Dynamic difficulty

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A comic depicting the rubber band AI of Mario Kart 64.

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 they're going, almost as if they were attached by a rubber band. Rubber band AI is most popular in racing games, but was adopted heavily in sports and fighting games recently, and isn't unheard of 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 surpass their maximum speed when a player is ahead of them, guaranteeing that they will always be close behind the player.

When rubber band AI is implemented evenly, I usually don't mind it, but when benefits the AI over the player allowing the computer to "cheat," I despise it. I almost always prefer when games have discrete difficulty levels.

Successful Implementation

I find that rubber band AI works best either when it benefits players and AI evenly, or when it helps keep the game on track. I also don't mind when the AI difficulty ramps up, but it's level is clearly described to the player.

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 In the jetbike race with Johnny, neither the player nor Johnny can ever get too far ahead or behind. Although this makes the "race" rather ridiculous, it is evenly balanced between the player and AI.
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, player who stay in the same place for too long are punished, etc. The game has built in difficulty levels, so why mess with the difficulty dynamically?
Mario Kart 64 1996-12-14 Has at least two forms. The first form occurs in item randomization. Drivers in the lead get weak items like single bananas or green koopa shells, while those in the read get good items like three red shells, stars, and blue shells. This is balanced for human and AI drivers.

The second occurs with the max speed of AI karts only. In multi-player races, the AI will get an speed boost allowing them to always hound the leader, but human-players will not receive this benefit. The AI speed-up is so flagrant that, even if you use the track jump 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.

Links

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