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分享以难度图表分析游戏难度变化的方法

发布时间:2011-11-21 16:24:58 Tags:,,,

作者:Rafael Vázquez

(Xibalba Studios首席游戏设计师Rafael Vázquez针对开发者难以评估游戏实际难度的问题,设计了一种考察游戏难度的方法,并以三款横向卷轴动作游戏为例进行说明。)

数月之前,我在一个新项目的原型制作阶段发现了一个问题。由于团队中不同成员对何谓完美游戏挑战的看法不尽相同,我们几乎无从判断自己的游戏怎样才能设置理想的难度级别。

我知道这对小型且具有多元文化的团队来说更是一个普遍问题,因为大家的技能水平并不一致,所以很难就此问题达成共识。

思索一段时间后,我突然觉得游戏难度不应该与玩家技能挂钩,而应该取决于游戏本身。我们不能将游戏难度视为一种静态的因素,而应该去观察它在游戏发展过程中的变化。

抱着这种想法,我尝试开发一种可以在不考虑玩家技能的前提下,衡量并比较游戏挑战的起伏情况。尽管这种方法还是不够准确,但我认为这至少是一个良好的开端,并希望它能获得开发群体的关注和讨论并因此得到完善。

设置恰到好处的难度并非易事。游戏设置过难,玩家就会很抓狂,过于容易,玩家很快就会感到无趣。这是众人皆知的常识,因此设计师一般都会选择中间路线。但多数时候,我们还是得凭胆量和感觉行事,而这种方式也似乎颇为可行。

游戏难易情况往往取决于游戏类型,以及我们从玩家那里得到的反馈结果。宁静安详的社交游戏和紧张激烈的FPS属于两个截然不同的世界,我们不可将它们的挑战性等量齐观。当你向团队成员解释游戏挑战性时,往往是费了几个小时的口舌,大家还是没有就其难度设置是否合理达成一致。

出现这种情况的原因在于,难度是一个主观因素,它在很大程度与每个人的技能水平存在关联。更糟糕的是,合理的难易程度也要取决于每个人的兴趣点,例如有些人就喜欢《超级食肉男孩》这种游戏。当你想开发一款游戏时,就会发现真的很难找到大家共同的切入点。

但这个问题也并非毫无解决之策——它就是我们能够找到共同基准的难度图表。在深入探讨这个问题之前,我要先声明这种方法并非用于创造游戏难度(设计师恐怕还是得凭直觉制定决策),而是分析游戏难度,以便你的团队成员就此展开有建设性的讨论。这些图表并不能指明游戏应该具备哪种难度,只能说明游戏本身的难易程度。

何为游戏难度图表

这些图表可以解析游戏过程的难度变化情况,主要可分为两种类型:基于时间和基于距离的难度图表。前者会根据玩家体验游戏的时间指出遭遇挑战的时刻(游戏邦注:这里排除玩家暂停和死亡的时间),后者则标注挑战出现的位置(假设游戏开始至结束是一条直线路径)。

这两者各有千秋,但要选择哪种方法需取决于你所测试的游戏类型。《Asteroids》或《几何战争》等街机游戏只能选择基于时间的难度图表,因为这些游戏并不存在奔向目标的位移过程。

但多数FPS中的敌人会在特定地点出现,所以最适合采用基于距离的图表。我个人比较偏爱基于时间的图表,具体原因稍后再谈。

吹毛吹疵者可能会指出,这两种方法实际上还是不能完全摆脱玩家因素的影响;玩家闯过一关的速度,以及完成一个挑战所用时间均与其技能水平有关。

但这种方法已经得到我们团队每一个成员的检验,也是目前我们最靠谱和可行的解决方案。我们只能从不同玩家多次玩游戏的情况中搜集数据,并计算其平均值。事实上,只有进行多次测试之后,难度曲线才会派上用场。足够数量的玩家试过游戏之后,你就会了解玩家的平均游戏水平。

正如前文所述,难度具有主观性,所以如果我们询问测试者是否觉得游戏过于困难,很可能就会得到五花八门的答案(但你还是得向他们提问)。如果游戏难度因人而异,那么我们如何才能衡量游戏难度?诀窍就是让游戏与自身作对比。

假设我们有一个低级敌人会产生X的破坏性,那我们就把它的威胁指数设置为1,这里的威胁指数是指敌人对玩家而言的难度。如果我们有另一个敌人火力是前者的2倍,但其他属性完全相同,那么我们就将它的威胁指数设置为2。

也许这种方法过于简化,虽然地点、位置和其他敌人出现情况等因素也会产生影响,但重点在于,我们得先将最简单的挑战作为一个标准(低级敌人),才能将它与游戏中其他挑战进行对比,这样才能在脱离玩家技能的情况下判断每个挑战的难度。无论你的技能有多高超,行动速度快三倍的敌人,总是比标准敌人更难对付。

但是,敌人的状态并非影响游戏难度的唯一因素。假如敌人是在玩家身后或者从天而降地复生,那么游戏难度也会因其复生地点而发生变化。即使是最微弱的低级敌人,他们突然冒泡的情况也会让玩家瞬间手足无措,形成不容忽视的挑战。

除此之外,会飞的敌人总是比陆地敌人更难缠,藏匿于暗处的敌人亦是如此。但那些心不在焉的敌人,或者缘木求鱼地寻找玩家的敌人,就基本上是小菜一碟了。我们不能低估这种情况对游戏挑战造成的影响,所以我们使用了一个“情况乘数”将其考虑入列。

情况乘数是一个可以根据某障碍物与玩家之间的空间关系说明其难度的数值。但每个乘数的属性主要取决于游戏设计。

在使用双摇杆控制方式的射击游戏中,飞行的敌人并不是个大问题,只要玩家能瞄准目标就能将其击落。所以每种乘数的重要性如何也要看设计师的想法,后者需根据游戏机制的需要检验这些乘数的合理性。

要想判断征服一个敌人的难易程度,有一个好方法就是统计玩家采取攻击行动的次数。杀死敌人的方式越直截了当,就说明敌人越容易对付。

我们可以将敌人难度价值与乘数相结合计算出结果。例如,敌人出现在玩家身后的乘数是1.2,该敌人的威胁指数是3,那么它在玩家身后复出时造成的新威胁数值就是3.6。

我们还要考虑到多数时候,敌人不会单兵作战,他们总会成群结队地出现。在这种情况下,我们就需要添加同一时间(或同一地点)出现的所有敌人难度。在此我们就需要计算一群敌人的难度系数。这也正是我为何更习惯使用基于时间图表的原因,因为玩家可能在同一地点遭遇一茬又一茬的敌人,而且多数情况下是倒下一批又涌现一波。基于距离的曲线图表会让敌人难度数值挤到一块,但这并不能反映实际情况。

所以计算一帮敌人难度的公式如下:

Ʃn=0 = (ETn)(ESn)

n是指一波敌军的特定敌人种类;

ET是指敌人威胁指数;

ES是指情况乘数。

找到一帮敌人的所有难度数值之后,你就可以查看玩家何时或何地遭遇挑战。针对每个关卡的所有敌人都要采用这种算法,这样你就能得到一份难度图表了。

增强功能(或道具)

我尚未提及的另一个层面就是增强功能。关于这个层面的话题已经足够它独立成章了,我们还是得在此提出它对游戏难度的影响。如上文所言,我们在比较一个敌人或一波敌人的难度时,都会先设定一些标准条件。

在那种情况下,我们是假设玩家角色的能力水平保持不变,在晋升等级过程中的技能维持原样。但事实并非如此,玩家在游戏过程中总会有得有失,掌握或者丧失某些能力。这就会产生一个问题,如果你给玩家一颗核弹,原来难对付的敌人可能就再也不是威胁了。增强功能在游戏中随时随地都可能出现,即使是遭遇战也不例外,因此也必须考虑这种因素。。

我们可以将增强功能粗略划分为两种类型:永久性和暂时性。永久增强功能是那些在游戏过程中获取之后就永不消失的功能,它包括新能力(游戏邦注:例如,“现在你可以连跳了!”)以及带有易获取弹药的新武器(例如敌人掉落的装备)。而暂时增强功能则是那些在游戏中容易丧失的东西,例如因死亡而丢失技能,或者弹药有限的武器(例如在整个关卡中仅有6颗子弹)。只要丢失了,它们就不见了。

遇到第一种情况时,我们可以假设游戏标准条件已经发生改变,你可以重新计算敌人在这种新条件下的威胁指数。原先需要打10枪的敌人,现在只需要2枪就能毙命。假如其他条件不变,那么这个敌人的威胁程度已经比原来小5倍。

暂时增强功能就比较棘手了。问题就在于你无法知道玩家在何时会获得这些功能。如果假设玩家都已经获得这些功能,并根据这个条件设计游戏,这势必让玩家因频频出现的高难度挑战而抓狂。解决这个问题并非易事,但我们可以先在之前的标准条件下(假设玩家从未获得这些功能)制作图表,然后用垂直线标出他们获得这些功能的位置。这样即使我们绘制的是最不利的游戏场景,也能够清楚看到何处功能会让玩家获益。

步骤

现在让我们复述刚才提到的步骤:

*首先,确定游戏的标准条件。它应该是玩家在特定游戏片段(游戏邦注:例如一个台阶、关卡或者整个游戏过程)中拥有的最低级威力,它也可以因永久增强功能而发生变化。

*找到基本敌人类型(通常是最弱小或者最普通的敌人)并将其威胁指数设置为1,然后依此为基准调整其他敌人类型的威胁指数。

*进行多次测试。让不同操作习惯的玩家在游戏中闯关,计算他们遭遇每个敌人时的平均游戏时间(基于时间的难度图表)或者平均距离(基于距离的难度图表)。

*查看敌人会出现在哪些情形中,然后根据游戏机制以及敌人的攻击行为,为这些情形分配一个数值。

*确定战况。算出在每场战役中,敌人出动多少次兵力,每次分别都是哪些类型的敌人。

*Ʃn=0 = (Etn)(ESn)将敌人威胁指数与情况乘数相乘,并将每批敌军危害性的结果相加。

*根据敌人首次出现的时间或者距离依次绘制出图表。

举例分析

接下来我要用以上方法为三款不同的2D横向卷轴射击游戏绘制基于时间的难度图表。这些游戏来自不同的掌机设备和年代,这样才能体现这种方法的通用性(这三者也是我最方便找到的游戏)。

Metal Slug(from kotaku.com)

Metal Slug(from kotaku.com)

这三款游戏分别是《合金弹头》(由SNK于1996年发行的街机游戏)、《闪克》(由Klei Entertainment于2010年开发,本测试采用的是其PC版游戏)以及《铁血兵团:反叛》(由Arc System Works于2011年开发的Xbox Live Arcade和PSN游戏)。

它们表面看起来相差无几,但在某些细节上却极为不同。因为难度图表并不会显示哪一款游戏更困难,所以我需要特别指出这一点。这些游戏难度的评判标准是其本身,不可与其他游戏相提并论,需知《闪克》中的威胁指数1与《合金弹头》中的威胁指数1并不是同种概念。

这些图表可以显示游戏过程中的难度变化情况,以便我们了解游戏发展速度及紧张刺激性。如果图表存在许多高峰,那就说明它是一款速度快,极具紧迫性的游戏;如果图表多数时候呈平缓状态,那就说明这是一款较为“平静”的游戏。

提示:如果想直接衡量游戏进程的速度,可以参考Ben Cousins计算玩家行动的方法。他在《Elementary Game Design》一文中描述了这种方法,你可以通过其个人网站www.bencousins.com了解详细内容。

这些图表从左到右显示了玩家在每款游戏第一关遇到boss之前的情况。我在这里排除了boss遭遇战的情况,因为它们的出现总会改变规则,玩家需要借助特定的机制才能获胜。如果直接将它们与关卡中其他情况作比较,就会让图表产生巨大的高峰,这并不能反映游戏的总体难度。

我们先以《合金弹头》为例,这张图表截取了约1分20秒的游戏内容,从中可以看出,玩家每隔两三秒就会遇到敌人。除了普通手枪之外,玩家刚开始时还可以使用一些手榴弹,虽然数量有限,但却很管用。与其他两款游戏不同的是,其特点是所有敌人的破坏力都一样,并提供了数量可观的暂时性增强功能,其中包括号称“Metal Slug”的坦克。在这款街机游戏中,死亡并非什么要紧的事情,因为玩家每回都可以在原来丧命的地方复生。

《合金弹头》难度图表(from gamasutra)

《合金弹头》难度图表(from gamasutra)

从图表中可以看出,游戏的基本趋势很接近横轴,其中夹杂着一些较为平缓的突起,其中有两个大型高峰代表直升机。在标准条件下,它们非常难对付(用普通手枪要打40弹左右才能将其击落)。遇到直升机时,镜头会停滞在此,玩家无法躲过这些敌人,只有击落直升机后才能继续前行。

为了让玩家突围,游戏在玩家遇到这种劲敌之前为其提供了增强道具(红色垂直线),因为游戏中的所有增强道具都是暂时性的,所以游戏会连着多投放一些增强道具,确保玩家至少能够拣到其中之一。

在这个环节将近尾声时,玩家又迎来了一个高峰,遭遇其他坦克的攻击。如果玩家的Metal Slug此时的战斗力尚存,那就无需担忧这个问题,但如果Metal Slug在对付直升机时就已经成了炮灰,那就要做好心理准备了。不过游戏此时会增加玩家坦克的健康值(绿色垂直线),以免玩家在遭遇这些敌军坦克时挂掉。

接下来要分析的是《闪克》,它的第一关约为5分30秒。与《合金弹头》不同的是,游戏设置了不少健康条,这样就不会让敌人的攻击过于致命。玩家在开始游戏时拥有4种武器(游戏邦注:手枪、刀、电锯和有限的手榴弹),你可以同时对付数个敌人。但不利之处在于,除了额外的健康条和手榴弹之外,玩家没有其他任何增强道具。

《闪克》难度曲线(from gamasutra)

《闪克》难度曲线(from gamasutra)

从图表中可以看出,《闪克》拥有不少阻碍玩家前行的遭遇战,所以它的游戏速度比《合金弹头》更慢。在多数遭遇战中,敌人是挨个逐渐出现,所以其图表曲线呈阶梯状走势。其明显用意是让玩家轻松进入并适应战斗状态,而不是同时应对所有的挑战。

在大型遭遇战之间(较高的曲线突起部分),我们发现敌人总是成双成对地出现,所以挑战性不会太低。

与《合金弹头》一样,它在将近尾声时也有一个高峰,但它的这个高峰恰好是该关卡中最困难的部分。在此之后,高峰迅速下滑,接近于0状态,所有敌人和障碍荡然无存,为便玩家做足准备,迎接后面的boss战役。

有趣的是,游戏中的健康包一般出现于遭遇战之中(它们一般是敌人掉落的健康包),并且多集中于该关卡的后半部分。与此同时,我们在这个过程中只能补充一次手榴弹,这可能是游戏有意让玩家多练习掌握其三种主要武器。

最后一款游戏是《铁血兵团:反叛》,玩家在其中拥有分段的健康条,初级敌人的每一击都会耗损整段健康条。虽然玩家刚开始时只有一把作用有限的机关枪,但他们可以拾取一些增强道具(但这些道具在玩家被命中时会丢失)。其首个关卡(敌人最少)大约6分钟20秒。

《铁血兵团》难度曲线(from gamasutra)

《铁血兵团》难度曲线(from gamasutra)

游戏虽然拥有一个街机模式(无创建角色的设置),但其最具吸引力的模式却是反叛模式,玩家在该模式中可获得经验值,即使毙命也仍可为角色升级。换句话说,该模式支持玩家刷任务。

首先我们可以看到图表中的直线高峰,它代表玩家遇到miniboss。miniboss威力极大,玩家需要多次命中和强大的攻击力才能把它放倒。但在与之决斗的过程中你会发现,玩家所处地势非常有利,所以miniboss并没有想象中的那般无敌。这也正是我为何建议将boss战役视为特例的原因,因为它们的游戏规则和机制与整个关卡存在较大差异。

撇开miniboss,我们会发现游戏还是具有极为突兀的难度曲线,有一些较为短促的平缓状态和大量的高低起伏,它显然是一款速度极快的游戏。除了高低起伏之外,其曲线在整个关卡中多呈走高趋势,玩家在特定时间段内要打一场需应对更多敌人的持久战。

在关卡开始之初,遭遇战的难度等级约为7,接近尾声时达到30,其难度差距较大,因此适合玩家以刷任务形式体验游戏。在近结尾时我们再次看到玩家遇到一次较大阻力,以及一次短暂的间歇。需要注意的是,游戏中的增强功能几乎是平均分配在这个关卡中,仿佛是游戏机制的一个组成部分。健康条出现频率虽然也很均衡,但总体数量却非常稀少。

结论

我们可以通过标准化的时间(分解其总体时间长度),在同一图表上绘制三款游戏难度曲线。再重申一次,该图表主要作用是显示游戏难度变化情况。在这三款游戏中,游戏难度曲线的平均值均低于10,这在多数游戏的第一关中极为常见。《铁血兵团:反叛》的难度变化最大,而《闪克》则较为稳定,《合金弹头》也较为稳定,只是偶尔会夹杂一些高峰。

Normalized_Chart(from gamasutra)

Normalized_Chart(from gamasutra)

如果从每款游戏的总体设计来看,我们就不难发现它们这些差别的合理性。《铁血兵团:反叛》的刷任务机制和持续的增强道具使其得以采用陡峭难度曲线,同时也有助于鼓励玩家失败后再重玩游戏。而《合金弹头》则依靠miniboss增加挑战性,允许玩家进行多次尝试。《闪克》却侧重于游戏的易用性,曲线较为平缓,以便新玩家顺利体验游戏。

分析难度图表不但可以让我们了解游戏构造,游戏粘性所在,而且还可以让我们触及每款游戏背后的设计原理。你在设计游戏的过程中,也可以采用这种方法找到合适的难度高峰,或者排除不合适的难度设置。

当然,这也并非衡量游戏难度的唯一可行方法,但却是排除玩家技能因素,独立考察游戏难度的有效工具。通过绘制图表找到结果后,开发团队中的每个成员都可以更容易看到游戏的难点,更容易共同解决问题。我真的建议你也来试试。(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

How Tough Is Your Game? Creating Difficulty Graphs

by Rafael Vázquez

[Having trouble figuring out how difficult your game really is? Xibalba Studios lead game designer Rafael Vázquez devises a method for plotting difficulty -- and uses three popular sidescrolling action games to test the theory, in the process explaining how difficulty meshes their overall design.]

A few months ago, during the prototype stage for a new project, I noticed a problem starting to brew. We were having lots of trouble identifying the ideal difficulty for our game, as different members of the team had very different ideas on what is the perfect challenge.

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I know this is a common issue for small, diverse teams, as what one member might think is a walk in the park another will find more akin to torture. The problem was that everyone had different skill levels, and because of this we weren’t going to reach an agreement anytime soon.

After thinking about this issue for some time, it came to me that difficulty in a game should not be related to player skill, but to the game itself. Instead of looking at the game’s difficulty as a static, all-encompassing threshold, we would do better to discuss how it changes throughout the game.

With this in mind, I tried to develop a method to measure and compare how challenge rises and falls across a game, independent of the player’s skill. This article tries to explain this method. It might still be a little rough around the edges, but I believe it’s a good start, and I hope it encourages discussion in the community so the method can be perfected.
Enter Difficulty Graphs

Getting difficulty right is tough. It might seem redundant, but that doesn’t make it any less true. Make the game too difficult, and people will get frustrated; make it too easy, and they get bored. This is common knowledge, and so designers typically strive for the middle ground. Most of the time, we do this by gut feeling, which is fine.

As it is, the game’s difficulty really depends on the type of game and the type of response we want to get out of the player. A calming social game and an adrenaline-pumping FPS are worlds apart in challenge. The problem comes when you try to talk about this challenge to your fellow team. Long hours pass, and no one seems to agree if that game section is just the right amount of tough.

This is because difficulty is subjective; it depends a great deal on an individual’s skill. To make things worse, the right difficulty also depends on what each person finds fun; some people like masocore games like Super Meat Boy. So when you’re trying to create a game, it’s a real problem to find common ground from where to start discussing.

There is a solution, however — a method to find a common base which we can all agree on: difficulty graphs. Before we dive in though, a quick disclaimer. This method is not for creating difficulty (that’s still up to the designer’s gut, I’m afraid), but for difficulty analysis, and as a means to start meaningful conversation with the rest of your team about it. These graphs don’t show you how a difficult a game should be — only how difficult it is.
So What Are They?

They are graphical representations of how difficulty changes throughout the game. This is to say that they plot how challenge changes over time. There are two main types, time-based and distance-based. The first places the spikes in challenge according to the time spent played (taking away paused time and death); while the second places them depending on where the challenges appear (assuming a direct route from start to goal).

Both have their advantages, and some work best depending on which type of game you’re testing. For arcade games like Asteroids or Geometry Wars, time-based is really the only way to go, as there is no real displacement towards a goal.

On the other hand, most FPS use location-based triggers for their enemies, so distance-based graphs work very well. Personally, I like time-based graphs better, and we’ll get to why in a moment.

Nitpickers might have already noticed that both of these still depend on the player. How fast the player goes through a level and how much time it takes him to get from one challenge to the other depends on how good he is at the game, and also at his play style (if he likes exploring, you can kiss time-based graphs goodbye).

The truth is that our medium is experienced differently by each and every one of us, and this is something we have to live with. The best we can do is get data from multiple playthroughs from different people and average them out. As a matter of fact, difficulty curves can only be applied after playtesting the game several times. After enough testers have tried the game, you’ll get a pretty good idea of what the average player experience is.

As you might recall, difficulty is subjective, and asking all the testers if they felt the game too difficult might give you wildly different answers (you should still do it, though). How then can we measure a game’s difficulty if it depends on the eye of the beholder? The trick is we compare it onto itself.

Say we have a basic enemy that deals x amount of damage. Let’s say he has a threat level of 1; this will be our base enemy. The threat level is basically how difficult an enemy is compared to the player. If, on the other hand, we have another enemy with twice the amount of firepower but the same in all other aspects, we can go ahead and say that it is twice as dangerous and give him a threat level of 2.

I know this might seem to oversimplify things, and that other factors like location, position, and the presence of other enemies matter… We’ll get there. The point is that, by taking the easiest challenge present as a standard (your base enemy), and comparing it with other challenges in the game we can quantify how tough each one is independent of skill. No matter how good you are, an enemy that is three times faster is tougher than the standard.

Recap

So, let’s recap the steps:

* First, you have to determine what the game’s standard conditions are. This is the minimum power level the player has during a determined segment of the game (be it a stage, a level or the whole game). This can vary often due to permanent power-ups.

* Find the base enemy (generally the weakest or most generic enemy) and set its threat level to 1, and adjust the threat level of all other enemies in accordance to its stats.

* Playtest. A lot. Have different players with different play styles move through the level so that you can get the average time (if doing time-based difficulty charts) or average distance (if the chart is distance-based) of each enemy encounter.

* Check which situations you find the enemies in, and assign values to those situations depending on how the mechanics and enemy behaviors allow you to fight them.

* Determine how the encounters work. Figure out how many waves there are in each encounter and what enemies they are composed of.

* Ʃn=0 = (Etn)(ESn) Multiply each enemy’s threat level by the situation multiplier and add them all up for each wave.

* Plot them in a graph according to the time when they appeared or the distance from the starting point where you meet them.

Examples

Just to show an example of how they look, I followed this method to plot three different 2D sidescrolling shooters using time-based difficulty graphs. The games chosen come from different consoles and different eras just to show that this method is universal (and because they’re the ones I had easy access to).

GameLoft

The example games are Metal Slug (published by SNK in 1996 for arcades), Shank (developed in 2010 by Klei Entertainment; this test is based on the PC version) and Hard Corps: Uprising (created by Arc System Works in 2011 for Xbox Live Arcade and PSN).

Now, they might seem quite similar on the surface, but they have several nuances that make them different. This is important to mention, because the difficulty graphs do not actually show which game is harder. Remember that the difficulty is measured based on the game itself; a 1 in Shank is not the same as a 1 in Metal Slug.

What the graphs do show us is how the difficulty changes throughout the game, and this in turn gives us clues on the pacing and tension building of the game. Lots of spikes and you can bet it’s a fast-paced, tense game; lots of plateaus and it’s likely it is calmer.

Note: A great method for directly measuring pacing is Ben Cousins’ method of counting player actions. He describes it in his article Elementary Game Design, which you can find at his personal website, http://www.bencousins.com.

These graphs show the first level of each game from start to right before the boss. I’m leaving out boss encounters because they generally change the rules, requiring specific mechanics for victory. Comparing them directly with the rest of the level can cause gargantuan spikes in the chart that are not really representative of how hard the game is overall.

First up, Metal Slug, the shortest of the bunch (clocking at around 1:20). We can see that it presents enemy encounters every two to three seconds. Besides your standard pistol, the game starts you with some grenades, which are a hard-hitting, though limited, weapon. Unlike the other two, it features one-hit-kills (another way to say all enemies do the same amount of damage) and hefty temporary power-ups, including the titular Metal Slug (a tank, basically). As an arcade game, death isn’t a big deal, as each continue will respawn you at the point of death. This is, of course, until you run out of quarters.

(click for full size)

We can see that the basic trend is quite close to the horizontal axis, with a smooth rise throughout the level. There are two large spikes which represent helicopters. Under standard conditions they are quite tough (taking around 40 shots from your basic pistol to take down). The camera freezes and stops your movement until you kill them, making sure you can’t just skip ahead, explaining the plateaus.

To help you out, the game provides you with power-ups (red vertical lines) right before facing them. As all power-ups in Metal Slug are temporary, the game makes sure you have at least one by dropping a bunch of them really close to one another.

There is another rather large spike near the end, once you start facing other tanks. Provided that you still have your Slug, they should be no problem; however, if you lost it to the helicopter, you could be in a tough spot. The game balances this by giving you health for your tank (the green vertical lines) right after the second chopper and just before the tank section, again trying to make sure you don’t miss out.

Next we have Shank, whose first level is around five and a half minutes long. Contrary to Metal Slug, you have a health bar in the game, so enemy attacks aren’t so devastating. Also the game starts you out with four weapons (pistols, knives, a chainsaw, and limited grenades) allowing you to take on several enemies at once. On the flipside, there are no power-ups except health and more grenades, so what you start with is what you get.

(click for full size)

At first glance, we can see that the game is a slower-paced than Metal Slug, due in large part to having clearly-defined encounters in which the player can’t move ahead. We can also see that in most encounters, enemies appear drip by drip, leading to the stair-like form of the curve. The obvious point of this is easing the player into combat, instead of presenting the challenge all at once.

Between large encounters (which can be distinguished by the high plateaus), we find that the game always presents a couple of enemies, so that the challenge never gets too low.

Just like in the previous example we have a spike near the end — however, this is an absolute spike, presenting the hardest part of the level. After this we see a steep decline until we get to zero, the complete absence of enemies and obstacles, in preparation for the boss battle.

Interestingly, we see that health packs are commonly found in the middle of the encounter (usually because they are enemy drops) and tend to be found more frequently at the later stages of the level. On the other hand, only once can we restock grenades; this is most likely due to the game trying to get the player to learn to use his three main weapons.

Lastly we have Hard Corps: Uprising. In this game, the player has a segmented health bar, with the base enemies taking a full segment with each hit. Though the player only starts with a weak machine gun, she is able to pick up several power-ups which are lost when hit. The first level (minus the boss) clocks in at around 6:20 and is easily the longest of the bunch.

Despite having an Arcade Mode (no character building), the game’s star attraction is the Rising Mode, which allows the player to gain experience and permanently upgrade her character even if she dies. In other words, it allows for grind.

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First off, you’ll notice the off-the-chart spike; I intentionally left it there. This is the miniboss (the typical Contra wall). Clearly its power is off the roof — it takes many hits to bring down and has powerful attacks. However, once you battle it, you’ll notice the terrain gives you a great advantage against it, so in the end it’s not really that tough. This is why it’s recommended to treat your boss fights as special cases. They work differently, often with different rules and game mechanics — hence, they don’t really relate with the whole level.

Putting the miniboss aside, we can see that the game has a very spiky difficulty curve, with short plateaus and lots of ups and downs. This is decidedly a fast-paced game. Despite its ups and downs, the mean does tend to go up throughout the level, with longer confrontations and more enemies being faced at a time.

While encounters near the beginning of the level are at around seven, near the end they reach 30. This is a massive slope in difficulty, perfect for a grind-friendly game. Near the end we once again find a major confrontation followed by a brief respite before fighting the boss. Notice that power-ups are evenly distributed throughout the level, as they form an integral part of the game mechanics. On the other hand, health is much rarer, though it is also found at equal intervals.

Conclusion
We can compare all games’ graphs by simply normalizing the time (dividing it by their total length) and plotting them together. Once more, this is not a measure of how difficult a game is, but how that difficulty changes. Here we can see that in all three games, the difficulty curve’s mean is below 10, which is quite usual for first levels. Hard Corps is the one with the highest change in difficulty, while Shank is a lot more constant. Metal Slug is also quite constant with few (though large) spikes.

(click for full size)

If we start thinking about the overall design of each game, we see that these differences make a lot of sense. The grind and permanent power-ups of Hard Corps allow it to have a steep difficulty curve throughout the level, and actually encourages the player to start over several times to pass it. Meanwhile, arcade Metal Slug, which is trying to suck you out of quarters, depends on minibosses to provide challenge, allowing you to beat them in multiple tries (as long as you pay). Shank, on the other hand, seeks accessibility, having a smooth curve, which allows new players to ease into the game.

Analysis by difficulty graphs tells us a lot about a game — not only about the way it’s structured, and the way it looks for engagement, but also about the design philosophy behind each game. It’s also useful while designing your own games as an analytical way to find spikes and valleys that shouldn’t be there.

Of course, this is not the only method of measuring difficulty; however, this has been very useful as it is independent of individual player skill. Once the results are in, everyone in a team can easily see where the trouble spots are, and it makes tackling them together a lot easier. I really recommend you give it a go. You’ll be surprised how much this makes difficulty easier.(source:gamasutra


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