The senses#

Eight senses, and each AI picks the ones it has. A guard, a guard dog and a blind cave creature are three profiles, not three pieces of code.

The eight senses#

Choosing an AI's senses#

Open the profile → Senses › Setup → Sense Classes, and add or remove senses. The order is the slot number; an AI uses at most 8.

• Sense Weights: a multiplier per sense (anything not listed is 1). A guard dog might weigh Smell 1.5, Hearing 1.2, Vision 0.6.
• Sense Intervals: seconds between a sense's evaluations, per sense (anything not listed uses the sense's own default: Vision 0.15 s, Hearing 0.1 s, Smell 0.25 s, Echolocation 0.3 s). Slowing an expensive sense is the cheapest performance win there is: vision at 0.3 s on background AI is usually indistinguishable from 0.15 s.

Copy-paste settings for each are in Ready-made AI.

Vision#

Automatic. If you're in range, in a cone, lit enough and not hidden, confidence rises. The Confidence page shows each factor in the demo; the rest of vision is here.

What it checks, in order#

1. CamouflageA Visibility Multiplier of 0 on the target's APS Target Component stops here, before any trace.
2. RangeDistance falloff.
3. ConesFocal, then peripheral, then rear. Outside all three: nothing.
4. How much showsTraces to the target's points: exposure, 0 to 1.
5. StanceEnough points visible for this stance?
6. LightGlobal, or measured at the target.
7. MotionA faster target is easier to spot.

Cones, and keyhole vision#

Use Separate Vertical FOV gives the cone its own pitch angle (Vision Vertical Half Angle Deg, 40): elliptical instead of round, for games with a lot of verticality.

Keyhole Vision (Keyhole Vision) makes the cone wide up close and narrow far away: Keyhole Near Angle (90) inside Keyhole Near Distance (400 cm), narrowing to Keyhole Far Angle (15) at full range. You can slip past a guard at 20 m by staying a little off its centre line; at 3 m it catches you in a wide arc. It reads far more naturally than a fixed cone.

Eyes in the head#

Unreal's own perception aims from the actor's root and faces its control rotation, which has nothing to do with the mesh: head turns and lean animations never move it, and the cone snaps to a Move To target before the body has turned. Detection › Eyes fixes both:

To use the eyes yourself (aim a flashlight, check what the AI could see), call Get Eye View Point: their live location, and the facing after Eye Turn Rate Deg Per Sec.

What blocks sight#

Only things that block count: a trigger volume that only overlaps never hides you, unless its surface is listed in Surface Vision Transmission.

Light#

The light on a target is the first of:

1. its APS Target Component Light Level Override, when it is 0 or more. This wins whatever the profile says, so a light gem works even with Use Per Target Light off;
2. with Use Per Target Light on, the sun-shadow trace (shadow can only darken, never brighten);
3. the global light.

Losing you: grace 0.3 s, direct cut, decays 3 times as fast. Its loss reason comes from what blocked its last look: Occluded, OutOfRange or SensorDropout (see Why it lost you).

Hearing#

Only hears what your game emits: there's no hearing from velocity. Call Emit Sound (from an actor) or Emit Sound at Location (from a place), with a Sound Type Definition that says how loud it is and how far it carries. Walls muffle, repeated noises add up, and a noise with no maker becomes a belief about a place. Everything is on its own page: Hearing & sounds.

Smell#

Automatic while in range, and indifferent to light: it works in total darkness. It reports where you are, but marked as imprecise (0.15 to 0.4), so any more precise sense wins the last known position.

The wind is the mechanic. When the wind carries your scent to the AI (it is downwind of you) it smells you up to twice as strongly (Downwind Bonus, 2). When the wind blows from it toward you, its signal drops to a tenth: keep the wind in your face and you can walk right up to it.

• Where the wind comes from: Set Wind State on the APS subsystem (direction and speed), or an APS Environment volume that overrides the wind. Indoors there is no wind. To give one kind of AI its own wind, set Wind Direction on a Blueprint child of Smell Sense: when it isn't zero, it wins.
• The world changes it: indoors scent is 1.2 times stronger; outdoors it weakens as the wind gets stronger, down to a quarter at 800 cm/s.
• Scent tags: On AI Smell gives a Scent Tag: the first actor tag on the target that starts with the Scent. prefix. Tag your player Scent.Human, a deer Scent.Prey, a body Scent.Blood, and branch on it.

Range and build-up come from the profile when set: Smell Max Range (400), Smell Accumulation Rate (0.1), Smell Decay Rate (0.05). The build-up only grows while you are in range, and fades only once you are out of it.

Confidence = max(instant, built up)
instant    = (Base Scent Intensity / Scent Threshold) × (1 − distance / range)
           × wind × exp(−Wall Absorption Coeff × walls) × weather × environment
wind       = Lerp(0.1, Downwind Bonus, (dot + 1) / 2)    (dot: the wind against target-to-AI)

Losing you: grace 20 s, no direct cut, decays slowly (×0.3), loss reason ScentLost. Scent lingers long after you've gone.

Vibration#

Footsteps through the floor. Passes through walls; no line of sight. The main sense for zombies, burrowing creatures and blind bosses (the clip at the top of this page).

Anything moving faster than Vibration Min Speed (10 cm/s) within Vibration Detect Range (800 cm) makes a signal: (1 − distance / range) × clamp(speed / 600, 0, 1). How fast you move matters more than how close you are: at 150 cm/s you make a quarter of the signal you make at 600.

• Surfaces: the target's actor tags Surface.Metal, Surface.Water or Surface.Ground say what it's walking on; On AI Sense Vibration passes it on.
• Things that aren't someone walking (a grenade, a collapsing wall): call Report Vibration Event (location, strength, surface). The AI feels it at that place (a place belief tagged Vibration, and On AI Sense Vibration), never as evidence about whoever stands nearby.
• Explosions shake the ground: vibration also feels sounds whose Sound Name is Explosion, from Emit Sound and Emit Sound at Location alike.

Losing you: grace 0 s, decays fast (×5), loss reason SensorDropout. Stand still and a vibration hunter loses you at once.

Echolocation#

Pulses that need no light and no line of sight. For bats, aliens, blind bosses and sonar robots. Anything within Echo Range (1500 cm) returns the pulse, every 0.3 s.

Confidence = (1 − distance / range)²: precise up close and dropping hard; at half range it is only 0.25. Echo Directional limits it to the front, loosely: it ignores targets more than about 101° off its facing (the same facing vision uses). Echolocation fires On AI See: to the AI, it is sight.

Losing you: grace 0.6 s, direct cut, loss reason OutOfRange.

Touch#

Contact: instant, and exact. Touch always places you with full precision (vision does only while all of you shows).

It wires itself: with Touch Sense in the profile and Auto Wire Touch Events on (the default), APS listens to the AI's own hits and overlaps.

• A character walking into the AI is felt as a Bump, stronger the faster it was moving: no physics settings needed.
• Only an AI whose root is physics-simulated needs Simulation Generates Hit Events; APS warns in the log if it's off.
• Auto Wire Overlap as Grab treats overlaps as a held Grab.
• A contact belongs to whoever made it, or whoever threw, fired or carries the thing that did. The AI bumping into a wall belongs to nobody.

Or report contacts yourself: Report Touch Contact (instigator, type, strength, persistent, location), Report Touch Contact with Impulse (from a physics hit: the impulse sets the strength), End Touch Contact (ends a held one). Contact types rank Explosion > Grab > Collision > Bump: when several land in one tick, the highest sets the confidence.

A held contact (a grab, an ongoing overlap) keeps building: strength + min(seconds held × Touch Persistent Accumulation Rate, Touch Max Persistent Bonus) (0.3 a second, up to 0.5).

Losing you: grace 0.5 s, decays ×2, loss reason SensorDropout.

Damage#

Whoever hurt it, at any distance. Add Damage Sense and every Unreal damage call (Apply Damage, Apply Point Damage, Apply Radial Damage) reaches it, no setup. Even a sniper 200 m away becomes a target the moment the shot lands, and the AI places the shooter almost exactly.

• Auto Confidence from Damage on (the default): confidence in the attacker rises to all the damage taken from them so far ÷ 100. 100 damage in one hit is instant full detection. If your rifle does 25, one shot is 0.25: enough for Suspected, not Detected; the second makes 0.5, Detected.
• Off: damage only fires On AI Damaged (instigator, amount, damage type tag, hit location), and you decide, for example with Set Target Confidence.

The Emotions & pain page shows the demo's D1 room: a thrown can tells the guard exactly where you stood.

Losing you: grace 8 s, decays slowly (×0.5). An AI remembers being shot.

Pain#

Pain is not health: it dulls the AI's senses. A flashbang leaves a guard near-blind for a few seconds; fire distracts its hearing. You create one Pain Type Definition per kind of pain (which senses it dulls, how fast it fades) and report it with Report Pain from Definition. Low health, read automatically from your character, dulls every sense too. All of it is on Emotions & pain.

APS and Unreal's AI Perception#