0:00:03.780,0:00:09.400
There aren’t many genres with such a strong[br]core fantasy, as the stealth game.

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

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

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These are games about spies, assassins,[br]and, uh, Batmen.

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

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enemy awareness, to information gathering,[br]to robust detection systems.

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

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So, welcome to the School of Stealth.

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

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

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

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and the end user experience.

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

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hidden.

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

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

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the two main human senses: sight and sound.

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

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entity that is stuck to the enemy’s face.

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If the player character enters the cone, they[br]get detected.

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

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

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enemy - so more complex shapes are often used.

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

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

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

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just over your shoulder.

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

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

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

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

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is in the way.

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

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

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spot him if a certain number are visible.

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

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

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

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

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staying perfectly still.

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

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

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

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

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help give the impression of intelligence and[br]awareness.

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Now, simulating hearing is a different problem.

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

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

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

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the sound.

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

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noises to be muffled by walls.

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

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

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

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through an environment in real life.

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

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

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based on what other enemies are up to.

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

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away from them, which gives disguises more[br]power.

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

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

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visual and auditory perception.

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

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

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by a wall.

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

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with - which can lead to friction and frustration.

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

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but the guard saw you anyway.

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

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these perception systems more obvious.

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The first is helpful interface elements.

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

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

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bottom of the screen to show your current[br]visibility.

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

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

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

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the guard’s awareness meter from earlier.

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

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you the location of the guard who’s seen[br]you.

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

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

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

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

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Then there are refuge spaces.

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These are places in the game world where,[br]in normal circumstances,

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you are unambiguously hidden.

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

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or crates and cupboards in Hitman.

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

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

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the player.

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

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NPC can see or hear from a simulation perspective.

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

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

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

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

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

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being spotted.

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

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

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feel very fair.

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

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totally alerted.

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

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these systems to the player.

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

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is about as unambiguous as you can get:

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Their vision cones are displayed on screen.

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

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

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

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whether your noisy distraction or sneaky getaway[br]will be successful.

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

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

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

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

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

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edges of the enemy’s view cones.

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GUARD: "Is someone up there?"

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

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

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

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

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if you stand up.

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

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be invisible.

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

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part touches your character, you’re spotted.

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

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

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

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

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

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But it’s not impossible.

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

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

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Cartoony, yes.

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But also, immediately readable.

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

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

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

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

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sure that you’re safe.

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

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

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from the enemy.

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

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risk of losing your tenuous advantage over[br]the enemy.

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

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by holding them on the cusp” of being found.

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

0:10:58.590,0:11:00.100
stealth elements.

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

0:11:04.850,0:11:06.410
Xenomorph was looking.

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

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But making the system completely obvious has[br]its own advantages.

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

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

0:11:27.280,0:11:32.440
As Ninja producer Jamie Cheng put it, “as[br]we were iterating, I found that I wasn't nearly

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

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in creating an elaborate death trap”.

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

0:11:44.750,0:11:50.010
refuge zones and by giving the player a bucketload[br]of gadgets and super powers - but the more

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

0:11:54.200,0:11:56.120
to that experience.

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So, that’s it for lesson one.

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

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

0:12:09.300,0:12:10.580
the player.

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

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

0:12:19.440,0:12:20.460
goes live.

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Hey. Thanks for watching.

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

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

0:12:33.192,0:12:35.730
and follow the necessary guidelines.

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