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

[@AIandGames]

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.

[@fireaza]

"Where it decides which child to execute based on some logic in the world" Man, this video got *dark* .

[@CCGS]

Sophie's choice in here all of a sudden...

[@illusions500]

"sequence nodes allow us to execute children one after another"

[@fatal_pastry1176]

Reminds me of my OS course, where we talked about zombie and orphan children and how/when to kill them. Ahh, CS. . .

[@Known_as_The_Ghost]

Execute. Them. All.

[@Gnurklesquimp]

FOR THE BEHAVIOUR TREE!

[@nathanaelgazzard7989]

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😊

[@Deadener]

I already know how BTs worked, but it's great to hear some of the reasons why you might use them over the alternatives. Your explanations are also fantastic, and I wish this video had been around when I was first learning them.

[@JanB1605]

We use something similar in automation technologies to program the behaviour of machines. We have a special programming language for this, called "SFC" or "Sequential Function Chart".

[@shayoko6]

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.

[@mftripz8445]

Not only are you a weeb but you are a rardo

[@DasAntiNaziBroetchen]

@@mftripz8445 Wtf is a rardo?

[@DasAntiNaziBroetchen]

@Awawawa CM Ok. Wtf is a rardo?

[@Crimbtw]

@Awawawa CM Im here to post a comment a year later... I shall await for another comment In a year

[@Crimbtw]

@Awawawa CM you too :)

[@zoecarlibur]

When this popped up on my feed and I saw the date. I was so happy for a new video!

[@AIandGames]

Still gotta case study lined up for this month too. Currently there's at least two videos a month until April. Provided I get them finished! 😅

[@MichaelJackson1479]

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

[@AIandGames]

Thanks for watching. GOAP will be getting it's own episode at some point, alongside other planning approaches. Plenty more to cover to keep me busy.

[@mek101whatif7]

> A finate state machine needs to be updated at every tick. If you know witch external states a FSM observes, you can just hook it up to those states changing as events and evaluate an internal state change only when any of those external events fire

[@blehblehson1194]

Oh good thanks! I am glad you made this video.

[@sam_making_games]

I was looking for best practices for using Behavior trees but this is fun to watch too.

[@NecrotekLabs]

Thank God for this channel and THIS video. Thank you mate, helped me loads. I'm working on an immersive sim and wasn't sure which technique to use.

[@JTaylor2K]

All my brain picks up in this is “child execution” and “ child manipulation “

[@DArnez-c5n]

Hahahahahahahahaha there really are a lot of scary vocabulary in our tech world hahahahahahahahaha

[@tjrizvi251]

....man...

[@BleachDemon707]

Tell us you're a PDFfile, without telling us 🙄

[@GameDevNerd]

Great overview of behavior trees. I may be taking a job working on complex behavior trees and I'm studying everything I can about it. Appreciate the video!

[@ObiAdeGaming]

Love your vids dude, came for the division, stayed for the informative AI explanations 👍👍

[@AIandGames]

Heh heh, stick about. I have some good stuff lined up between now and March.

[@squanguslobotomus7890]

I would love to see games add outside forces that are like stats but instead of changing power, it would change how they approach scenarios. Let's say one enemy has high self preservation, but low bravery. They would usually stay under cover and run away when you get close. Or what about high bravery and low self preservation. This ai would rush you at low health and usually leave cover alot. This allows for a much more random experience with ai, but still somewhat predictable due to different enemy types keeping traits close to their purpose, like defense always having higher bravery but high self preservation, meaning they will stay under cover but sometimes move away from it. Obviously there would be more factors but this is just a basic idea.

[@Fantastic_Mr_Fox]

I'm quite certain this kind of system has been used in games before. It's too obvious to not have been.

[@sergiuferentz9125]

I was listening to this on 0.75x for some reason. Thought Tommy had one too many pints down at the pub before filming this video. Great content regardless of level of sobriety !

[@AIandGames]

Just listened to it myself and yeah... sounds like I've had a little too much to drink. 😂

[@iw_legendary_sayain2215]

@ 1:19 the background marine says “They killed the Chief! You bastards!” That’s gotta be a south park reference.

[@mftripz8445]

? I don't remember no line said like that from sp

[@skyacaniadev2229]

You mean chef?

[@Gnurklesquimp]

For people wondering, this is in reference to the iconic ''They killed Kenny'' line

[@pierreo33]

@@Gnurklesquimp Thanks cpt obvious

[@Gnurklesquimp]

@@pierreo33 You realize people were asking? Or do you not understand how captain obvious works? Reminds me of how people use r/woosh, it isn't that complicated

[@apresmidi153]

Great video and concise explanation of how I can use a data structure I already know to create scalable AI. Bravo!

[@perkkie]

You have way too low sub count. This channel needs more spotlight. Great job mate.

[@rebirth4119]

Its really great that nowadays certain game engines use visual node scripting so you can easily isolate the different actions of AI based upon what data is provided to them via the player of the world around them. It allows people to see whole tree and easily make changes when needed.

[@AthenaMarriesDionysus]

He held my lecture today!

[@saturn7_dev]

Yeah , good video. I'm currently using finite state machine to make an enemy AI. The Blackboard you mentioned for me is really just global variables used by any and all enemies that's updated once from the players Behavior. Some good points though as yes, had a bit of mass jungle of things needing to code for when sharing functions and code across states....I can see why they use trees. I tend to use 2 states - the main state and its sub state (bit like tree structure) to keep track of what to update when else there is a lot of conflicts and difficult debugging issues...

[@andycarr3711]

I'm only a hobbyist developer but I'd suggest it doesn't entirely replace the object's state machine but allows it to run a very simple one, state.move, state.reload etc.

[@DanielKarbach]

"Decide which child to execute" hmmm, trolley problem in AI?

[@johngoatie]

You should look into Utility AI for a future video, I haven't seen it mentioned here but from my own work it looks to be an incredibly powerful and scalable solution.

[@AIandGames]

Definetely on my to-do list (somewhere).

[@johngoatie]

@@AIandGames Awesome and great content, I love what you're doing.

[@JohnVanderbeck]

@@AIandGames I too would love to see a video on utility systems.

[@RyanPowell-t8s]

my favorite new channel!

[@blackcitadelstudios]

Currently deciding which to use on my enemy ai. Finite state machines or behaviour trees. 😅

[@Kestas_X]

7:02 That Tapir Just being like "Oh there's a spear in my temple. Well, guess I'll Go simewhere Else, then"

[@CommieG]

Damn, this channel is so underrated

[@yukikun9685]

best thing ever. thank you for this!

[@pricelessair]

Happy find!👍

[@SkyYurt]

I'm currently developing an Tank AI using Behaviour Trees. I have found it to be very challenging to debug my AI as I work on it. My solution was to make a simple Debug leaf node, that would output custom text whenever the behaviour tree hit that point of the tree. What other techniques do you use to debug your behaviour trees?

[@AaditDoshi]

It really depends on what system ur using. Unreals behavior trees let u put break points on the nodes. U could display the behavior trees current chain of running nodes above the AI.

[@SkyYurt]

​@@AaditDoshi I'm using Unity as the game engine and writing the behaviour tree from scratch using C#, so I was looking for more of an idea of a solution, which I then could implement. Displaying the current chain of nodes sounds like an good idea. I imagine I could have each node return both their status and their identifier and then make the root node output the chain of identifiers to the user.

[@jassem3441]

Thank you !

[@JamesKelly89]

I wish I could find more information on techniques and the design of behavior trees. Everything I can find seems to be conceptual.

[@Ye4rZero]

What an awesome unique channel! +1 sub

[@blorp-]

Super helpful video. Thanks!!

[@BaseRealityVR]

Excellent Video

[@bezerker2173]

So question, im trying to make a game, and im not new to coding but very very new to game design. Im trying to make an old school final fantasy style rpg, with 2d sprites and turn based combat. My question is, are behavior trees a good idea for the enemies in combat? I was gonna use a sort of rng to have enemies decide what attack to use, but if I wanted to give it more intelligence and have it react to the player's strategies and status effects on both sides, would it be a good idea to use a behavior tree or a different method instead?

[@xavdest5481]

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.

[@aliquewilliams3080]

Behavior trees are certainly more scalable than state machines (even hierarchical FSMs and pushdown automata), but many of the negatives you mentioned with FSM really isn't that true. For example, states are highly reusable, can be composed to contain other states, and can be executed concurrently. Creating new states should be as easy as composing new states together, each handling one small task. Memoisation can also be used with FSM as an optimization technique. They’re not exclusive to trees.

[@fareastarmadildo5093]

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?

[@arcadeassassin7176]

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.

[@peremoyaserra2749]

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?

[@kd45286]

Everyone thinks of AI but behaviour trees work wonderfully for player states

[@LeonardoRamos01]

Hi! Great video! I really like your content 👍🏽. I got a little confused with the Reusability part, couldn't it be that Finite State Machines reuse more general states? Why is it just available for Behavior Trees?

[@BobTheBob9]

i believe doom eternal actually uses behavior trees that can be programmed visually by designers, given the advantages it makes sense that they'd move away from 2016's state machines

[@jamesostrander8022]

Any books or websites to check on how to construct a behavior tree using C#? I would love to do this for a game in Unity Engine.

[@RyanScottForReal]

Love to have a couple examples before telling us how it works.

[@DarthMizaru]

Not sure if it was explained but I wpuld like to kown how the AI aims to your bodypart in fps games.

[@AaditDoshi]

Umm, it has nothing to do with this video. BUT. You get the player mesh, get the bone of the body part you want it to SHOOT at. Get the bones position. Tell it to shoot at that position.

[@DarthMizaru]

@@AaditDoshi he did say to comment what we would like to see in future videos. I was also wondering how the AI doesnt always go for a headshot, moving targets, how to miss, etc.

[@laseede17]

@@DarthMizaru maybe with probabilities. But good idea for a vid.

[@burhansancakl351]

@@DarthMizaru because developers want you to feel powerful, it would be boring if ai just headshot you all the day and didnt miss any of its shots

[@vanivanov9571]

I wish the graphs were more details, giving examples of the topics discussed.

[@everythingwhatyouwan]

Summarising: Selector is a question (is this condition satisfied) and sequence is a procedure( do this and that)....... AI is basically questions and procedures.

[@dmdizzy]

Fix your playlist order before it's too late!

[@DanZhukovin]

Is this a re-upload...?

[@MrJaaaaake]

Can you explain why the AI in most video games is so bad when we've had such big improvements in every other area?

[@tratbagd4500]

At the moment, and especially for some games, it's not that difficult to implement an AI in a game that can't be beaten. However, this is a turnoff for gamers because it's not fun when you know you are going to lose everytime you play. Games are entertainment and people play them for fun.

[@dendon3384]

@@tratbagd4500 I've heard this rebuttal many times before. It's not about an AI that can beat the player, it's about a believable AI. Such as not seeing the same behaviors every time, or the AI taking additional steps to stay alive, or use more tactics than rushing the enemy. There is so much potential, and no one is tapping into it.

[@vincentpol]

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.

[@YawLighthouse]

Will you be at some point going over Utility AI(even though its really IA ;) )?

[@AIandGames]

Yeah I think Utility is worth covering. It's not a particularly well understood subject.

[@foolwise4703]

Hmm, so how can it learn?

[@thomasnied7722]

How's about the Director from the Left 4 Dead series?

[@tabeatamm3594]

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.

[@duramirez]

We need Doctor Sung, to give us positronic brain ^^

[@tjrizvi251]

"No children, just parents" Hell yeah SINK life!

[@undisclosedmusic4969]

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?

[@AIandGames]

No they're static. Behaviour Trees are hand-made by designers to solve very specific situations. What you're suggesting - dynamic optimisation of tree-structures for decision making - is more in-line with things like Monte Carlo Tree Search (MCTS) which is only now being used more frequently in video games.

[@ls200076]

oh

[@clovislfrc9881]

3:02

[@DeathxStrike18]

You basically explained a simple AI neuron

[@riley8280]

Bet!

[@pshyco6874]

Sabonner

[@vaaz_rodriguez]

Not again dyslexia... I was expecting to see different trees in games...

[@Glockmog2007]

In my opinion state machines are better if you want to make very in depth ai. Yes behavior trees are easier to follow and therefore simpler to debug but you cant have too many different branches before they become a complete mess if you want to add more functionality. State machines are much more scalable. And since I actually write the code rather than using visual tools theyre just as readable to me.

[@tyridge7714]

I'd disagree with this, simple finite state machines(especially if they aren't hierarchical) are equally if not more of a nightmare as massive behavior trees GOAP, HTN Planning, or similar planning techniques however definitely allow you to make AI that isn't a complete mess, if you keep your search space small and compact for planning and delegate more complex conditionals and logic to something like behavior trees. I think the main goal should be to use both behavior trees with other techniques, they all do a good job at something and a poor job at others, so it's a good idea to combine them in a way that makes sense and gets the best of both worlds

[@Restart-Gaming]

Great video none of this works in unreal all of it is bought from the asset store ue4 is not setup to do very advance AI

[@loot6]

You should never execute children.

[@localgames1]

@Cyberpunk2077

[@phobos2077_]

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.

[@vast634]

The reason to use them is more that non programmers can read and understand them more easily. How performant they run is always down to their implementation anyways.

[@piotrr5439]

pointless, it is still a FSM just not all transitions are explicitly coded. Easier to debug? surely.

[@matthewboyd8689]

I can't wait till they make AI using simple Venn diagram style of play rather than a behavior tree. And by that I mean the idea of two separate things happening and it figuring out which of the two is best based on experience and behaving more realistically. Like, which path to take The tigher corridor with cover but is the long way Or the wide open space thats direct Depending on how tense the situation it could make those gray area dicisions more realistically. Weve all seen NPCs make dumb decisions because of this lack of understanding gray areas

[@Marceilus]

The fact you put dumb Arkham henchman in the video 😂

[@Patttiat]

4 min only explaining how to read this tree???!!! do you think that i´m stupid???

[@badradish2116]

behavior trees are a narrativist abomination lmfao

[@badradish2116]

this is a terrible idea!! why would this be the most popular method right now??