Soldier of Fortune 2:Double Helix

Paris YorkCIS 588

Jumping, Climbing, and Tactical Reasoning: How to

Get More Out of a Navigation System

• Christopher Reed and Benjamin GeislerRaven Software/Activision

“using navigation to give AI more than just routes from one place to another.”

Image Sources: https://zerowing.idsoftware.com/svn/radiant/Sof2Pack/trunk/docs/Nav%20Point%20System.htm

• Navigation system is graph-based– Points - represent locations– Edges - represent safe routes

Enhanced Navigation Data

• Points and edges have static attributes attached to them, these are available to the AI in a look up table

• The data allows the appearance of more intelligent NPCs, and allows the system to be less resource intensive

• The path is selected by at a higher level, traversing points and edges is handled separately

Example: Bounding Overwatch– NPCs act in tandem, one covers while the

other advances; roles then switch until the goal is reached

– Points are marked as safe cover, the path selection is then biased towards these safe points.

• Example: Throwing Grenades– Problem -- How do we get the AI to know the best

place to throw a grenade?– Answer -- Compute all possible grenade throws in

advance and encode results in navigation system

Obstacle

Closest Point to Player

Closest Point to Agent

Standard Edge

Throw Edge over Obstacle

Agent

Player

• Biasing Edge Cost– Bias edge cost to prevent NPCs from

taking undesirable routes simply by increasing edge cost.

– Edges can also be biased for unique behaviour, such as flying

ParentPoint = OpenList.GetFromOpenList()

For each edge from ParentPoint

Switch Edge.type()

Case FLY_EDGE:

If (actor.CanFly() )

Edge.cost = actor.FlyBiasCost()

OpenList.AddToOpenList(Edge)

• Dynamic Movement: Opening a Door– The path is already generated, so

encountering a door is a simple matter.– The edge has the door attribute, the AI

automatically selects the appropriate animation, then continues on the path

– This also applies to using ladders, special doors, etc.

The Navigation Point

• Points provide discrete locations in space where agents can move to and from

• Can be any shape, sphere is easier to store -- point, radius

• Shape covers navigable area around point -- no obstacles, holes, etc

• Obstacle Visibility -- Side to Side and Vertical – If an obstacle is detected close to a

navigation point, information about this obstacle, such as height and width, can be pre-computed and stored in the nav system

• A low wall is good cover, so is the side of a building

The Navigation Edge

• Edges connect points to tell the nav system how to safely move between points

• Certain characters may not be able to traverse an edge, or may require a special action to do so

Examples of Edge Data

• Size– Sometimes characters are big, really big

and can’t fit down some paths.– Using collision detection, the maximum

size of a character that could travel a path can be determined and encoded in the nav system

• Jumping or Flying– If points in the air and the ground are

connected, they need to be encoded so that only flying characters attempt to follow

– If points connect over a hole, the edge can be encoded as a jump edge, restricted by game animation and physics

• Through Moveable Obstacles– Actions necessary to remove the obstacle

are encoded into the edge• Opening a door• Destroying a crate