Build the Car Primitive

Welcome to Scene Graph: Car Primitive — the final scene-graph domain. Over four lessons you'll build a car you can paint, climb into, and drive, all from compile-clean Verse. But first, an honest word about what a "car" is in UEFN.

Two very different "cars"

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Primitive Handle

Fortnite has real, fully-simulated vehicles — engine, suspension, four wheels that grip the road. In build 41.00 those are assembled from modular_vehicle_component, vehicle_engine_component, vehicle_wheel_component, and friends. Here's the catch: every one of those component classes is <epic_internal>. A creator cannot build a wheeled vehicle from scene-graph components in Verse. (You can place a prebuilt vehicle with a vehicle_spawner_*_device — we'll point at that — but you can't author the sim.)

So we build the version you can drive from Verse: a primitive. A primitive is just a placed prop — a mesh body — that we claim as "the car", get a live handle on, and move on command. That handle is the foundation for everything that follows: colour it (Part 2), climb in and switch to first person (Part 3), and actually drive it (the capstone).

Honesty rule for this whole domain: when a capability is Epic-internal, we say so and teach the closest public thing instead.

Finding the car without wiring it

We'll use tag discovery — the same idiom behind every working island. Instead of dragging object references into @editable slots, the device finds its collaborators at runtime by tag:

• the car body is any placed prop carrying car_body_tag,
• the ignition is any button carrying car_ignition_tag.

A tag is just a class deriving from tag:

car_body_tag := class(tag) {}
car_ignition_tag := class(tag) {}

At begin, we ask the world for everything wearing each tag and cast it to the concrete type we need:

for (Obj : FindCreativeObjectsWithTag(car_ignition_tag)):
    if (Ignition := button_device[Obj]):
        Ignition.InteractedWithEvent.Subscribe(OnIgnition)

FindCreativeObjectsWithTag returns creative_object_interface values; button_device[Obj] casts one to a button (the [...] form fails gracefully if it isn't a button). Same trick gives us the car body as a creative_prop.

Proving we have a live handle: nudge it forward

Once we hold a creative_prop, we can move it. The proof-of-life for Part 1 is simple — press the ignition and the car eases forward along the way it's already facing:

NudgeOne(Body : creative_prop)<suspends> : void =
    Current : transform = Body.GetTransform()
    Forward : vector3 = Current.Rotation.GetLocalForward()
    Target : vector3 = Current.Translation + Forward * NudgeDistance
    Body.MoveTo(Target, Current.Rotation, NudgeSeconds)

GetTransform() reads where the prop is right now. Current.Rotation.GetLocalForward() is the prop's own +X axis as a unit vector — "the way the car points". Scale it by NudgeDistance, add it to the current position, and MoveTo glides the prop there over NudgeSeconds, keeping its rotation so it doesn't spin. Because OnIgnition can't wait, it spawns the work.

Why not physics? creative_prop does expose ApplyForce / SetLinearVelocity, but those no-op unless the prop carries a FortPhysicsComponent, which isn't Verse-exposed — you'd have to author the prop physics-enabled in the editor. MoveTo works on any prop, so it's the reliable primitive-motion tool for a lesson.

The full device

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Car Primitive Assembly

This device is compile-verified in UEFN 5.8 (the whole project builds clean with it installed). Grab it as 03-device.verse, place a prop for the body (tag it car_body_tag) and a Button for the ignition (tag it car_ignition_tag), and press it.

Recap

• A creator-built wheeled vehicle is Epic-internal; the public version is a primitive — a placed prop you drive from Verse.
• Tag discovery (FindCreativeObjectsWithTag + a cast like creative_prop[Obj]) gets a live handle with no @editable wiring.
• GetTransform() + GetLocalForward() + MoveTo is the reliable way to move a prop along its own facing.
• Prefer MoveTo over physics for a prop primitive — physics calls need an editor-authored FortPhysicsComponent.