How Stealth Game Guards See and Hear | School of Stealth Part 1

There aren’t many genres with such a strong
core fantasy, as the stealth game.

These are games about staying unseen, and
then striking from the shadows.

About outsmarting an entire army of enemies,
without them even knowing you exist.

These are games about spies, assassins,
and, uh, Batmen.

But making this fantasy work means balancing
a number of complicated game systems: from

enemy awareness, to information gathering,
to robust detection systems.

Get any of them wrong, and the whole thing
can crumple in on itself.

So, welcome to the School of Stealth.

This is a short, GMTK mini-series about how
stealth games work.

In each episode, I’m going to take one system
from the stealth game formula and break down how

it works - looking, where necessary, at the
technical side of things, the design considerations,

and the end user experience.

For episode one, we need to start where most
stealth games begin: with the player being

hidden.

And then ask ourselves: how do guards actually
see and hear the player?

Ultimately, guards in games are given virtual
eyes and ears that are designed to simulate

the two main human senses: sight and sound.

To simulate vision, video game guards typically
have a viewcone - which is an invisible, cheese-like

entity that is stuck to the enemy’s face.

If the player character enters the cone, they
get detected.

It’s a touch more complicated than that,
of course.

A simple cone would allow characters to stay
unseen even if they were right next to the

enemy - so more complex shapes are often used.

In Splinter Cell Blacklist, there’s a basic
vision cone for the guard’s primary sightline,

but a second, much wider box to simulate peripheral
vision.

And even a small area behind the guard to
mimic that sixth sense of knowing when someone’s

just over your shoulder.

Developers will also need to consider the
height of the cone, depending on whether the

character should be able to hide when they’re
above enemies.

To know if the player is in cover, a game
will typically use a raycast - which is basically

when an invisible line is drawn between two
elements to see - in this case - if anything

is in the way.

You can make this more complex to catch moments
of partial cover: so, in Splinter Cell, the

enemy raycasts to eight different bones in
Sam Fisher’s player model - and will only

spot him if a certain number are visible.

Now, if the player enters the cone and isn’t
in cover, they probably don’t get immediately spotted.

Instead, the guard’s awareness of the player
starts to grow.

The speed at which this meter fills might
be slower if the character is further away,

or only in the guard’s peripheral vision,
or in low light, or crouching down, or

staying perfectly still.

When the meter tops out, though, the guard
will know exactly where you are.

It’s also important to note that guards
can be aware of more than just the player

character - such as open doors, interesting
objects, or dead bodies.

This can be used to make interesting plans
like traps and distractions - but it can also

help give the impression of intelligence and
awareness.

Now, simulating hearing is a different problem.

When you make a sound, like firing a gun,
walking on a loud floor board, or throwing

a stone - the sound will be given a distance
- related to the volume of that noise.

Any guard who is within that distance can
then be told to go check out the source of

the sound.

However, a straight line between the sound
and the guard won’t work, because we expect

noises to be muffled by walls.

So the typical solution is to use the game’s
pathfinding system - the same tech that allows

an enemy to find their way around a world
without bumping into objects.

Make the sound travel across that, and you’ll
more realistically capture the way sound propagates

through an environment in real life.

That’s the gist of things, then, but more
complex stuff might be included in certain

games - for example, in Thief, guards can
have second-hand information about the player

based on what other enemies are up to.

And in Hitman 2, enforcer characters are way
less perceptive of Agent 47 if he’s facing

away from them, which gives disguises more
power.

Links to more detailed technical information
can be found in the description beneath this video.

When done well, this system should create
a pretty realistic representation of a human’s

visual and auditory perception.

You can then make educated decisions about
where you will be safe, using your real-world

knowledge of how sight works in different
light conditions, or how sound might be muffled

by a wall.

But there’s always going to be a certain
level of ambiguity for the player to deal

with - which can lead to friction and frustration.

I’m sure you’ve played a stealth game
where you thought you were totally invisible,

but the guard saw you anyway.

So to help players make sense of this stuff,
there’s a few smart ways that devs can make

these perception systems more obvious.

The first is helpful interface elements.

Even way back in Thief, the developers knew
that it was tough for players to understand

how lit their character was from a first-person
perspective, so there’s a light gem at the

bottom of the screen to show your current
visibility.

And in Splinter Cell, the awkward challenge
of knowing how much sound you’re making,

is helped by a visualiser on Sam’s head’s
up display.

Also, most games have some kind of detection
indicator on their interface, which mimics

the guard’s awareness meter from earlier.

This helps the player know that they’re
about to be made - and sometimes even shows

you the location of the guard who’s seen
you.

Next up is using animation and audio to help
communicate a guard’s status to the player.

A guard who is idly lazing about might suggest
that they have pretty weak perception, but

a suspicious enemy with their weapon raised
will be way more alert to potential threats.

Audio barks also let you know that the guard
is starting to become aware of you.

Then there are refuge spaces.

These are places in the game world where,
in normal circumstances,

you are unambiguously hidden.

That might be the high-up gargoyles in Batman,
or areas of long grass in Assassin’s Creed,

or crates and cupboards in Hitman.

These give you at least one place where you
can scout and plan from a position of total safety.

Another big solution is player favouring - which
is the art of handicapping systems to bias

the player.

As Splinter Cell Blacklist programmer Martin
Walsh says, “it doesn’t matter what the

NPC can see or hear from a simulation perspective.

It’s what the player thinks the NPC should
be able to see or hear”.

So in his game, a guard’s hearing is reduced
by half when they’re offscreen, because

it feels unfair to be heard by someone you
can’t even see.

And in The Last of Us, enemies typically raycast
to Joel’s head to determine line of sight

- but that changes to his chest when he’s
crouching, to let him peep over cover without

being spotted.

And then the biggest help of all, as discussed
earlier, is a fuzzy detection system.

If you were immediately spotted when you touched
the guard’s vision cone, that wouldn’t

feel very fair.

So it makes sense that guards take a few moments
to become aware of your presence before being

totally alerted.

Now there’s one final, and rather bold solution
to this problem: and that’s to simply reveal

these systems to the player.

In the excellent side-scrolling sneak ‘em
up Mark of the Ninja, the guard’s perception

is about as unambiguous as you can get:

Their vision cones are displayed on screen.

The ninja is either in shadow or in light
- and that’s shown on the character’s sprite.

And when you make noises, you can see them
emanating from the source as big round pulses.

This is also shown to you before you even
make the noise, which is helpful for knowing

whether your noisy distraction or sneaky getaway
will be successful.

With the info on screen, there’s no arguing
about what’s happening in the system.

You’re either in the cone, or you’re not.

And that sound either reached the guard’s
ears, or it didn’t.

And so Ninja’s binary perception system
can be paired up with a totally binary detection

system of instant awareness - though, there
is a slight analogue fuzziness on the very

edges of the enemy’s view cones.

GUARD: "Is someone up there?"

For a slightly more nuanced take on this,
check out Shadow Tactics: Blades of the Shogun.

Here, the vision cone is split into three
zones: the bright green part near the enemy’s

face is the danger zone and leads to a pretty
instantaneous detection.

In the dark green part, you can stay hidden
if you’re crouched down, but will be spotted

if you stand up.

And the dotted part is for refuge zones like
bushes and high grass, where you will always

be invisible.

If you do trip the viewcone, the whole cone
will fill up with yellow - and if the yellow

part touches your character, you’re spotted.

It’s a very elegant way of displaying all
the necessary information, right there on screen.

Of course, it’s a lot harder to show this
sort of stuff in a fully 3D game.

The original Metal Gear Solid’s solution
was to simply photocopy the game world into

a top-down, 2D representation on your radar,
and then draw the vision cones on that.

It’s a sorta hand-wavey solution that’s
still being used in games like Deus Ex: Mankind Divided.

But it’s not impossible.

The Sly Cooper series has guards with torches
that cast obvious pools of yellow light.

If you find yourself inside the light, you
get spotted - but otherwise you’re safe.

Cartoony, yes.

But also, immediately readable.

The most important thing, though, is the experience
that these different perception systems lead to.

When the system is made analogue and ambiguous,
the player must evaluate the environment with

an immersive and realistic understanding of
light, shadow, distance, and sound.

And it also gives the game a certain level
of tension - where you can never been 100%

sure that you’re safe.

And I think this fits quite nicely with the
core stealth fantasy: these are games where

your power doesn’t come through sheer brute
force, but only through your ability to hide

from the enemy.

So having your sneaky status be fragile and
fuzzy reminds you that you’re always at

risk of losing your tenuous advantage over
the enemy.

As Thief programmer Tom Leonard says, “it's
about getting the player's heart pounding

by holding them on the cusp” of being found.

And it’s especially important to hide this
stuff in survival horror games that borrow

stealth elements.

In a game like Alien Isolation, it would be
rubbish if you could see exactly where the

Xenomorph was looking.

A huge amount of fear and anxiety is derived
from your shaky knowledge of the alien’s senses.

But making the system completely obvious has
its own advantages.

It puts way more power in your hands, and
allows you to play with a huge amount of confidence.

You can feel more like an apex predator, luring
enemies into traps or sneaking in for a silent kill.

As Ninja producer Jamie Cheng put it, “as
we were iterating, I found that I wasn't nearly

as interested in guessing whether a guard
would hear me or not, and way more interested

in creating an elaborate death trap”.

Of course, the predator feel can be achieved
through other methods, like those aforementioned

refuge zones and by giving the player a bucketload
of gadgets and super powers - but the more

accurately they can predict the enemy’s
perception, the quicker the player will get

to that experience.

So, that’s it for lesson one.

Stealth game guards see and hear through a
system of simulated eyes and ears - and developers

can create very different experiences, depending
on how much of that system they surface to

the player.

Come back next time for more deep dives into
sneak ‘em up design.

And if you subscribe to my channel, you’ll
get access to the new episode as soon as it

goes live.

Hey. Thanks for watching.

I hope you’re all doing okay in the midst
of this awful coronavirus pandemic.

It’s such a scary situation, so please stay
safe, wash your hands,

and follow the necessary guidelines.

I’ll do my best to keep making interesting
stuff to keep you busy and entertained.