NPC Interaction in Video Games Robert Higginbotham

What is an NPC and what does it do? An NPC, or Non-Player Character, is an agent in a game that typically represents a creature or person in a virtual world. The roles of NPCs can range from being something as simple as a rodent crawling around a city to something as important as a primary character in the game’s storyline. The behavior of any NPC is typically governed by a predetermined set of rules.

Why is interaction between NPCs important? In single player games it is common for the player to be the center of the universe. Most NPC actions will only take place in the presence of the player and usually only in response to some action taken by the player. This style of game design can be entertaining, but it lacks depth. Interaction between NPCs can go a long way towards helping a virtual world seem more alive.

How can this be accomplished? NPC interaction requires an agent function that determines how an NPC will act towards another NPC in a given situation. An excellent example of this is known as the “Radiant AI” system developed for the game Oblivion by Bethesda Softworks.

Radiant AI Radiant AI provides every NPC with a set of goals that they will try to accomplish. Goals are determined by things such as:  needs for food and rest  desire to reach a destination  will to survive if the NPC is attacked

Radiant AI continued How an NPC chooses to reach its goal is determined by a list of variables that includes things such as:  Aggression – How easily the NPC is provoked to fight.  Confidence – How likely the NPC is to flee or defend itself in combat.  Energy Level – How fatigued an NPC is.  Responsibility – How willing the NPC is to commit a crime.  Schedule – How often an NPC will feel inclined to perform a certain task such as eating, sleeping, or traveling.

Radiant AI continued The Radiant AI provides a list of packages of AI instructions. Each NPC is given a subset of these packages that it is allowed to choose from. An NPC will choose the first package in its list that is valid based on the current conditions.

Radiant AI continued Once a package is selected, conditions are again used to determine when and how to execute the instructions in that package. An NPC will continue to execute instructions from its chosen package until it the package is determined to have been completed (in other words, the NPC’s goal has been reached) or another package is determined to be of a higher priority.

Radiant AI examples A resident of a town might become hungry in the evening and decide to walk down to the nearest tavern in search of food. On the way there, the NPC might notice a guard on duty and engage him in a brief conversation before continuing to the tavern. Once the NPC reaches the tavern he will obtain food either by paying for it or possibly stealing it. The NPC would likely prefer to sit while eating and if he notices an unoccupied chair, he will finish his meal there. The NPC is also likely to converse with other patrons during his time at the tavern.

Not so Radiant AI examples Some of the unexpected behavior of NPCs that were discovered while testing the Radiant AI system were rather humorous. One character was given a rake and the goal "rake leaves"; another was given a broom and the goal "sweep paths," and this worked smoothly. Then they swapped the items, so that the raker was given a broom and the sweeper was given the rake. In the end, one of them killed the other so he could get the proper item. Another test had an on-duty NPC guard become hungry. The guard went into the forest to hunt for food. The other guards also left to arrest the truant guard, leaving the town unprotected. The villager NPCs then looted all of the shops, due to the lack of law enforcement.

Limitations and Improvements One limitation of the Radiant AI system that becomes obvious after spending some time playing the game is the shallow pool of dialogue that is available to NPCs who engage in conversation. This can result in some fairly awkward and very brief conversations. One way this could be improved is through giving NPCs different knowledge bases initially to determine what they are willing to talk about as well as implementing a learning function to allow them to increase their knowledge base.