Dynamic difficulty

A comic depicting the rubber band AI of Mario Kart.

In video games, dynamic difficulty, frequently referred to as rubber band AI, is a term which describes when a game's difficulty adjusts itself based on the player's performance in order to ensure a constant challenge regardless of the player's skill level. Stated simply, the better a player does, the harder the game becomes. Dynamic difficulty 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.

The term "rubber band AI" comes from how dynamic difficulty is implemented in racing games. It feels like the AI opponents are attached to the player with invisible rubber bands so that the larger the lead a player gets, the faster the AI springs back to close the gap.

Implementation

Dynamic difficulty can be implemented in all sorts of different ways, but there are two primary methods based on whether it affects all players equally or only if it only benefits the AI. I will use the Mario Kart series to explain both methods.

An example of dynamic difficulty that is implemented evenly, can be seen with how items in question mark blocks are awarded. Those players in the lead will get less-valuable items 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 dynamic difficulty because it tends to keep the game interesting even when a weaker player is playing against a stronger player. A sense of balance is maintained by allowing stragglers to catch up and preventing the leader from getting too far ahead, but, even the the stronger player makes a mistake and winds up in last place, they can rest assured in the knowledge they will now benefit from the better items.

An example of dynamic difficulty that only benefits the AI can be seen in the maximum speed of the karts. Each kart has a maximum speed when a human player is driving it, but the AI is allowed to exceed the maximum speed by wide margins whenever they're not in first place, regardless of the game's difficulty setting. This form of dynamic difficulty has a negative impact on the game in multiple ways. The first is a morale problem. If human players know the AI doesn't have to play by the same rules, it creates a feeling of unfairness which hurts enjoyment. Second, this tends to disproportionately hurt weaker human players. Because the best player will set the pace for the pack, AI players are guaranteed to match their rank, but the weaker human players will continue to fall further behind because they do not receive the same benefits. Third, dynamic difficulty of this type usually indicates lazy coding on behalf of the designers. If the game 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 dynamic difficulty:

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.
Gun Fight	1975-??-??	Probably the very first example of dynamic difficulty used in a video game. Each time a player is shot, the game adds to the defenses of their side of the screen, helping to prevent a more-skilled player from dominating a weaker player.
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 game's difficulty. 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 the difficulty levels set the range of the random encounters 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 brutal. 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. This means the dynamic difficulty forces a specific play style, which I don't like. The game has built-in difficulty levels, so why mess with the difficulty so dynamically?
Mario Kart 64	1996-12-14	I like the dynamic difficulty in how items are randomly awarded because it helps out weaker players, but increasing the maximum speed of AI carts beyond what 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!

Media

Videos

• youtube.com/watch?v=0JV-kMYLYCo - Demonstrating the rubber band AI in Mario Kart 64.

Links

   

• giantbomb.com/rubber-band-ai/3015-35/games - Giant Bomb.