Game Engine Architecture

45

sistent, online virtual world hosted by a powerful batt ery of central

servers.

The multiplayer networking layer is shown in Figure 1.28.
Multiplayer games are quite similar in many ways to their single-player
counterparts. However, support for multiple players can have a profound
impact on the design of certain game engine components. The game world
object model, renderer, human input device system, player control system,
and animation systems are all aff ected. Retrofi tt ing multiplayer features into
a pre-existing single-player engine is certainly not impossible, although it can
be a daunting task. Still, many game teams have done it successfully. That
said, it is usually bett er to design multiplayer features from day one, if you
have that luxury.
It is interesting to note that going the other way—converting a multi-
player game into a single-player game—is typically trivial. In fact, many game
engines treat single-player mode as a special case of a multiplayer game, in
which there happens to be only one player. The Quake engine is well known
for its client-on-top-of-server mode, in which a single executable, running on a
single PC, acts both as the client and the server in single-player campaigns.

1.6.15. Gameplay Foundation Systems

The term gameplay refers to the action that takes place in the game, the rules
that govern the virtual world in which the game takes place, the abilities of
the player character(s) (known as player mechanics) and of the other characters
and objects in the world, and the goals and objectives of the player(s). Game-
play is typically implemented either in the native language in which the rest
of the engine is writt en, or in a high-level scripting language—or sometimes
both. To bridge the gap between the gameplay code and the low-level engine
systems that we’ve discussed thus far, most game engines introduce a layer

1.6. Runtime Engine Architecture

Gameplay Foundations

Event/Messaging

System

Dynamic Game

Object Model

Scripting System

World Loading /

Streaming

Static World

Elements

Real-Time Agent-

Based Simulation

High-Level Game Flow System/FSM

Hierarchical

Object Attachment

Figure 1.29. Gameplay foundation systems.

Online Multiplayer

Match-Making &

Game Mgmt.

Game State

Replication

Object Authority

Policy

Figure 1.28. On-

line multiplayer

subsystem.