作者：Brice Morrison

游戏中的“滑坡效应”是指当你落后时，你还会因此落到更后面；遭受损失时，这个损失还会继续对你造成伤害（有点像“雪上加霜”的意思）。（请点击此处阅读本文姐妹篇《解析“无限恢复”机制在游戏设计中的运用》）

比如，篮球赛游戏中，每当一方得分时，另一方则损失一名队员。在这里，落后的损害是双倍的，一是每次投篮都是计分的，二是落后方更加不可能得分。尽管现实的篮球赛并没有这个别扭的特征，所以真正的篮球赛中并不存在“滑坡效应”。在现实的篮球赛中，得分只是让你更接近胜利，但完全不会损害对方的得分能力。

“滑坡效应”的另一个名称是正反馈，就是一个扩大自身效应的环路。因为人们总是很容易就混淆正反馈与负反馈，所以我更倾向于称之为“滑坡效应”。

slippery_slope(from thegameprodigy)

滑坡效应在游戏中造成的结果通常是不好的（游戏邦注：当然，这是对对“滑坡方”而言）。如果游戏的滑坡效应太过强大，这意味着当一名玩家稍微领先一点，他就更可能离最终的胜利再进一步，再再进一步。像这样的游戏，赢家其实早有定数，无论你是玩家还是看客，游戏的过程基本上是多余的。

《星际争霸》和象棋中确实存在滑坡效应，但撇开这个消极属性不提，这两者都是好游戏。在象棋中，玩家损失一个棋子，他的攻击能力、防御能力和控制能力都会稍逊一筹。当然，象棋中还有其他许多因素——定位、势头、布阵等，决定着玩家到底是不是“失败”，当然损失一枚棋子也确实产生了一定的效应。显然，损失太多棋子，比如8个，玩家就彻底处于劣势了。要赢回来简直是猴子捞月了。所谓的胜利，其实是“赢”了很多、很多步，最后来一个“致命一击”的累积效应。

这就是为什么象棋中存在那么多惩罚。如果意识到对方最终会赢，好玩家其实不会再作无谓的挣扎。象棋玩家认为游戏存在这种经常性惩罚很好，没什么不合适，但与不存在滑坡效应的游戏相比，这是个令人扫兴的特点。但不论如何，象棋仍然是一种好游戏。

《星际争霸》中也有滑坡效应。当你损失一个单位时，你受到的是双重打击。第一，你离最终的失败更近一步了（完全没有单位事实上相当于损失了所有建筑）；第二，你更难进攻和防御，因为你不仅损失了得分，用于进攻和防御的单位也减少了。

在篮球赛中，得分与玩法完全是分离开的。你的得分能力并不取决于当前的得分。无论你是领先20分还是落后20分，再次得分的机会都是一样的。在《星际争霸》（和象棋）中，得分与玩法密切相关。损失一个单位意味着离失败更近一步，并且更难反击。

说到游戏的经济系列方面，《星际争霸》的滑坡效应表现更为明显。假设对方提前进攻你，你hold住了。其他方面损失不相上下，但你还多损失了一个生产单位。放到其他游戏中，这大约就相当于落后一分。但在《星际争霸》里，后果就严重多了，因为采矿量的增长接近于指数型，你的对手在资源上只领先你一个生产单位，收益就比你翻了好几番。在你损失那个生产单位起，你就已经在斜坡上往下滚了，此时的劣势效应正在无限放大。

格斗游戏

格斗游戏中一般不存在滑坡效应。比如在《街霸》中，你的角色即使只剩一口气了，仍然行动自如。被打中只是让你的命值（得分）受损，但并不会限制你的行动选择，这点和象棋中损失一个棋子或在《星际》中损失一个工作单位造成的恶果是不一样的。《武士之刃》中倒是意外地存在滑坡效应。在游戏中，被打中腿，玩家就会行走蹒跚；被打中手臂，可能那手臂膊就算是残了。这在格斗游戏中是极为罕见的。

虽然，从现实一点的角度讲，快死的角色也是可能慢慢地爬着，至于能爬多远就另当别论了，但这样的游戏也太没意思了。（游戏邦注：至少在《武士之刃》中，这部分会持续数秒钟，然后你才挂掉。）在《街霸》中，复原是很频繁的，所以所有玩家都能“笑到最后”。其实《街霸》中还是存在一点点滑坡效应的（如果你气数将近，你肯定很担忧被堵在角落，而当你的血条全满时，你从来不担心这个），但总的来说，这是个中性的滑坡效应。

有一款格斗游戏因为是个例外而显得格外突出，它就是《Marvel vs. Capcom 2》。在这款游戏中，各名玩家选择3名角色。在任意给定的时间内，屏幕是只出显一名可活动的角色，其余二名则在屏幕之外的地方恢复受损的精力。玩家可以召唤屏幕之外的角色来辅助主角色，之后再切换屏幕。主角色可以与辅助角色一起发动攻击，从而丰富进攻策略和技巧。玩家可以任意转换活动角色，但如果他已经损失了所有角色，就算是失败了。在这里，滑坡效应就出现了。当玩家只剩最后一个角色时，而其对方仍然有两个甚至全部角色无损，那么前者就明显处于下风。玩家的当前角色没有办法得到辅助攻击，胜算可谓微乎其微。恢复在这款游戏中相当少，游戏往往在玩家“技穷”以前就结束了。

带有“出圈即败”设计的格斗游戏，如《Virtua Fighter》和《Soul Calibur》就尤其不具有滑坡特征了。在这些游戏中，如果一名玩家的角色被推出圆圈，则玩家马上失败，无论此时角色的血条还有多长。从根本上说，无论你目前落后对手多少、无论你的血条还有多少，出了圆圈对你的造成的伤害都是100%的。很久以前，我曾认为这个概念并不高明，除了速战速决，不见得有什么好处，但事实上，“出圈即败”的危险给游戏加分不少。因为“出圈即败”的危险度太高了，无形中给游戏增加了一个“定位”的玩法；也就是，玩家必须在打击对手的同时稳住自己的位置，以免被推出圆圈。

有限的滑坡效应

格斗游戏的滑坡效应范围非常小、非常有限，这对格斗游戏来说是一个优良的特点。如果游戏在任何时候都不存在滑坡效应，那么各个环节之间可能会让人觉得比较脱节。虽然，如果你做出的决定始终会引发某种结果，进而影响到后面的游戏，那样会比较有趣。但问题是，如果这种影响发展成了“滚雪球”呢？

在受限的滑坡效应中，你能“滑”多远存在上限，造成的后果也是暂时的。在《街霸》中，被击倒了确实会引发一点滑坡效应。你的命值（得分）受损，同时，角色的活动受到短暂的限制。角色摔倒，显然是劣势情况。这里我们要注意到两点：1、击倒结束后，你恢复所有动作；2、你不能进一步被击倒。

Street Fighter(from thegameprodigy)

图中的Ken被击倒了，暂时处于下风，但这个劣势不会像滚雪球一样越滚越大。

将对手击倒的影响确实在后面体现出来了，但这个优势很快就被重新组合，不会再“滚雪球”了，因为不存在“进一步击倒”对手这种事。如果你已经摔倒了，你当然不可能“再摔倒”。

另一个例子是把对手逼到角落（游戏界面的边缘）。如果你这么做了，你就占据了“地利”，因为对手的行动受限了。但还是存在一个限度——一旦对手被逼到角落了，他不可能再被逼进“角落的角落”了。对手所处的劣势程度是有限度的。

再举一个更直接的例子，任何时候你采取阻挡动作，你就得到一定的复原量。此时，你的恢复速度会领先于正在进攻的对手，所以在下一次进攻时，你在时间上有可能先出手。这是你的优势，因为如果你们双方都打算发动相同速度的进攻，你的角色会赢（因为它会先行动）。你的阻挡动作的成效就在这里表现出来了，但这个效益是转瞬即逝的，可能仅仅过了一秒，优势就不复存在了。

所以，格斗游戏中充满了小型滑坡效应，这确实给游戏增添了乐趣。但从宏观的角度看，这些不算真正意义上的滑坡效应，因为其效果不会像滚雪球般随着游戏进程渐渐变大。与象棋相比，相当于你是在几个回合后就把失去的棋子拿回来了。

没有滑坡效应的RTS

这里我有一个想法，就是把完全的滑坡效应（通常是恶性的）变成有限的滑坡效应（通常是良性的）。双方玩家一开始均持有相当的资本去购买单位。当你的单位被摧毁后你的资本就会得到偿还。一方面，偿还需要一定的时间，另一方面，重新生产新单位需要一定的时间，这两方面意味着损失单位确实产生了消极影响，但这种劣势会渐渐消失，这与格斗游戏中的被击倒是一样的。即时策略游戏《World in Conflict》正是这么做的，不过我本人没有玩过。

我说这些的目的不是评判《World in Conflict》这款游戏好不好，或者讨论上述的“恢复系统”可取不可取。我只是想表明，如果你能努力研究一下，消除RTS中的滑坡效应还是可能的。那些非常乐衷于此的人可能会想出更高明的解决方案吧，然后一款更高深的游戏就此诞生。

不知各位是否已在自己开发的游戏中运用了滑坡效应呢？（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）

Game Design Profile: Slippery Slope (Part I)

by Brice Morrison

[Editor: In this guest post from David Sirlin, he discusses a common design that shows up often in games called the slippery slope, and how it works.]
If a game has slippery slope, it means that falling behind causes you to fall even further behind.

For example, imagine that every time your team scored in basketball that the opponent’s team lost a player. In that game, falling behind is doubly bad because each basket counts for score AND it makes the opposing team less able to score points of its own. The actual game of basketball does not have this screwy feature though, so real basketball does not have slippery slope. Scoring in real basketball puts you closer to winning but does not at all hamper your opponents’ ability to score.

Slippery slope is another name for positive feedback, a loop that amplifies itself as in a nuclear reaction. Because people confuse the terms positive and negative feedback so easily, I prefer the more descriptive term slippery slope.

Slippery slope is usually a bad property in a game. If a game has a powerful slippery slope effect, that means that when one player gets a small early lead, he is more likely to get an even bigger lead, which in turn makes him more likely still to get yet an even bigger lead, and so on. In a game like this, the real victor of the game is decided early on, and the rest of the game is futile to play out (or to watch).

StarCraft and Chess do have slippery slope. They manage to be good games anyway, despite this anti-climactic property. In Chess, when a player loses a piece, his ability to attack, defend, and control space on the board is slightly reduced. Sure, there are many other factors in Chess–positioning, momentum, pawn structure–that determine if a player is actually “losing,” but losing a piece does have an effect. Clearly, losing a lot of pieces, say 8, puts a player at a significant disadvantage. It’s pretty hard to make a comeback in Chess, and a game is usually “won” many, many moves before the actual checkmate move.

This is why there are a lot of forfeits in Chess. Good players don’t actually play out the pointless part of the endgame when they recognize the opponent will definitely win. Chess players would say that forfeits being a regular part of the game is fine and not awkward, but it’s a disappointing quality compared to games without slippery slope. Still, Chess is a pretty good game anyway.

This guy just lost a Chess piece.

StarCraft also has slippery slope. When you lose a unit, you are penalized doubly. First, you are closer to losing (having no units at all is so crippling as to be virtually the same as the actual loss condition of losing all your buildings). Second, you are less able to attack and defend because the unit you lost was not just part of a score, but also part of the actual gameplay of attacking and defending.

In basketball, the score is completely separate from the gameplay. Your ability to score points doesn’t depend at all on what the current score is. You could be ahead by 20 points or behind by 20 points and have the same chances of scoring more points. But in StarCraft (and Chess), the score is bound up with the gameplay. Losing units pushes you closer to loss AND makes it harder to fight back.

StarCraft has even more severe slippery slope when it comes to the game’s economy. Imagine that your opponent rushes you (sends an early attack to your base) and you fend it off. Let’s say you each lost about the same value of units in the exchange, except that you also lost one worker unit. In a different type of game, this might equate to being one “point” behind. But in StarCraft, that can be a crippling loss because gathering minerals is nearly exponential. Your opponent is ahead of you in the resource curve, increasing his earnings faster than you are. You’ve fallen down a very slippery slope here, where an early disadvantage becomes more magnified as the game goes on.

Fighting Games

Fighting games don’t usually have slippery slope. In Street Fighter, for example, your character still has all of his moves even when he’s about to lose. Getting hit puts you behind in life totals (in “score”) but doesn’t limit your gameplay options in the way that losing a piece in Chess does or losing a unit in StarCraft does. An unusual example of a fighting game that does have slippery slope is Bushido Blade. In that game, getting hit can cause you limp around or lose the use of an arm. This is extremely rare in the fighting game genre though, and for good reason.

While it might be “realistic” for a nearly dead character to limp, move slowly, and have generally less effective moves, it’s not fun. (At least in Bushido Blade’s case, this part of the game lasts only a couple seconds, then you lose.) Meanwhile in Street Fighter, comebacks are frequent and games are often “anybody’s game” until the last moment. Street Fighter does have some very minimal slippery slope aspects (if you’re very near death you have to worry about taking damage from blocked moves which aren’t a threat if you have full life), but overall it’s pretty “slippery slope neutral.”

There is one fighting game that stands out as an exception: Marvel vs. Capcom 2. In this game, each player chooses 3 characters. At any given time, one character is active and on-screen, and the other two are off-screen, healing back some lost energy. The off-screen characters can be called in to do an assist move, then the jump off screen again. The main character can attack in parallel with the assist character, allowing for a wide variety of tricks and traps. The player can switch the active character at any time, and he loses the game when he loses all three characters. But here, slippery slope rears its bitter head. When one player is down to his last character and the other player has two or even all three of his characters, the first player is at a huge disadvantage. The first player has can no longer attack in parallel with his assists, which often means he has no hope of winning. Comebacks in MvC2 are quite rare and games often “end” before they are technically over.

Fighting games with “ring out” such as Virtua Fighter and Soul Calibur as especially devoid of slippery slope properties. In these games, a player instantly loses if his character is ever pushed out of the ring, no matter how much energy he has. Basically, no matter how far behind you are, no matter how close you are to losing, you always have a 100% damage move: ring out. Long ago, I thought this concept was “cheap” and served only to shorten games while adding little benefit, but actually the threat of ring out adds quite a bit to both these games. Since the threat of ring out is so great, another whole element of positioning is added to the game. A player must fight both to do damage to his opponent, and fight for position to avoid ring out.

Limited Slippery Slope

Fighting games do have very localized, limited kind of slippery slope that’s actually a good quality. If a game truly has no slippery slope whatsoever at any point, then it can feel like a series of disconnected decisions. It’s interesting though, if a decision you make at one point in a game echoes forward through time, and can influence later moves in the game. The problem is if this influence is allowed to snowball into a greater and greater advantage.

In limited slippery slope, there is a cap on how far you can slip and the effect is temporary. In Street Fighter, getting knocked down (hit by a sweep) does have a bit of slippery slope. You lose health (“score”) but you also have temporary limitations on what your character can do. Your character falls down, then gets up into what is usually a disadvantageous situation. The two things that are important about this are: 1) after the knockdown is over, you regain all your moves and 2) you cannot get doubly knocked down.

Ken is at a temporary disadvantage here from being knocked down, but the disadvantage can’t snowball into deeper levels of knockdown (there aren’t any) and it fades with time.

Hitting the opponent with a sweep does echo forward through time, but this advantage is reset soon after and can’t snowball into “getting REALLY knocked down” because there is no such thing as degrees of knockdown. If you are already knocked down, you can’t be knocked down “even more.”

Another example is backing the opponent into the corner (the edge of the stage). If you do this, you have a natural advantage because the opponent has fewer movement options. But again, there’s a limit here. Once the opponent is in the corner, he can’t be “more in the corner.” There’s a limit to how disadvantaged he can get.
An even more basic example is anytime you block a move that has a fair amount of recovery. In these case, you recover from your block stun before the opponent recovers from his move, so you have a few frames to act first. This gives you an advantage because if you both try to do a move of the same speed, yours will win (it will start first). Your good decision to block echoed forward into the future, but the effect is very fleeting. Even one second later, this advantage fades.

So fighting games are full of small, temporary slippery slope effects that actually help the game. And yet, on the macro level, they do not have the real kind of slippery slope, the permanent kind that snowballs until the game ends. Compare this to Chess where you don’t just get your captured pieces back a few turns later.

And RTS Without Slippery Slope

Here’s an idea for turning the full-on slippery slope (usually bad) into the limited kind (usually good). Both players start with the same amount of resources to buy units. When your units are destroyed, your resources are refunded. A delay in the timing of this refund combined with the build-time for making new units means that losing units really is a disadvantage, but that the disadvantage fades over time, similar in nature to getting knocked down in a fighting game. The real-time strategy game World in Conflict does exactly this, but I’ve never actually played it.

My point here isn’t about whether World in Conflict is a good game, or even whether the exact refund system stated above is good. It just shows that it is possible to remove slippery slope from an RTS if you try hard enough. Someone very dedicated to that problem could probably come up with an even better way to remove it that results in a deeper game, rather than a shallower one.

Readers, have you used a slippery slope in a game you’ve developed?  How might you use (or avoid using) a slippery slope?（source:thegameprodigy）