Game Development Reference
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without emotions? The Sims do not have emotions, but they do have needs,
preferences (traits), and relationships. Friends, lovers, and enemies are a function
of the relationships between the Sims. The psychological model for the Sims is
that a strong positive relationship is created between Sims that meet each other's
needs through shared interests. As a basis for relationships, real-world experience
suggests that this one is pretty bulletproof. The Sims all have needs such as needs
for food, comfort, fun, and social interactivity. Each individual Sim has a small
number of traits selected from a much longer list of possible traits. These traits
provide each Sim with individual preferences. Each Sim keeps its own rela-
tionship score with every other Sim it has met. Relationship scores need not be
mutual. The relationship score runs from 100 to þ 100.
Positive interactions build the relationship score, and negative interactions
reduce it. All Sims act to meet their most pressing need. The interaction that one
Sim prefers to use to meet a need might not be an interaction preferred by the
other Sim. The preferences color the interaction, changing each Sim's relation-
ship score with the other. If both Sims like the interaction, they both react
positively. Thus, meeting needs through shared preferences builds positive
relationships. It is not modeling emotions, but it certainly has proven effective.
Instead of modeling needs, other systems directly model emotions. The same
100 to þ 100 range that The Sims uses for needs is instead used for emotions.
Often thought of as ''sliders'' (vertical scrollbars), each one carries a pair of
opposed emotions. One might be joy versus sadness. Other pairs include
acceptance versus disgust or fear versus anger. A small number suffices because
the number of combinations grows very rapidly as more sliders are added.
The system deals with all of the emotions combined, so look at the combinations
to see if two emotions that are directly modeled give you an emotion that you are
thinking of adding.
On this core data, an input and output system is required. It is pointless to model
an emotion that the system cannot show. It is equally pointless to model a feeling
that is only subtly different from other feelings. If the player cannot tell the
difference, there is no difference. Just as in personality modeling, a broad brush is
required. A few archetypes suffice. The system can output feelings directly into
expression and posture if the animation assets to support them exist. It can output
them indirectly through the kind of techniques described earlier in this chapter.
The input system has to make emotional sense of the world. Strong emotions
fade over time, but the AI has to react to events in the world around it. The
 
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