Adaptive AI Engine for RTS Games

Discussing the theory and practice

On-line Planning for Resource Production in Real-Time Strategy Games -part2

Posted by ferasferas on November 2, 2010

The RTS Resource Production Domain

Human players typically have no difficulty selecting actions that at least achieve a particular set of resource goals. However, it is much more difficult, and requires much more expertise, to find close to minimal makespan plans.

As an example consider the seemingly simple problem of collecting a large amount of gold starting with a single peasant and “townhall”. One could simply repeatedly collect gold with the single peasant, which would eventually achieve the goal, but it’s far from optimal where another player can choose to execute one or more actions, defined from the action set of the game. Each action produces a certain amount of products, but also consumes a certain amount of other resources, and requires that some preconditions are met before it can be executed.

Actions are Durative : take a certain amount of time to finish upon which the products are added to the game state.

In RTS games, resource-production actions are usually deterministic, and the preconditions, effects, and durations of each action are usually given or can be easily discovered through game-play. For certain actions, where a unit has to travel to a destination for an action to take place, the duration of the action will vary due to the spatial properties of a game map. However, for simplicity we assume we have a constant duration for each instance of an action. On average over the many actions taken during a game turns out to be a reasonable assumption.

Game State Representation at time t :

( 1 ) For each resource Ri , the amount ri possessed by the agent.

( 2 ) List of actions Ai , i = 1, . . . , m ,currently being executed along with the start and end times “ts” and “te” for each action ( ts < t < te ).

Goal State Reached When :

All actions currently executing in S have terminated as the projected game state, denoted by Proj(S). This state is timestamped with t = maximum te of all defined actions, has resources updated according to the effects of the actions Ai , and no actions being executed. reach a certain resource goal, G = {R1 ≥ g1 , . . . , Rn ≥ gn },defined as constraints on the resources, from the current game state. In essence, the player must determine a plan, which is a list of actions, ((A1 , t1 s , t1 e ), . . . , (Ak , tk s , tk e )), where Ai is an action that starts at time ti s and ends at time ti e .

Four Resource Tags

Require: An action requires a certain amount of a resource if it needs to be present throughout the execution of the action.

For example, the action collect-gold requires the presence of the townhall In this case, the same town-hall can be used for concurrent collect-gold actions, as the specifications townhall is not “locked up” by the collect-gold actions. Thus, the requires tag allows for sharing of resources.

Borrow: An action borrows a certain amount of a re-production source if it requires that the resource amount be “locked up” throught the execution of the action, so that no other action is allowed to borrow those resources during it’s execution. After the action has completed the resource amount is freed up for use by other actions.

For example, the collect-gold action borrows a peasant. During the execution of collect-gold action, the borrowed peasant may not be borrowed by another action. After the collect-gold action is finished, the peasant becomes available again and can be used for other action. Therefore to allow concurrent collect-gold actions,multiple peasants must be used.

Consume: An action consumes a certain amount of a resource at the start of it’s execution, as this amount is deducted from the game state. As the game state must obey the constraint that every resource value is non-negative the inferred precondition of the action is that this resource amount must be present at the start of the action.

For example, the build-barracks action consumes 700 units of gold and 450 units of wood.

Produce: An action produces a certain amount of a resource at the end of it’s execution, as this amount is added to the game state.

Resources are Divided into two Classes:

-Consumable Resources: are those that are those consumed by actions, such as gold, wood, and supply(a peasant of footman cannot be built unless there is unused supply).

-Renewable Resources: are those that are required or borrowed by actions, such as peasants, town halls and barracks.

Model Domain Characteristics :

1- Dependency Structure between resources is such that, if the initial state has a townhall and a peasant( and assuming the world map has enough consumable resources like gold and wood), there always exists a plan for any resource goal. Further if such a state can’t be reached, then no such plan exists.

2- The amount of renewable resources in a problem never decreases, since no unit is destroyed in our scenarios.

3- By Wargus action specification, all effects at the start of an action are subtractive effects, while all effects at the end of an action are additive effects.

4- Again by Wargus action specification foreach resource there exactly one action that produces it. This property implies that every plan that produces the goal resource from a game state must contain the same set of actions( though possibly not in the same sequence).


One Response to “On-line Planning for Resource Production in Real-Time Strategy Games -part2”

  1. Tahsin said

    Step-by-step easy steps of application of game tactics. There are simple and straightforward methods of free bonuses also have a strategy wow schools for ideas.

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