Behavior Trees in TGB Experiment

If you are interested in game AI and haven't heard of behavior trees you might want to check out AIGameDev.com. In short they are the new, sexier finite state machines, basically a good way of laying out behaviors for your game agents in a nice modular and reusable way.

I felt like messing around with them and maybe a few other AI techniques for some experience, and ended up with this...


Some "monster" agents getting hunted down by a "hero" agent.



To give you an idea what it looks like to put together a behavior tree in script...
Note that there's more code behind the scenes doing work of course, in fact some in C++.

function FighterBehaviorTree::create( %agent )
{
   %tree = new BehaviorTree()
   {
      class = "FighterBehaviorTree";
      agent = %agent; 
   };
      
   %root = SelectorBehavior::create();
   %tree.add( %root );        

   // Eat some Food
   %beh = %root.add( SequenceBehavior::create() );
      %beh.add( Condition::create( "HungryCondition", false, true ) );                  
      %beh.add( ChatBehavior::create( "I'm hungry..." ) );      
      %sel = %beh.add( SelectorBehavior::create() );
         %beh = %sel.add( SequenceBehavior::create() );      
            %beh.add( Condition::create( "HasFoodCondition", false, true ) );         
            %beh.add( ChatBehavior::create( "Eating some food!" ) );
            %beh.add( LeafBehavior::create( "EatFoodBehavior" ) );
            %beh.add( WaitBehavior::create( 2000 ) );
         %beh = %sel.add( SequenceBehavior::create() );
            %beh.add( Condition::create( "HasFoodCondition", false, false ) );        
            %beh.add( ChatBehavior::create( "Going to buy food!" ) ); 
            %beh.add( MoveToPositionBehavior::create( $TavernPos, 0.1 ) );
            %beh.add( LeafBehavior::create( "BuyFoodBehavior" ) );
   
   // Kill a Monster
   %beh = %root.add( SequenceBehavior::create() );
      %beh.add( Condition::create( "MonstersNearbyCondition", true, true ) );
         %sel = %beh.add( SelectorBehavior::create( "FightOrFleeSelector" ) );
            %beh = %sel.add( SequenceBehavior::create() );               
               %beh.add( ChatBehavior::create( "Hunting a Monster!" ) );               
               %par = %beh.add( ParallelBehavior::create() );          
                  %par.add( LeafBehavior::create( "TargetClosestEnemyBehavior" ) );
                  %par.add( FollowTargetBehavior::create( 15 ) );        
                  %beh = %par.add( LoopBehavior::create( "FOREVER" ) );                  
                     %beh = %beh.add( SequenceBehavior::create() );
                        %beh.add( LeafBehavior::create( "ChooseWeaponBehavior" ) );
                        %beh.add( ChatBehavior::create( "Attacking!" ) );
                        %beh.add( LeafBehavior::create( "AttackTargetBehavior" ) );
                        %beh.add( WaitBehavior::create( 1000 ) );               
   
   // Rest at Inn
   %beh = %root.add( SequenceBehavior::create() );
      %beh.add( Condition::create( "TiredCondition", false ) );
      %beh.add( Condition::create( "MonstersNearbyCondition", true, false ) );
      %beh.add( MoveToPositionBehavior::create( $InnPos, 0.1 ) );
      %beh.add( LeafBehavior::create( "SleepBehavior" ) );
      
   // Wander
   %seq = %root.add( SequenceBehavior::create() );   
      %seq.add( Condition::create( "MonstersNearbyCondition", true, false ) );
      %beh = %seq.add( TimerBehavior::create( 3000 ) );
         %beh.add( LeafBehavior::create( "WanderBehavior" ) );   
      %seq.add( WaitBehavior::create( 1500 ) );

   return %tree;
}

So that looks fairly organized, maybe even a little "tree" like right? Still, creating them could be easier. So I've been working on an editor to help out.



Looking at the FighterBehaviorTree in game


So I'd say my review of behavior trees is "thumbs up". Once you have the framework in place, putting together a new tree consisting of a few sequences and selectors is a breeze.

Still I'm not doing anything very complex with these yet. Some more complex things I'd still like to try...
* building behavior trees dynamically, perhaps under the direction of a planner

And lots of stuff to add to the editor
* substituting one behavior for another ( correctly transferring over children )
* show the execution path in the tree editor
* Saving assembled trees to a managed script file

Of course there's plenty of other ai topics I'd like to look into
* fuzzy logic ( possibly used in a SelectorBehavior )
* a system for working and permanent memory of agents
* etc ad infinitum