[http://www.giantbomb.com/videos/quick-look-alien-isolation/2300-9543/]
Recently I purchased Alien: Isolation on Steam, which had been on my wish list for quite some time. 75% off, game + 3 DLC for $20. Praise Lord Gaben!
I'd heard that the game was very good at achieving its goal, which is to say, scaring the living crap out of you. I had also heard that a large part of its success came from the alien, whose AI algorithm gave it very believable, lifelike behavior; it was (so I'd heard) like being hunted by a thinking breathing, perfect organism.
So when I booted up Alien: Isolation, at about midnight for maximum immersion, I was feeling hopeful and excited. Firstly, since I am a long-time fan of Horror Games, it had to tick all the right boxes in regards to what a horror game should do. Secondly, I was interested to see if this Alien AI was really all that it had been talked up to be. Would it really think strategically, react to its environment dynamically, and generally behave in a lifelike manner?
*4 hours later*
WELL. That was... that was something.
The alien, to put it elegantly, is freaking terrifying. It appears at any moment, slithering out of ceiling vents like some great serpent. Once on the scene, the AI performs in a way that I feel is very unique in terms of video game AI systems. Points I found particularly interesting about it are:
- The AI seems to act on a FOV basis, meaning it cannot spot you if you are not visible. Hiding behind boxes and walls is a core mechanic used to avoid the alien, and if it cannot see you it may walk straight past you.
- The alien hunts you based on sound, meaning noises such as footsteps and colliding objects attract it. I found that as long as you stay completely still, it would often be lost. Conversely, make any sort of noise at all and the AI will pinpoint our exact location and come running.
- Items such as flares, flashbangs and noisemakers are introduced, creating the gameplay mechanic of 'distract it while you run'. However, this seemed to lose effectiveness over time, as the alien learned from its past experiences.
- The AI always seemed to know the general are of where I was, even if it couldn't find my exact location. Whether this is another clever AI aspect or simple rubber-banding I'm unsure.
Overall the AI was pretty amazing, and as I played I began to wonder how the developers had accomplished this complex, lifelike behaviour. Unfortunately google search didn't come up with anything too useful, which is understandable. Since the game has only recently been released the developers are not willing to unveil too many behind-the-scenes aspects.
I did, however, manage to find several interviews withe the devs talking about how the AI system works. Based on this info, I can scrape together my theory on Alien: Isolation's AI system.
Interview A (skip to about half-way)
Interview B / Quotes
Interview C (no. 6 onwards)
My Theory / Concept
Generally, the AI system functions based on a complex decision tree to act upon outside stimuli. In this case, that stimuli is audio cues such as player footsteps, doors closing, and objects being knocked about. Once the AI 'hears' these noises, it will decide to investigate. How it does so is, I imagine, similar in approach to GOAP; it can simply walk over, use a vent, or even take a circuitous route to try and delay its arrival.
Next comes the more interesting aspect, reactionary decision-making. According to interviews, the system possesses a series of pre-defined behaviour patterns that are locked off from the main loop. These auxiliary instructions are unlocked in reaction to player behavior and remain as permanent options for the decision tree.
For example if the player is using a lot of distraction devices, normally the AI will be obliged to investigate the noise in accordance with its basic behaviour. However, after a certain number the 'doubt sources of noise' flag is turned on and an option to ignore that noise and look elsewhere is activated. From that point, on, distractions become less effective since the AI can choose to ignore it and look elsewhere. Improving this even further would be to guess where the player is likely to be based on where the distraction was placed (ie. the player wants me to go over there, lets go in the opposite direction).
Of course these are just theories, and I may be completely wrong. But I also think that, even if I am, the basis for a very clever AI system is in here somewhere, and these ideas I came up with will likely be designed and developed when the oppoortunity presents itself. I think I'll add this AI concept to my ideas notebook.
I think it's a lot more complex than just pruning the decision tree with flags turned off. Reading at the interviews, they really use the word "learn" - and they also say that they don't know what the alien is going to do the first time they put him in a room.
ReplyDeleteI think they might use some kind of machine learning algorithm - where the alien uses his previous experiences to provide a more interesting gameplay.
First of all, thanks for reading! I didn't know anyone actually read this thing, let alone was interested enough to comment.
ReplyDeleteAfter reading what I wrote again, I agree that my theory is probably not what they used. Decision Tree learning seems too simple and binary compared to what the in-game AI shows.
Other forms of machine learning that are more likely? Hmm. Association Learning, Deep Learning, or maybe some grand ensemble of different algorithms. All I know is I'd love to get a look at it; I bet it's glorious!