Perception

What it is#

Perception is how an agent finds out that something is there without anybody telling it. Five senses — sight, hearing, damage, touch and what an ally reports — over entities that opted in to being perceivable, producing a per-listener list of what is known and how long ago it was known.

It lives in Vixen.Ai.Perception, which is a second assembly rather than a folder in Vixen.Ai, because it needs Vixen.Engine for where things are and Vixen.Physics for what is between them. A game that wants behaviour trees without a physics world links Vixen.Ai and stops.

What it is for#

The loop every stealth game, shooter and survival game has: a guard on a patrol notices the player, chases, loses sight, searches where the player *was*, gives up. Every piece of that is here — the cone, the lose-sight radius, the last known location and the stimulus age — so a tree branches on keys instead of a game writing its own memory management.

You do *not* want it for a trigger volume, a scripted ambush or a boss that always knows where you are. Those are one line each, and a sense that is really a script is a sense whose settings mean nothing.

Using it#

Three things: a PerceptionConfig per kind of agent, an AiPerception on whoever notices, and an AiStimuliSource on whoever can be noticed.

⚠ A listener does not have to be an agent. A camera, a trap and a trigger all want to notice things without deciding anything; the blackboard binding is the only part that needs an AiAgent, and it is skipped when there is not one.

The senses#

| Sense | What it is | How it is found | |---|---|---| | Sight | inside a radius, inside a cone, with nothing solid in the way | sampled every pass | | Hearing | a noise somebody made, inside a radius scaled by its loudness | reported — ReportNoise | | Damage | was hurt by it. No radius, no cone, nothing in the way | reported — ReportDamage | | Touch | within a small radius, through anything | sampled every pass | | Team | an ally within range perceived it and said so | derived, after the other four |

⚠ Hearing and damage are events; sight and touch are states. An entity is continuously visible, so sight can be asked at any moment. An entity is not "audible" — an *event* is — so a hearing sense that sampled would have to sample the exact frame the shot happened, which at 4 Hz is one shot in six. Events are kept for PerceptionSystem.EventMemory seconds and each listener consumes what it has not seen, so a slow listener hears every shot a little late rather than hearing one in six.

⚠ Damage goes through the filter regardless of team. An agent shot by its own side has to notice, or friendly fire is invisible to the AI and a squad walks through its own grenades.

The lose-sight radius#

SightSettings has two radii and the second one is not a refinement:

⚠ With one radius, a target loitering on the boundary is found and lost several times a second. Each of those is a blackboard write, and each write aborts every branch whose decorator observes it — so the symptom is a guard that stutters between patrolling and chasing, and it reads as a bug in the behaviour tree rather than in the sense. It is the first thing every hand-rolled implementation gets wrong and it costs one field.

What a pass costs, and the three bounds on it#

Sight is O(listeners × sources), and the schedule is the whole design. Three things bound it and all three are mandatory rather than tuning:

1. A broad phase. StimuliGrid is a uniform grid over the sources, rebuilt every frame — every source moves every frame, so the incremental update a tree needs to earn its structure is the case that never happens. Five hundred listeners against five hundred sources examine 7 960 sources instead of 250 000. 2. A per-listener update rate, with a random deviation. ⚠ A deviation of zero puts every agent spawned in the same frame on the same tick for ever, which is a frame that costs the whole population. 3. Distance LOD. DistanceLodGovernor stretches the interval in three bands; with the shipped 0.1 s interval the far band lands on 4 Hz.

PerceptionSystem.LastStats reports what the last frame actually cost, and it is a deliverable rather than a diagnostic — a perception system that quietly stopped noticing things is a frame budget met by an AI nobody agreed to.

What reaches a tree#

An IBlackboardBinding is the join. Perception writes through Blackboard.Set*, so every decorator observing those keys gets its abort for free — a target appearing *is* the mechanism that interrupts a patrol, with nothing in the perception pass knowing a behaviour tree exists.

TargetLocationAgeBinding writes the default triple: who, where they were, and how long ago.

⚠ The target key stays set after the target is lost, and the age key is how a tree tells. Clearing it would make "chase him" and "search where he was" two branches over two keys, and the second would need its own copy of the position and its own timer — which is the hand-written memory management this exists to remove. A branch that wants a live target tests age < 0.5; a branch that wants to search tests age > 0.5; both read one key.

PerceivedCountBinding is the other shape: a flag and a number, naming no target at all. That is what a turret, an alarm or a "you are outnumbered" branch actually reads.

The three nodes#

PerceptionNodes.Register adds them to a schema and teaches a resolver to build them, which is how a .vxbt can name a node whose implementation is in another assembly:

| Node | Slot | Does | |---|---|---| | PerceivedTarget | decorator | this sense perceives something, or perceived it recently enough | | NearestPerceived | service | writes the nearest currently-perceived target into a key | | MakeNoise | task | emits a hearing stimulus where the agent is |

⚠ **PerceivedTarget reads the perceived list but *observes* a key**, and that pairing is the whole trick. A perceived list changing is not an event a tree can see; only a blackboard write is. So the binding writes the key, the key's observers interrupt the branch, and the decorator then answers the finer question — which sense, how stale — that a key could not carry.

Examples#

Wiring occlusion to a real physics world, and slowing down the agents nobody is looking at:

A guard that chases what it can see and searches where it last saw it — one key, two branches:

Reading a listener's list directly, which is what a debug overlay does:

A filter a game writes itself, for factions that are a table rather than a byte:

See also#

• Behaviour trees — what the keys a sense writes actually interrupt.
• The blackboard — the six key types, and why a version bumps only on a real change.
• Authoring a behaviour tree — adding a node type of your own, which is what
• the three nodes here do.