factors, how this feature operates is very dependent on the theme the game wants to project and the kind of stealth play it wants to encourage. However, this feature isn’t always essential and some games exclude these systems entirely.

Other than mini-map systems there are also avatar driven mechanics and gadgets to help the player in further acquiring information beyond her field of view. In Tom Clancy’s Splinter Cell series, these gadgets can come in the form of external cameras that the player can look through, such as snake cameras beneath a door or sticky cameras that can be thrown and stuck onto walls. These are used when a player has little to no access to information about what is ahead.

Mitigating the avatar’s detectability is thus the main player challenge throughout stealth play. So far we’ve covered the avatar assigned abilities to remain undetected whilst navigating hostile environments and the guard’s ability to pick up on the presence of the avatar through emissions they leave behind. The avatar has to breach securely guarded areas in order to reach her goal, so the player needs to manage the degree to which, adversaries are able to pick up on her presence by any means provided.

Unlike other action games, stealth gives the player a narrow failure margin before completely failing at stealth play. Once a player fails at stealth by having their avatar discovered, the game transitions into a different kind of play where the player has to choose between fight or flight. The examination of stealth play doesn’t really end here. There are more topics covered in my thesis, including level design, discussing how to pace stealth levels, balancing multi-path solutions and so on. More research has to be done into what kind stealth play styles there are within the spectrum of stealth centric games. You can also find more information on how I went about my inquiry and the kind sources I used in reaching these conclusions in my thesis, Examining the Essentials of Stealth Game Design.(source:gamasutra)

 


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