Ellen Ripley: How do we kill it, Ash?
There's gotta be a way of killing it. How do we do it?
Ash: You can't.
You still don't understand what you're dealing with, do you?
A perfect organism.
A survivor.
Unclouded by conscience, remorse… or delusions of morality.
I can't lie to you about your chances, but…
You have my sympathies.
One of the biggest AI-driven titles of modern times, Alien: Isolation brings the terror
of the xenomorph to video games. While the alien, LV-426 and many other key elements
of the franchise have previously been explored, Alien: Isolation takes a different route.
A horror game steeped in the aesthetic and style of Ridley Scott's classic 1979 movie:
in which the player is trapped on the Sevastapol space station with a sole intelligent, near-invincible
alien. For this experience to play out as expected, the alien itself has to carry a
large amount of the experience: a demanding feat for any AI in an industry in which non-player
characters are either heavily scripted or suffer really short life spans.
So how did developers Creative Assembly pull it off? The short answer is through tried
and tested AI techniques, designed in a manner that breaks from so many traditional game
design concepts. So stay tuned as we dive in and find out what it makes the Alien tick.
The team behind Alien: Isolation built the game around a simple premise: to survive an
encounter with Ridley Scott's original alien. Such a feat is pretty difficult to balance,
given that you need a strong story to carry the player through, but also to establish
pacing such that the tension varies throughout your gameplay. Neither of these are easy when
you introduce a systemic AI into play: a collection of decision making systems that react to what
is happening in the game. It needs to know when it can participate in the experience
and it what level it should do so. In order to do this, it required a unique design perspective.
As explained by Andy Bray in his talk at the nucl.ai conference, the player can't be scared
shitless all the time, otherwise they'll just give up. The tension had to vary but also
maintain an air of unpredictability in the xenomorph itself. This led to the developers
seeking inspiration naturally from Ridley Scott's Alien movie, but also looking classic
horror movies as well adventure movies such as Jurassic Park. The resulting philosophy,
could be considered as 'psychopathic serendipity': in which the alien always find itself in the
right place at the right time. Even if you're in hiding and the monster can't see you, and
that it doesn't know your ultimate objective, it will still find a way to mess with your
plans. However, the key to pulling this off was to ensure it wasn't scripted. Co-ordinated
jump scares while fruitful the first time, are cheap and don't yield any further reward
in subsequent playthroughs. Instead, the player should have a sense of powerlessness, but
still know that while the alien is smart, they are smarter.
But all of this goes against the grain of a lot of AAA games development: a world in
which player empowerment is key. This means challenging a lot of design tenets that have held
in pretty much every Alien game - from Alien 3 and Alien: Trilogy to Aliens: Colonial Marines.
The biggest departure from previous games, undoubtedly, is the fact that you cannot kill
the alien. You can distract it and in time find ways to hurt it and scare it off,
but you cannot kill it. Ever. But in another significant departure, there's something of
a role reversal: the alien will not only kill you frequently, but it does so with one hit.
Ultimately, this creature must be a constant, believable and threatening presence.
In order for this alien to work, Creative Assembly fell back on reliable and trusted
AI techniques. Building systems that ensure the alien serves its purpose, but interestingly,
also ensures it plays according to expectations.
To achieve this, the game requires two distinct behaviour management systems: the director
AI and the alien AI. The director AI observes the player throughout the game: always watching,
always knowing where you are. Meanwhile the alien AI is driven by a series of sensors
and behaviours that allow it to hunt down and find the player. The director's job is
to point the alien in the direction of the player periodically, to give it a hint as
to where it should be looking. Despite this, the alien AI is never allowed to cheat.
While the director always knows where you are, the alien has to figure it out for itself.
You can fool it, surprise it, and even can escape it.
But let's break this down in more detail...
The director AI of Alien: Isolation was responsible managing what was known as the menace gauge:
a metric that allowed the system to understand how much it was pressuring the player. This
is rather similar to an approach made in Left 4 Dead in which a stress gauge was used to
prioritise enemy attacks. However in this case, menace would come not from attacking
the player, but being in their proximity and thus ratchet up the tension.
As such, the system would periodically wish to invoke menace upon the player and tell
the alien to head to a particular location. While the alien is near the player, the menace
gauge would increase depending on specific factors: such as whether the creature is within
a short walking distance of the player, whether the player has actual line of sight and whether
the alien was close on the motion tracker, but could also reach the player quickly. This
one is important, as the alien could technically be close on the motion tracker but in another
room. As such, the menace won't increase as much given there is no real threat.
Naturally once this menace metric has reached too high a level, the director backs off and
sends the alien elsewhere. Either into neighbouring areas or up into the air vents. The general
principle being that when the alien is closer, the game is scarier, but we need to give players
a break after a while. Not only to ensure they stop freaking out, but to enable them
to achieve some progress in the game.
This whole process is managed via a Job system - previously used in the likes of BioShock: Infinite
for the companion character Elizabeth - which dictates the task to complete, the
location the alien should visit and the priority with which it should do so. The priority dictates
to the alien whether it should finish what it is doing now, immediately move towards
doing this action (and blend the required animations) or just outright interrupt everything
and execute right now!
The job system allows the alien to operate in two main states of active and passive.
The active state or front stage mode, is when it is told to sweep areas of the map or search
specific locations after a certain noise or trigger occurred. Meanwhile the passive or
backstage mode occurs once menace levels peak for too long and it falls back into the air vents.
Sometimes, it will then head for air vents up ahead if it thinks you're going to move towards.
So how do this translate into action execution? Let's look at the xenomorph itself.
The aliens behaviour is reliant upon behaviour trees: a technique we've previously highlighted
in our video on the AI of Halo. The monster has over 100 nodes hidden within its behaviour
tree system, with the top level of around 30 nodes responsible for selecting what type
of behaviour it was about to execute. These top level nodes would then have the alien
execute within large sub-sections of the tree responsible for specific sub-behaviours relating
to specific tasks.
To add to this, certain parts of the behaviour tree are - on starting the game - locked off.
The system gradually unlocks these behaviours in the tree as certain conditions are met
throughout the game. This ensures that the longer you play, that the alien starts to
to exhibit new behavioural traits to keep you on edge. In a sense, it gives the impression
that the alien is beginning to learn from experience, given the players behaviour is
triggering the conditions that unlock it. Though as Andy Bray was keen to point out:
none of these conditions come from events that lead to a player's death. Otherwise the
AI may develop an unfair advantage: given you might never figure out what it is that
it's learned from your behaviour, but also that this game is designed for people to die
in a lot and every time you do, this thing would be getting smarter. One concession however,
is there are triggers in the final build of the game that unlock behaviours at specific
points in your campaign if they haven't already, that way the creature can keep pace with the
player as they begin to get better at playing.
When given a specific job to complete from the director, the alien is reliant upon a
fast and efficient pathfinding system: whose heuristics help to dictate the values of particular
sensors the alien has. The alien has a variety of sensors that allow it to identify with
varying accuracy player footsteps when walking or running, gunshots, and even the motion
tracker when the player is around 1.5 meters away. Now the question that burned in my mind
when I played it was whether the alien had sensors built into the tail, so as to ensure
you don't try and step on it. The answer is actually no, but in a sneaky move by Creative
Assembly, the alien has very, very short range sensors that look behind it. It quite literally
has eyes in the back of its head, I am told with the intention of ensuring that players
do not avoid detection by walking around behind it.
So the last real thing to cover is the alien movement patterns: those who have played Alien: Isolation
will know that the alien is prone to pacing around an environment for several minutes.
This thing isn't moving towards a specific location in an optimal fashion; it's searching, it's hunting.
This is achieved by giving the alien particular areas of interest
- either those hand tuned by designers or generated dynamically in the environment - to
head for. Sometimes these are specific locations the designers want the alien to visit, and
other times it's responding to players given they made too much noise. A perimeter of nearby
locations is established and the alien will devise a path to visit them all in a travelling
salesman fashion - meaning it will visit every point, but not with any guarantee it does
so in an optimal fashion, given it's they are prioritised based on their visibility by
the alien. This sub-optimal execution results periodically in the alien backtracking on
itself: a behaviour designed on purpose to give the impression it's double checking or
doubting itself. These locations are separated into search locations: those that the alien
will visit, and spot locations: where it will stay where it is currently standing and then
turn to look at the area of interest. For safety, the designers were able to dictate
specific areas the alien never looks, so as it doesn't catch you out in unfair situations.
Considering all this design effort, you'd be quick to think that despite it all, the
game designers would still allow for the alien AI to cheat periodically by either teleporting
or being clairvoyant in its knowledge. According to Bray, this is categorically untrue. Throughout
the entire 12-18 hour campaign, the alien only teleports twice to be in very specific
locations and is only done to enable it appear in cutscenes.
Alien: Isolation is an achievement: an AI-driven game that holds your attention for the most
part through the experience of interacting with a primary AI-driven antagonist. A real feat in an of itself,
the fact that this is then achieved within the context of a horror game is all
the more impressive. Fortunately this was largely agreed upon by critics, who hailed
the alien and its AI behaviour as a real driving force behind the game and certainly
worth checking out if you haven't done so already (I mean seriously, if you're watching
this channel, just play the games I tell you to, doctor's orders!).
We're only just cracking the surface of how this system works. For those more tech savvy,
be sure to catch the full talk by Andy Bray from nucl.ai. My personal thanks to both Andy
as well as Derek Fagan from Creative Assembly who were kind enough to humour me and answer
some questions I had about the game.
With that we bring this video to a close, I'm Tommy Thompson and this has been AI and
Games: a series that doesn't happen without the help and support of our sponsors listed
now on patreon.com. Thanks for watching and we'll see you all next time!