UEFN Scene Graph Sight Detection: Make Guards See Entities

UEFN Scene Graph sight detection uses an emitter on the object being watched and a detector on the observer. In Fortnite 43.10, the experimental Perception API can report when an entity enters or leaves a viewing cone. It doesn’t provide an entire guard AI, and islands using the experimental feature cannot be published.

To build a guard prototype, subscribe to the detector’s events and decide what to do when it sees a target. You can trigger an alert or store the last sighting in your own logic. A detector on its own won’t make an NPC chase or investigate anything.

How UEFN Scene Graph sight detection is wired

Attach a sight_emitter_component to the entity you want observed. On the observing entity, use a sight_detector_component. The detector describes a viewing cone along the entity’s forward direction. Set both HalfAngle and Range in Verse, then adjust near clipping if needed. A receiver must subscribe to the detector with an appropriate filter.

Without a subscription, the detector does not do the detection work you expect, so an otherwise correct scene can look completely inert.

Set the sight cone for UEFN Scene Graph sight detection

In UEFN Scene Graph sight detection, Range controls how far away an entity can be noticed. The cone angle controls how far to the side the detector can see. Near clipping avoids detecting objects impossibly close to the observer, and hysteresis can prevent visibility flickering when a target hovers at the edge of the cone.

Test with the target directly in front of the observer, then move sideways and behind it. Only after those boundaries look right should you connect detections to an alert mechanic.

Understand Active, Inactive and Instant events

Perception events use a shared state model, but not every sense relies on every state in the same way. For a sight workflow, pay attention to the start and end of visibility and the detail carried with the event, including how clearly something was perceived and where it was.

A guard can use those states to mark the beginning of an alert and to stop treating a vanished target as currently visible. That last part is your gameplay logic, not an automatic memory system.

UEFN Scene Graph sight detection: why the detector might see nothing

First check that the target actually has the sight emitter and that the observer has the matching detector, not the hearing or touch component by mistake. Confirm that a subscription exists and that its filter does not reject every event. Next, inspect position and forward direction, then test a larger range and a wider cone.

A mesh is not a prerequisite for the basic sight setup in the way collidable meshes are for the built-in touch sense.

Why HalfAngle and Range are required in Verse

A common UEFN Scene Graph sight detection compile error comes from leaving out HalfAngle or Range when constructing sight_detector_component. Neither has a default. HalfAngle uses radians, while Range and NearClipRadius use centimetres. Start with a short range and a target directly ahead, then widen the cone after events arrive.

HysteresisAngle and HysteresisRange help prevent detection from flickering at the edge. Objects between the observer and target can also block sight.

Test sight through walls and around corners

A guard should not report a clear view of an object through a solid wall. Place a sight emitter directly in front of your detector and confirm that Active arrives. Then move the target behind an obstacle and check whether visibility ends. Repeat near the edge of the sight cone to see whether the detector flips rapidly between states.

If the detector seems unreliable, change only one setting at a time. Increase the range, widen HalfAngle or adjust the hysteresis after you know the subscription works. Epic’s data also reports how clearly an entity was perceived and where it was seen. Use that information for your own alert logic instead of treating every sighting as the same event.

UEFN Scene Graph sight detection FAQ

Why will my sight detector not compile? When constructing it in Verse, set both HalfAngle and Range. Epic says neither has a default.

Can sight detection automatically make an NPC chase something? No. It reports detections; you write the movement or alert behaviour.

Can an island using this system be published? Not with the experimental 43.10 Perception API.

Official source: Epic Games documentation.