The definition of the planning domain, more specifically the semantic of actions and propositions included, plays a critical role over the quality of the unfolded plans, and thus on the genre of generated narratives. The first step is the elicitation of all the planning knowledge required and starts by providing a complete propositional representation of the world.
The nature of the propositions included depends on the different types of predicates and are represented using a dimensional representation. Each dimension contains a set of discreet values, which represents the values contained within the planning operators. The propositions are best represented with the formalism which is composed by:
- a Dimension: a predicate that represents the symbolic nature of the dimension, e.g. anger, hunger, location, time, fear, etc.
- (Optionally) a list of symbolic parameters separated by comas containing:
- a list of characters - or objects - representing subject and target(s) of the proposition, e.g. LRRH, WOLF, inside_house, etc.
- an intensity - or the discreet value - qualifying the proposition; e.g. LOW, MEDIUM, HIGH, 1, 2, 3, etc.
The propositions are either unary such as hunger(WOLF, HIGH) and are related to one character, but they also can be binary such as fear(LRRH, WOLF, HIGH). Now, the nature of the predicate present in the dimension allows the classification of the propositions into three different categories:
- Physical propositions
They are used to represent physical or temporal facts such as characters’ location, object properties or time-related property; e.g. Location(LRRH, inside_house), time(morning), etc.
- Contextual propositions
They represent particular situations that can trigger certain types of actions and are usually composed by solely a predicate; e.g. IS_IN_CONVERSATION(LRRH, WOLF), etc.
- Emotional propositions
They define a set of ground mental states and, in a certain way, represent the elementary psychological traits driving the characters’ decisions; e.g. fear(LRRH, WOLF, HIGH), etc.
The initial and goal states are then represented by conjuncts of propositions and correspond to the scene’s or characters’ objectives. The planning operators are represented using a STRIPS-like formalism (i.e. a set of propositions as preconditions and effects) and correspond to actions that can be performed by the characters.