Ultimate Game Design : Building game worlds

Building Game Worlds
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U L T I M A T E G A M E D E S I G N
USING TRIGGERS
As we’ve seen, some behaviors are handled with scripts built in JavaScript/JScript,
Python, Visual Basic, or another proprietary language. Increasingly, however, to
complement simple script systems, complex trigger systems are used. A trigger is simply
a switch that can cause an event, or series of events, to occur when tripped. This is
sometimes called event handling.
Building up and using complex trigger systems has a couple of practical advantages.
First, value and parameter editors to control trigger properties can be built into Windows-based
editors (GUI-based editors) quite easily. Second, designers and artists
rarely have more than basic programming abilities (if any), so the property-editing
nature and workflow of setting up trigger behavior is much easier for the
nonprogrammer.
The most powerful world-editing systems combine trigger functionality with
line-item scripting (like the shooter behaviors using a sine wave). Triggers are usually
placed in a scene and represented by icons in a 3-D scene within an editor. They are
completely invisible to the player. Triggers can be used in a number of interesting
ways to build up game behaviors.
In most world-editing software, trigger properties can be set up to include several
types of associated information. These include
� True and false modes (to set the logical state or condition of the trigger)
� Value fields (to enter radius, distance, strength, and offset information)
� Text fields (to print a message to the screen for a player, or for testing purposes)
� File pointers (to point to a file resource as part of a trigger event)
Let’s look at a quick example from the very popular UnrealEd (version 3.0) showing
the power of this kind of trigger-based editing. In many games, when an actor is shot
by a weapon, we frequently want to launch a resulting behavior of some kind. For a
shoot trigger, a trigger that trips when it has been shot by a projectile weapon, we
want to launch a behavior—maybe a further scripted event—or maybe we want to
display a simple text message.
Figure 6-5 shows that for an assigned shoot trigger type (TT_Shoot), when the
trigger is tripped, a text message will be displayed: “Ouch, you shot me!”
This is a simple example to be sure, but it reveals the power of having a field-edited
trigger system at your disposal for shaping actor-based behaviors. Many game developers
create similar systems to enable trigger-based behavior editing.
It’s important to remember that triggers can be edited in a field-edited GUI-type
tool (like the UnrealEd example), edited directly in the script code, or edited in both.