A More Advanced Character

References

A text based tutorial
An article on Behavior Trees
Another Tutorial Series. This one is very good. I reccomend it.
And some from the book.

A Behavior Tree
- Like a FSM, this is a mathematical model for modeling behavior.
  - You construct a tree to model the behavior.
  - This tree gets "executed" whenever the AI is called.
- They are represented as a tree
  - A special root node.
  - Selector nodes.
  - Sequence nodes
  - Parallel nodes
  - Decorator nodes.
  - Task nodes.
- Note: All nodes return a state
  - Success
  - Failure
  - Running
  - Error
The Root node only marks the start of a computation.
- It only has one child.
- In a "classical" behavior tree, it is the start of an execution.

A Selector node has one or more children.

Attempts to execute the children in order
Exits on the success of the first child.

for i = first child  to n do
    childstatus = Tick(child(i))
    if childstatus = running
       return running
    else if childstatus = success
       return success
end
return failure

A Sequence node has one or more children

for i from 1 to n do
    childstatus = Tick(child(i))
    if childstatus = running
       return running
    else if childstatus = failure
       return failure
end
return success

Parallel Nodes execute all children at the same time.

for i = 1 to N:
    state_i = Tick(child[i])

if nSucc(state) >= S:
    return SUCCESS
else if nFail(state) >= F:
    return FAILURE
else
    return RUNNING

Taks Nodes perform some task, an action.
Decorator Nodes perform other functions.

In Unreal, nodes are stateful
- If a task is running, they do not execure again.
- But if something else takes over, they abort.
Run the AI level and watch this.