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By William van der Sterren
Gamasutra
[Author's Bio]
September 12, 2001

Terrain Reasoning

Waypoint Graphs

Analyzing Terrain and Adapting Tactics

Other Terrain Reasoning Applications

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This feature originally appeared in the Game Developers Conference 2001 Proceedings.

 

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Features

Terrain Reasoning for 3D Action Games

Waypoint Graphs
Waypoint graphs convey a lot more information than just the shortest path from A to B. For example, the waypoint graph in Figure 1 below suggests terrain that is composed of three rooms, connected by a few hallways.



Waypoints expressing the shape of a level (left), and waypoints in-game (right)

All waypoints together not only communicate the shape of the accessible terrain. If enhanced with line-of-sight and line-of-fire information, waypoint graphs can be used to express many ingredients that make up tactics.

Picking Tactics Apart
Tactics in 3D action games frequently deal with the terrain. For example, in deathmatch games, the winner typically is the one able to seize and hold the location where the most powerful weapon, item or power-up respawns (such as the "quad damage").

In 'capture-the-flag' (CTF) team games, the team able to identify and defend the bottleneck areas near the home base will stand a good chance. In a tactical shooter, the squad that manages to reach the objective via an concealed approach will have good chances of surprising the enemy.

In all these examples, terrain plays a big role. Actually, the relation is often big enough to enable tactics ingredients to be expressed in terms of waypoints and their relations.

Look at the game terrain, pick two locations, and ask the question: Why is one a better (or worse) location than the other for a specific tactic?

Relating Tactics, Terrain and Waypoints
In deathmatch games, p1 often is better if it is closer to a key power up than p2, or if enables to the actor to see and lay fire on power up locations. "Closer to" can be measured in travel time along the shortest path, and the presence of a line of sight between two waypoints often equals the availability of the line of fire between the corresponding locations.

The bottleneck area in CTF is typically on the access route to the base. Access routes can be identified from the number of paths to the base that pass through that area. The bottleneck area also offers little cover from observation and incoming rockets. Expressed in waypoints, that means that waypoints in or near the bottleneck area all have a line-of-sight and line-of-fire from a number of waypoints in the base area.

A concealed approach consists of a path to the destination that cannot be observed for long periods from locations near the destination. In other words, solely a small number of path waypoints can be observed from waypoints near the destination, and it never involves a long string of consecutive waypoints. Note that the quality of a tactical expression in terms of waypoints depends on the density of waypoints: the denser the waypoint graph covering the terrain, the more accurate the expression (see also section 6.1).

The expressions also make assumptions about the effects of weapon, and player movement abilities.

Waypoint Calculations
The direct relation between tactical concepts and the waypoint graph enables us to turn these concepts into computable properties. For meaningful results, those computations should resemble the human tactical interpretation.

A waypoint has four different kinds of characteristics that all should be taken into account:

  • The local environment. Waypoints describe, for example, how dark the location is, which types of movement are required (crouching, swimming, using a ladder), whether a door or button entity is present.
  • The membership(s) of higher-level terrain concepts. Waypoints may be grouped to represent terrain concepts such as a room, a lake, or the blue team's base.
  • The relations with other waypoints. Waypoints have relations with other waypoints: is there a valid line-of-sight, how long does it take to get from one waypoint to the other.
  • The focus (in the relations with other waypoints). A waypoint whose relations, such as line-of-sight or easy access, are clustered primarily in a single narrow direction has focus in that direction.


Waypoint characteristics: local environment, group membership, relations and focus


Of all the four characteristics, focus, is the most complex to grasp. The following case studies will illustrate why it is needed.

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Analyzing Terrain and Adapting Tactics


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