Event-reaction model proposal

Subject: Event-reaction model proposal
From: Jari P Saukkonen (jsaukkon@cs.Helsinki.FI)
Date: Thu Nov 18 1999 - 23:24:52 EET

In this document I propose a following model for handling actions in the
world. It might get a bit technical at times (to explain implementation to
other developers), but I'll try to keep this readable for others too :).

First, some terminology:

Event, is any action that some object (player, npc, monster, ...) does.
For example movement, item handling, speaking and basically everything
that includes action detectable by senses (sight, hearing, ...). 

A reaction is a piece of code that is performed when object receives an
event. Object does not receive any events that it cannot react to.

In practice, events are classes that have different attributes depending
on the event.

I'm not that good in explaining so let's start with an example.

Plot: Yorkaturr is on a sunday-walk, and happens to walk through a small

When he moves, 'movement' event is dispatched. In case Yorkaturr wasn't 
already in the vicinity (ie. villagers haven't yet noticed him), an
'arrive' event is generated. 

Villager's code has a reaction defined for the 'arrive' event. It checks
the source of the event, Yorkaturr, and starts examining his outlook. 
It determines that Yorkaturr is a quite scaring sight, and thus the
villagers starts to flee in panic.

In case the villager would have been interactive (a PC) the code would
have catched the 'arrive' event and sent a command to client to add
Yorkaturr's object to the scene. Further movement events would cause the
object to move in the client. 

For the implementation we will probably need a huge tree structure that
divides the world in equally-spaced cells. Nodes of the tree contain
references to objects. When object moves, it's position is updated in the
tree structure accordingly, and it's very quick to determine the nearest
objects (that an event needs to be dispatched to) by traversing the tree
up a couple of levels (depending on the event sending range) and checking
all nodes with the subtree.

With this universal model, we can get over the problem of keeping the
clients in sync, and provide a framework for building decent AI.

If we start extrapolating, it could be possible to build a fighting AI
that is based on reacting to opponent's movements and strikes. This is
however purely theoretical, and would be quite cpu-intensive, but the
framework would make it possible :).

Hmm, maybe that's it, South Park is on TV :).. Ask if something is still

++ Dazzt

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