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

There are not many genres with a core fantasy as strong as the stealth game. These are games about staying unseen, then striking from the shadows. They are about outsmarting an entire army of enemies without them even knowing you exist. They are about spies, assassins, and, occasionally, Batman.

Making that fantasy work means balancing a number of complicated systems: enemy awareness, information gathering, and robust detection. Get any of them wrong, and the whole thing can crumple in on itself.

Most stealth games begin with the player being hidden. The central question is what happens next: how do guards actually see and hear the player?

Ultimately, guards in games are given virtual eyes and ears designed to simulate the two main human senses: sight and sound.

To simulate vision, video game guards typically have a viewcone: an invisible, cheese-like shape stuck to the enemy's face. If the player character enters the cone, they can be detected. It is a little more complicated than that, of course. A simple cone would allow characters to stay unseen even if they were standing right next to the enemy, so more complex shapes are often used.

In Splinter Cell Blacklist, there is a basic vision cone for the guard's primary sightline, 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 is just over your shoulder.

Developers also need to consider the height of the cone, depending on whether the character should be able to hide above enemies. To know if the player is in cover, a game will typically use a raycast: an invisible line drawn between two elements to see whether anything is in the way. Splinter Cell makes this more complex by raycasting to eight different bones in Sam Fisher's player model, then spotting him only if a certain number are visible.

If the player enters the cone and is not in cover, they usually do not get spotted immediately. Instead, the guard's awareness starts to grow. The speed of that meter might be slower if the character is farther away, only in peripheral vision, standing in low light, crouching down, or staying perfectly still. When the meter tops out, the guard knows exactly where the player is.

Guards can also be aware of more than the player character. They might notice open doors, interesting objects, or dead bodies. That can help players make plans with traps and distractions, and it also gives the impression that the guards are intelligent and aware of the wider world.

Hearing creates a different problem. When the player makes a sound, such as firing a gun, walking on a loud floorboard, or throwing a stone, that sound is given a distance related to its volume. Any guard within that distance can be told to investigate the source of the sound.

A straight line between the sound and the guard will not work, because players expect noises to be muffled by walls. The typical solution is to use the game's pathfinding system: the same technology that allows an enemy to find its way around the world without bumping into objects. Letting sound travel across that pathfinding data more realistically captures the way sound propagates through an environment.

Some games layer in more complex perception rules. In Thief, guards can have second-hand information about the player based on what other enemies are doing. In Hitman 2, enforcer characters are much less perceptive of Agent 47 if he is facing away from them, which gives disguises more power.

When these systems work well, they create a pretty realistic representation of human visual and auditory perception. Players can make educated decisions about where they will be safe, using real-world knowledge of how sight changes in different light conditions or how sound might be muffled by a wall.

There will always be some ambiguity, though, and that ambiguity can create friction. Many players have had the experience of thinking they were totally invisible, only for a guard to see them anyway. To help players make sense of perception systems, developers can make them more obvious in a few smart ways.

The first method is interface feedback. Even in Thief, the developers knew it was tough for players to understand how lit their character was from a first-person perspective, so the game uses a light gem at the bottom of the screen to show current visibility. Splinter Cell uses a display on Sam's heads-up interface to show how much sound he is making. Most games also have some kind of detection indicator, which mirrors the guard's awareness meter and sometimes points to the guard who has seen the player.

Animation and audio can communicate a guard's status too. A guard idly lazing about might suggest weak perception, while a suspicious enemy with a raised weapon feels much more alert to potential threats. Audio barks also tell the player that a guard is starting to become aware of them.

Then there are refuge spaces: places in the game world where, in normal circumstances, the player is unambiguously hidden. That might be the high gargoyles in Batman, areas of long grass in Assassin's Creed, or crates and cupboards in Hitman. These spaces give the player at least one place where they can scout and plan from a position of total safety.

Another major solution is player favouring: the art of handicapping systems to bias the player. As Splinter Cell Blacklist programmer Martin Walsh put it, "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."

In that game, a guard's hearing is reduced by half when the guard is offscreen, because it feels unfair to be heard by someone the player cannot even see. 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 is crouching, letting him peek over cover without being spotted.

The biggest help of all is a fuzzy detection system. If the player were immediately spotted the moment they touched a guard's vision cone, it would not feel very fair. It makes sense that guards take a few moments to become aware of the player's presence before becoming fully alerted.

One bold solution is to simply reveal the perception systems to the player. In the side-scrolling sneak 'em up Mark of the Ninja, guard perception is about as unambiguous as it gets. Vision cones are displayed on screen. The ninja is either in shadow or in light, and that state is shown directly on the character's sprite. When noises are made, they appear as big round pulses from the source. The game even shows the noise radius before the player makes the sound, which helps them know whether a distraction or getaway will work.

With all that information on screen, there is no arguing about what is happening in the system. The player is either in the cone or not. The sound either reached the guard's ears or it did not. Because the perception system is so clear, the game can pair it with a mostly binary detection system of instant awareness, with only slight analogue fuzziness on the edges of the viewcones.

Shadow Tactics: Blades of the Shogun offers a more nuanced version. Its vision cone is split into three zones. The bright green area near the enemy's face is the danger zone and leads to fast detection. In the dark green area, the player can stay hidden while crouched but will be spotted when standing. The dotted section covers refuge zones like bushes and high grass, where the player will always be invisible. If the player trips the viewcone, the whole cone fills with yellow, and if the yellow reaches the character, they are spotted.

It is much harder to show this kind of information in a fully 3D game, but it is not impossible. The original Metal Gear Solid effectively copies the game world into a top-down 2D radar and draws vision cones on top of that. Deus Ex: Mankind Divided uses a similar kind of abstraction. The Sly Cooper series takes another route by giving guards torches that cast obvious pools of yellow light. If the player is inside the light, they are spotted. Otherwise, they are safe. It is cartoony, but immediately readable.

The most important thing is the experience these different perception systems create. When the system is analogue and ambiguous, the player must evaluate the environment with an immersive, realistic understanding of light, shadow, distance, and sound.

That gives the game tension because the player can never be 100 percent sure they are safe. This fits the core stealth fantasy nicely. These games are not about power through brute force. They are about power through the ability to hide from the enemy. Keeping the player's hidden status fragile and fuzzy reminds them that they are always at risk of losing their tenuous advantage.

Thief programmer Tom Leonard described it as "getting the player's heart pounding by holding them on the cusp" of being found. That hidden uncertainty is especially important in survival horror games that borrow stealth elements. In a game like Alien: Isolation, it would be terrible if the player could see exactly where the Xenomorph was looking. A huge amount of fear and anxiety comes from shaky knowledge of the alien's senses.

Making the system completely obvious has its own advantages. It puts more power in the player's hands and allows them to play with much more confidence. The player can feel like an apex predator, luring enemies into traps or sneaking in for a silent kill.

Mark of the Ninja producer Jamie Cheng explained the appeal this way: "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."

That predator feeling can be achieved through other methods, such as refuge zones, gadgets, and super powers. But the more accurately players can predict enemy perception, the quicker they reach that confident, planning-heavy experience.

Stealth game guards see and hear through systems of simulated eyes and ears. Developers can create very different experiences depending on how much of those systems they surface to the player.

Hide the systems, and stealth becomes tense, fragile, and uncertain. Show the systems, and stealth becomes tactical, confident, and precise. Neither answer is automatically better. The right choice depends on whether the game wants the player to feel hunted, powerful, careful, clever, or some shifting mix of all four.

The design challenge is not just to make guards see and hear. It is to decide what the player should understand about that perception, what they should be forced to infer, and how fair the moment feels when the guard finally turns their head.