Dynamic difficulty

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A comic depicting the rubber band AI of Mario Kart.

Rubber band AI is a video game term which describes a dynamically changing difficulty level based on the player's performance in order to maintain a constant challenge. Stated simply, the better a player does, the harder the game becomes. The term "rubber band AI" is based on the fact that computer-controlled opponents in racing games which implement this behavior seem to always catch up to the player no matter how fast the player goes, almost as if they were attached by a rubber band. Rubber band AI is most popular in racing games, but has been adopted heavily in sports and fighting games, and can be seen in various other genres as well. One of the most famous implementations of rubber band AI is in the Mario Kart series which uses it in various ways, the most obvious is how AI racers have their speed increased beyond what human players can achieve in order to prevent the player from ever getting too far ahead of them.

Implementation

Rubber band AI can be divided into two forms based on whether it affects all players equally or only benefits the AI. I will use Mario Kart 64 to explain both forms.

An example of rubber band AI that is implemented evenly, can be seen with how items from question mark blocks are awarded. Those players in the lead with get a less-valuable item like a banana peel or a green turtle shell, while those in last place will receive more-valuable items like lightning bolts, stars, or a blue shell. I usually like this form of rubber band AI because it tends to keep the game interesting even when a weaker player is playing against a stronger player. This form helps keep a sense of balance in the game by allowing stragglers to catch up and preventing the leader from getting too far ahead. And, even the the stronger player makes a mistake and winds up in last place, they can rest assured knowing they too will benefit from the better items.

An example of rubber band AI that only benefits the AI can be seen in the maximum speed of the karts. In Mario Kart 64, each kart has a maximum speed when a human player is driving it, but the AI always exceed the maximum speed by wide margins whenever they're not in first place, regardless of the game's difficulty setting. This form of rubber band AI has a negative impact on the game in multiple ways. The first is a morale problem. If human players know that the AI doesn't have to play by the same rules as they do, it creates a feeling of unfairness which hurts enjoyment. Second, at least in racing games, this tends to hurt the weaker human players because the best player will set the pace for all the other AIs guaranteeing them to never fall behind, while the weaker human players will continue to fall further behind because they do not benefit from the speed increase. Third, rubber band AI is frequently a result of lazy coding on behalf of the developers. If the developers wrote a better AI, they could still scale the difficulty of the AI racers without having to resort to violating the game's rules.

Games

Here is a list of games that are important to me which utilize rubber band AI:

Title Released Description
Chrono Trigger 1995-03-11 The jetbike race with Johnny, is more like a rail shooter since you don't even control your acceleration, and neither the player nor Johnny can ever get very far ahead or behind. This makes it difficult to call what you're doing a "race," since using a boost just before the finish line guarantees victory, but at least it is implemented fairly. There are also items which can be acquired if the player is able to prevent Johnny from taking the lead by maneuvering around him properly.
Diddy Kong Racing 1997-10-21 Although it uses rubber band AI, I have found it to be far more tolerable than what is used in Mario Kart 64.
Left 4 Dead 2008-11-17 Left 4 Dead uses what is called the "AI director" which is just a fancy name for an algorithm which tries to balance the difficulty of the game. Essentially, if the players aren't doing so well, more healing items will be found, but if they're doing really well, uncommon infected will spawn faster. I don't mind this too much, since the game needs to have a fair amount randomness built in to keep the stages interesting, but I would have preferred set difficulty levels to make the accomplishment of victory feel more earned.
Left 4 Dead 2 2009-11-17 From what I've read, the AI director is much more sophisticated in this game, but also more punishing. Players who venture too far away from the pack are punished with powerful uncommon infected, players who stay in the same place for too long are punished with hordes, etc. The game has built in difficulty levels, so why mess with the difficulty dynamically?
Mario Kart 64 1996-12-14 I like the rubber band AI in the item randomization because it helps out weaker players, but increasing the maximum speed of AI carts far more than human players are allowed ruins the game for me. The AI speed-up is so flagrant that, even if you successfully jump the track in Rainbow Road on each lap, and E-slide the rest of the course, the AI will still catch up with you by the end.

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