An EXAG Science III

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(This is a series of short ‘previews’ of papers to be presented at the upcoming Experimental AI for Games workshop at AIIDE 2014. Tune in live on Twitch on October 8th to catch the presentations of these papers, or find the PDFs online at

Game stories often have an intended path for the player to follow. But players don’t always play along. Sometimes players just miss the main story thread. Other times players even try to foil the intended story arc. Is there a way to adjust the story or world to keep players on track? Can an interactive narrative give players unconstrained choices while maintaining the intended story? Justus Robertson and R. Michael Young, in their paper “Gameplay as Online Mediation Search“, present the General Mediation Engine system (GME) to guide players along an intended story in a game world. Read on for a preview of how the system works to guide players along an author’s intended story path. 

Story authors give the GME an intended narrative trajectory (a sequence of in-game events) to guide the player along. The GME tracks the game world and considers how player actions advance or prevent the desired narrative. The authors’ key insight is to consider an interactive narrative as a game between the player and the GME. In this model, every player action changes the game world and might alter the possible narratives that can result. In response, the GME directs in-game NPCs to ensure the desired trajectory can occur.

To play this game GME considers three types of player actions. Exceptional actions prevent the narrative from occurring. Constituent actions move along the narrative trajectory. And consistent actions are irrelevant to helping or hurting the narrative. GME responds to player actions by using a story oracle to know how NPCs should guide the player toward the desired trajectory. Here, the story oracle is a planner, though other systems are possible. GME has different strategies to respond to each type of player action to maintain the desired narrative. When players prevent the desired narrative, GME can use a process called accommodation to devise a new narrative trajectory similar to the original.

Modeling interactive narrative as game tree search opens many possibilities for future development. Currently, GME only uses accommodation. Other techniques to handle exceptional actions include intervention and proactive intervention. Intervention prevents the effects of taking exceptional actions to enforce a desired narrative. Proactive intervention is more subtle: it adjusts world conditions to stop players from taking exceptional actions. The paper delves into more details on how to guide interactive fiction narratives using the GME.

To see the paper’s full version, you can visit The paper will be presented at EXAG, and streamed live on Twitch on October 4th at

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