Behaviour Trees: The Cornerstone of Modern Game AI | AI 101

As the new series of AI 101 continues I take a look at behaviour trees – arguably the dominant AI technique in AAA games – and tell you how they work.

If you’re interested in learning more about behaviour trees here are some links to some resources you might find useful:

Chris Simpson: “Behaviour Trees for AI – How They Work”

Alex Champandard: “Understanding Behaviour Trees”

Epic Games: “Behaviour Tree (Unreal Engine 4 Documentation)”

AI and Games is series on research and applications of Artifical Intelligence in video games. It’s is supported through and wouldn’t be possible wthout the wonderful people who support it on Patreon.

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Games in this episode (in order of appearance):

Halo 2
Far Cry Primal
Tom Clancy’s The Division
Halo 3
Spec Ops: The Line
Ghost Recon: Wildlands
Batman: Arkham Asylum
BioShock: Infinite
Alien: Isolation

Soundtrack for this episode is “Twilight Metropolis A” and “This is My City” by TeknoAxe under the terms of a Creative Commons 4.0 license.

#gamedev #ArtificialIntelligence #BehaviourTrees


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  • AI 101 is back for 2019 with a new episode dedicated to Behaviour Trees: one of the most commonly adopted AI technologies in modern video game development. Long-term viewers of the show will have noticed how frequently Behaviour Trees have appeared – with them cropping up in Spec Ops: The Line, Far Cry, Halo and The Division last year alone. So let's take a moment to explain how they work and why developers and designers like to use them.

    AI and Games June 12, 2020 9:42 pm Reply
  • best thing ever. thank you for this!

    Joanna Parungao June 12, 2020 9:42 pm Reply
  • 4 min only explaining how to read this tree???!!!
    do you think that i´m stupid???

    Patttiat 2000 June 12, 2020 9:42 pm Reply
  • I think your videos can be very informative, but I usually find myself not knowing what the hell you were talking about in the end because it is way too technical for me.

    Tabea Tamm June 12, 2020 9:42 pm Reply
  • Excellent Video

    AlphaWoIF June 12, 2020 9:42 pm Reply
  • Summarising: Selector is a question (is this condition satisfied) and sequence is a procedure( do this and that)……. AI is basically questions and procedures.

    Sanchay Joshi June 12, 2020 9:42 pm Reply
  • Can you explain why the AI in most video games is so bad when we've had such big improvements in every other area?

    MrJaaaaake June 12, 2020 9:42 pm Reply
  • this is a terrible idea!! why would this be the most popular method right now??

    Aaron Van Meter June 12, 2020 9:42 pm Reply
  • behavior trees could be good for a story driven game. because assuming each character has their own tree they could be programmed to behave in a fashion that is far more consistent with their character for example in a shooter game if a character needs reviving an character who doesn't know them very well will only attempt to revive them if their near by but someone who they are close friends with would immediately drop what they are doing and go out of their way to revive them.

    Arcade Assassin June 12, 2020 9:42 pm Reply
  • As an aspiring game developer, there are moments when I feel particularly excited and motivated to make games. For example, every time I listen to the silent hill 2 soundtrack. In this instance, it's new insights and knowledge from you that is driving that inspiration. Thanks Tommy😊

    Nathanael Gazzard June 12, 2020 9:42 pm Reply
  • You basically explained a simple AI neuron

    Death Strike Gaming June 12, 2020 9:42 pm Reply
  • One thing I didn't get the 4:11 caching thing . I am making a game where the enemy_director tells each instance of a particular enemy what to do each frame using a (small)behaviour tree. The thing is taht I am traversing the tree and making the pertinent checks every frame; how can I store data about the state in wich it was and still check wether it shoud choose an other path?

    pere moya serra June 12, 2020 9:42 pm Reply
  • Do they learn on the fly? Like you would train a decision tree algorithm using some optimization technique? Also, is there a concept of a "forest" of behaviour trees, like a random forest in decision trees?

    Undisclosed Music June 12, 2020 9:42 pm Reply
  • Nothing stopping you from using the same event-driven optimizations in state-driven system. So the whole "performance" argument is kinda wrong. The same about reusability – nothing stops from writing states and transitions in an isolated, reusable fashion. You are comparing the bare bones state machines with real optimized implementations of BT's which is just incorrect.

    phobos2077 June 12, 2020 9:42 pm Reply
  • Super helpful video. Thanks!!

    Blorp June 12, 2020 9:42 pm Reply
  • Since using Behavior Trees depends on the type of game and its scale, what is recommended to use in a simple tactical rpg? I have made a finite state machine but I think it's lacking and the code will get really complicated if I get into this, so are behaviour trees more appropriate?

    FarEast Armadildo June 12, 2020 9:42 pm Reply
  • How's about the Director from the Left 4 Dead series?

    Thomas Nied June 12, 2020 9:42 pm Reply
  • He held my lecture today!

    420MonkyTime June 12, 2020 9:42 pm Reply
  • Fix your playlist order before it's too late!

    dmdz June 12, 2020 9:42 pm Reply
  • Damn, this channel is so underrated

    CommieG June 12, 2020 9:42 pm Reply
  • behavior trees are a narrativist abomination lmfao

    Aaron Van Meter June 12, 2020 9:42 pm Reply
  • can you fkin learn to link videos you mention? doesn't help with watching u

    Forien June 12, 2020 9:42 pm Reply
  • I wish the graphs were more details, giving examples of the topics discussed.

    Van Ivanov June 12, 2020 9:42 pm Reply
  • AIs are quite fascinating. without them we wouldn't have video games. i'm grateful for all the talented people through the years which have been responsible for creating such wonderful things.

    Akemi Homura June 12, 2020 9:42 pm Reply
  • The root can also be a selector. It doesn't have to be Root->selector, it can be root(selector) where the root is an event in the world that has a selector inside it.

    Xav Dest June 12, 2020 9:42 pm Reply
  • I may be focusing on the wrong issue here, but wouldn't it make more sense to say Behavior Trees are the 'powerhouse' of modern game AI. Clearly the 'cornerstone' of modern game AI is the state machine, which is used to build said powerhouse.

    I can already sense I'm giving this way more thought that I should. Great video as always.

    vincentpol June 12, 2020 9:42 pm Reply
  • This is monumental work, thank you for crafting these videos with so much care. The only issue in my opinion was in the comparison between the methods, you gave behaviour trees and FSMs so much attention but almost forgot GOAP :c

    Pedro Paternostro June 12, 2020 9:42 pm Reply
  • What an awesome unique channel! +1 sub

    Ye4rZero June 12, 2020 9:42 pm Reply
  • pointless, it is still a FSM just not all transitions are explicitly coded. Easier to debug? surely.

    Piotr R June 12, 2020 9:42 pm Reply
  • You have way too low sub count. This channel needs more spotlight. Great job mate.

    Perkkie June 12, 2020 9:42 pm Reply
  • Not again dyslexia… I was expecting to see different trees in games…

    Vaaz Rodríguez June 12, 2020 9:42 pm Reply
  • Thank you for this video! If I had spare money I'd give you some patreon cash. :/

    Stephan Ennen June 12, 2020 9:42 pm Reply
  • "Where it decides which child to execute based on some logic in the world"
    Man, this video got dark .

    fireaza June 12, 2020 9:42 pm Reply
  • @ 1:19 the background marine says “They killed the Chief! You bastards!”

    That’s gotta be a south park reference.

    IW_LEGENDARY_SAYaiN22 June 12, 2020 9:42 pm Reply
  • This is great and you are right about UE4 and it's Behavior Trees.
    I used them for nearly all my games.

    Kinos141 June 12, 2020 9:42 pm Reply
  • Sabonner

    PSHYCO June 12, 2020 9:42 pm Reply
  • Not sure if it was explained but I wpuld like to kown how the AI aims to your bodypart in fps games.

    Sebastian Alvarez June 12, 2020 9:42 pm Reply
  • First dislike :^)

    Juppe June 12, 2020 9:42 pm Reply

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