Conceptos Nucleares del estudio

Like PAStE, CANVAS relies on its own set of abstractions to describe a story sequence as the author designs it. The fundamental building block for CANVAS is the event instance (as described in Section 5.3.2).

6.1.1

Event Authoring and Story Structures

Events themselves are authored as PBTs with parameter fields for their participating actors and objects. Because of their graphical nature, behavior trees are easy to author using a graphical user interface and accessible commercial products for doing so are widely available [14]. Because graphical authoring is an explored problem (while still taking considerable development resources to implement), for our purposes we author behavior trees using a structured code format in C#. An example PBT authored in C# appears as follows (with minor syntax simplifications):

public static Node Subtree_DistractAndIncapacitate( SmartCharacter distractor, SmartCharacter aggressor, SmartCharacter target) { return Sequence( distractor.GoTo(target.position), ParallelSequence( Subtree_Distract(distractor, target), Sequence( Wait(17000), aggressor.GoTo(target.backWaypoint))), aggressor.UseAffordance(target, "Knockout")); }

public static Node Subtree_Distract( SmartCharacter distractor, SmartCharacter target) { return Sequence( distractor.Icon("speaking"), distractor.Gesture("callover", 3000), target.GoTo(distractor.transform.position), distractor.UseAffordance(target, "Talk"), distractor.Icon(null)); }

This code snippet presents most of the logic involved in the “Distract and Incapacitate” task, where two characters coordinate in such a way where one disractor calls the tar- get over towards them, while the aggressor moves behind the target and incapacitates them. Some events have no animation component, and exist to silently change the state of the world (like BreakAlliance, which enables two previously allied characters to take aggressive actions against one another by changing their relationship flags). This system exposes the most power to advanced users, but could certainly be converted to a more accessible user interface for end-users with little conceptual difficulty. Once these events

are authored, we organize them into the system with appropriate metadata and begin to assemble more complex narrative structures from them.

Story Beats. A Story Beatβ = {I1, . . . , In}is a collection of event instances for events occurring simultaneously at a particular juncture in the story. The preconditions of a beat

β = {I1. . . In} are a conjunction of the CNF preconditions of all its event instances:

φβ = φe1 ∧φe2,∧. . .∧φen. Multiple event instances within the same beat cannot share

any object participant between them (as all of the events in that beat will begin in parallel). A beat β is valid if all instances I ∈ β are valid. Recall that some of the instances of a beat may be partially defined, in that their candidate sets are missing members for a full successful execution of the event.

Story Arcs. A Story Arcα= (β0, β1, . . . , βm)is an ordered sequence of beats represent- ing a story, where events can occur both sequentially and simultaneously throughout that story’s execution. The beat β0 is the initial configuration of the world as defined by the author, and operates on the initial authored world stateSW.

6.1.2

The Story Sequence

Story arcs are authored using a structure called the Story Sequence Diagram (SSD) in the CANVAS graphical authoring interface. These diagrams chart the progression of the beats within the story arc, and illustrate which objects participate in each beat’s event instances. An SSD is a directed acyclic graphQ=hV,Di.

The vertices of an SSD are divided into two classesV =VW∪V_I. Object vertices

VWrefer to objects from the world, i.e.,VW ≈ {w1, . . . , w_n} ⊆W, while event vertices

three classes D = Dπ ∪Dφ ∪Dτ that indicate three relationship types. Participation edgesDπ ⊆ VW×VI denote a “participates in” relationship between an object and an event instance as a participant. Precedent edges Dφ ⊆ VI ×VI denote a “comes after” relationship between two event instances and are used to align events to different story beats. Termination edges Dτ ⊆ VI ×VI denote a termination dependency, where an edge between(Ii, Ij)indicates that event instanceIj is terminated as soon as instanceIi finishes executing (typically used for repeating background activity). Note thatIi andIj must be in the same story beat for this relationship to be created.

I(2,1) I(1,1) I(1, 2) w1 w2 w3 wn I(q,1) I(q,2)

β

β

β

Figure 6.2:An illustration of the various arcs and vertices that describe a Story Sequence Diagram.

Precedent edges can be manually added by the author to define separate story beats. The CANVAS system also automatically detects when the a single object is used twice in the same beat, and will move one of the two involving event instants to a subsequent beat with a new precedent link between them. We display the SSD in such a way that each horizontal row of event instances delineates a beat, and the ordered sequence of beats represents the resulting story arc. Figure 6.2 illustrates a generic story arc represented as a story sequence diagram.