Topics for Today

• Task planning for non-player characters• Coping with player character interactions and

their effect on narrative

• In Hamlet on the Holodeck, many issues related to character development and player’s interactions with characters/narratives were discussed– But how do we implement real solutions?

Rich Gold’s Little Computer People: Sega 1985

Will Wright’s The Sims (2000), The Sims 2 (2004), and The Sims 3 (2009)

Creating Characters

• How to design/develop the characters in interactive stories?

• Characters in games have routines and respond to triggers

• We want more depth, like in written scripts (e.g. books, tv, movies) but interactive

• What makes characters in traditional media believable?

The Friends Engine

• Character-based storytelling– Story emerges from interaction among autonomous

characters• Planning– Hierarchical task network (HTN)

• Interaction?– Changes to the environment (objects in environment)– Talking to character (to give information, instructions,

advice)

High-level design

• Original intent/design like audience yelling to actors

• Character actions driven by goal(s)– Goals can be achieved through alternative

sets/sequences of actions– Environmental features and other characters can

affect if a action is successful• Interleaving planning with execution and

opportunities for interaction

Planning Engine

• Actions have weights attached describing their characteristics (sociability, rudeness)– Character traits use these weights when selecting

among alternative actions– Different characters would try different plans

• Total order HTN planning to reduce/remove task interaction– Tries to avoid the crazy wolf scenario where

character rapidly switches back and forth between two plans

Interaction with World

• User as spectator– Can explore the space via invisible avatar

• Cannot interfere with characters directly• Can interact with objects in space– Can cause replanning by characters

• “Butterfly effect”– Inconsequential changes can cause narrative

changes indirectly

Communication with Characters

• Natural language intervention• Templates for– Instructions (“talk to Pheobe”)– Information (“the diary is in the living room”)– Advice (“be nice to Pheobe”)

• Heuristics to differentiate types of advice and instructions– Negative statements tend to be generic advice (“don’t be

rude”)– Positive statements are more likely to be instructions (“talk

to Monica”)

Summary

• Task modeling to enable weighting of activities and options

• Human interaction with characters related to activities and options

Preserving Narratives with Player Characters

User as Character

• Interactive Narrative-Oriented Systems– Users interact with animated agents in virtual

world• User’s actions can affect narrative– User has partial knowledge of narrative– Must manage actions to ensure story continues

• How to identify problems and their resolution?

Control and Coherence

• Competing goals• Control– Increases engagement– Not interactive without some control

• Coherence – Scenes and actions should relate to the overall

story– User actions can alter/break the story

Mimesis Planning and Architecture

• Hierarchical partial-order planner– Requires representation of all actions possible by

all characters (including user)• Causal links– Connects plan steps with condition

Monitoring User Activity

• User activity must be evaluated with respect to the planned narrative

• Constituent to the plan– Matches some action in the plan

• Consistent with the plan– Not part of but does not interfere with plan

• Exceptional to the plan– Interferes with plan

• System must recognize exceptions and respond

Responding to Exceptions

• Accommodation– Let the user’s action stand– Replan to keep narrative

• Example: user does not go to planned location– Response: find an appropriate alternative location

• Example: user discards weapon– Response: non-player character finds weapon or a

different weapon is chosen• Accommodation may not always be possible

Intervention

• Replace user action with a failure mode instance of the action

• Example of coin and vending machine– Broken machine– Could have created second coin …

• Mimesis constructs table with all possible exceptions and mediation policy– Only possible because of having representation of

all possible user actions

Run-Time Management

• Execution Manager receives plan and table of mediation policies

• Non-player characters go about initial plan• System waits for user to begin their

constituent actions and watches for exceptions

Accommodation example

Questions

• Consider yourself as user. How would you react to such intervention? What would your reaction depend on?

• How does this compare to what you would expect in the imagined holodeck?