Behavior Trees in Unreal

• To build a behavior tree in unreal you need three items
  • An AI - (AI Controller Blueprint) - ChaserAI
  • A Blackboard - ChaserBlackboard
  • An Behavior tree - ChaserTree
• The blackboard is the easist piece.
  • It is just a place to hold data
  • It is used to share data between nodes in the Behavior tree.
  • In this case there are four variables.
    • TargetToFollow, an object, thing the AI will chase.
    • SelfActor, an object, a pointer to our character.
    • HomeLocation, a vector, the AI's "base"
    • TargetLocation, a vector, the last place the Character was spotted before it was lost.
• The AI
  • Remember, an NPC has an AI
  • Sets the blackboard to use
    • 
  • Sets a value on this blackboard
    • HomeLocation is a vector in the blackboard.
    • So query for it.
    • And set it by name.
    • 
  • Fires the behavior tree.
    • 
    • Note the setting of the tree
• Later we will get and set values from the blackboard.
  • This net sets the Target Location to a computed vector.
  • 
  • This net gets the object stored in TargetToFollow
  • 
• The behavior tree
  • Click to add nodes.
  • Drag to connect.
  • Composite nodes can have services or decorators.
    • A Service will execute while the branch is being executed.
      • In this AI it is responsible for looking for the character.
      • If it finds the character
        • It sets the character location
        • And marks the character as the thing to follow.
      • For now, we will skip this service. It is quite complex.
    • A decorator
      • A conditional in the behavior tree.
      • It is evaluated before the node performs the assigned task.
      • If it is false, the node will not fire.
      • I have built a very simple decorator, A dice check.
        • I use this to decided if I should perform some random tasks in my chaser.
        • It takes two parameters, the sides, and a success value
        • Generates a random number
        • And returs true if the random number is greater than the success value.
        • 
        • Sides and SuccessAbove are integer variables.
        • They are exposed to the next level up.
        • And will act as parameters.
        • 
      • I call this in two places in my behavior tree.
        • First to see if my character should turn a bit when he is at home.
        • Second to see if he should go on patrol.
        • 
        • Note the use of "Sides" and "SuccessAbove"
  • Tasks
    • There are a set of premade tasks
    • 
    • Or we can build or own.