A sample BillboardComponent for RealityKit in visionOS

BillboardComponent.swift

//  BillboardComponent.swift

import ARKit
import RealityKit
import SwiftUI

public struct BillboardComponent: Component, Codable {
    public init() {}
}

public struct BillboardSystem: System {
    static let query = EntityQuery(where: .has(BillboardComponent.self))
    private let arkitSession = ARKitSession()
    private let worldTrackingProvider = WorldTrackingProvider()

    public init(scene: RealityKit.Scene) {
        setUpSession()
    }

    func setUpSession() {
        Task {
            do {
                try await arkitSession.run([worldTrackingProvider])
            } catch {
                print("Error: \(error)")
            }
        }
    }

    public func update(context: SceneUpdateContext) {
        let entities = context.scene.performQuery(Self.query).map({ $0 })
        guard !entities.isEmpty,
           let deviceAnchor = worldTrackingProvider.queryDeviceAnchor(atTimestamp:
                                                CACurrentMediaTime()) else { return }
        let cameraTransform = Transform(matrix: deviceAnchor.originFromAnchorTransform)

        for entity in entities {
            let translation = entity.transform.translation
            entity.look(at: cameraTransform.translation,
                        from: entity.position(relativeTo: nil),
                        relativeTo: nil,
                        forward: .positiveZ)
            entity.transform.translation = translation
        }
    }
}

BillboardComponentTestApp.swift

// BillboardComponentTestApp.swift
import SwiftUI

@main
struct BillboardComponentTestApp: App {
    init() {
        BillboardComponent.registerComponent()
        BillboardSystem.registerSystem()
    }

    var body: some Scene {
        WindowGroup {
            ContentView()
        }

        ImmersiveSpace(id: "ImmersiveSpace") {
            ImmersiveView()
        }.immersionStyle(selection:
                .constant(.mixed), in: .mixed)
    }
}

ContentView.swift

//  ContentView.swift

import SwiftUI
import RealityKit
import RealityKitContent

struct ContentView: View {

    @State private var showImmersiveSpace = false
    @State private var immersiveSpaceIsShown = false

    @Environment(\.openImmersiveSpace) var openImmersiveSpace
    @Environment(\.dismissImmersiveSpace) var dismissImmersiveSpace

    var body: some View {
        VStack {
            Model3D(named: "Scene", bundle: realityKitContentBundle)
                .padding(.bottom, 50)

            Text("Hello, world!")

            Toggle("Show Immersive Space", isOn: $showImmersiveSpace)
                .toggleStyle(.button)
                .padding(.top, 50)
        }
        .padding()
        .onChange(of: showImmersiveSpace) { _, newValue in
            Task {
                if newValue {
                    switch await openImmersiveSpace(id: "ImmersiveSpace") {
                    case .opened:
                        immersiveSpaceIsShown = true
                    case .error, .userCancelled:
                        fallthrough
                    @unknown default:
                        immersiveSpaceIsShown = false
                        showImmersiveSpace = false
                    }
                } else if immersiveSpaceIsShown {
                    await dismissImmersiveSpace()
                    immersiveSpaceIsShown = false
                }
            }
        }
    }
}

#Preview(windowStyle: .automatic) {
    ContentView()
}

ImmersiveView.swift

//  ImmersiveView.swift

import SwiftUI
import RealityKit
import RealityKitContent

struct ImmersiveView: View {
    var body: some View {
        RealityView { content, attachments in
            if let entity = attachments.entity(for: "Board") {
                entity.components[BillboardComponent.self] = BillboardComponent()
                entity.position = SIMD3<Float>(0, 1, -2)
                content.add(entity)

                // Add an ImageBasedLight for the immersive content here
                // Put skybox here
            }
        } attachments: {
            Attachment(id: "Board") {
                Text("Billboard")
                    .font(.system(size: 100))
                    .padding(64)
                    .background(.green)
            }
        }
    }
}

#Preview {
    ImmersiveView()
        .previewLayout(.sizeThatFits)
}