Import USDZ and manipulate materials for each surface separately at runtime - swift

Question

Basically, my requirement is to import a usdz, for an example, a car. Then apply different materials for each body part. Eg: material A to body, material B to side mirrors, material C to wheels etc.
I could iterate through the materials in the ModelEntity's ModelComponent and assign material as below.
func updateUIView(_ uiView: ARView, context: Context) {
let entity = try! Entity.loadModel(named: "CarScene")
var material = PhysicallyBasedMaterial()
material.baseColor = PhysicallyBasedMaterial.BaseColor(tint:.red)
material.roughness = PhysicallyBasedMaterial.Roughness(floatLiteral: 0.0)
material.metallic = PhysicallyBasedMaterial.Metallic(floatLiteral: 1.0)
for i in 0...(entity.model?.materials.count ?? 1) - 1 {
entity.model?.materials[i] = material
}
let anchor = AnchorEntity(plane: .horizontal)
anchor.addChild(entity)
uiView.scene.addAnchor(carAnchor)
}
But, I don't know which material is which. If I have multiple usdz files, I want to be able to accurately assign materials for each body part. Is that doable?
Do I need to break the usdz model and assign identifiers before importing to my Xcode project? Any help would be really appreciated.

Answer 1

Use the following sample code which shows you how to modify materials at runtime:
import SwiftUI
import RealityKit
struct ARViewContainer: UIViewRepresentable {
let arView = ARView(frame: .zero)
let fiat = try! Entity.load(named: "Fiat_Uno")
func makeUIView(context: Context) -> ARView {
print(fiat)
fiat.scale /= 5
fiat.orientation = .init(angle: .pi/1.5, axis: [0,1,0])
let anchor = AnchorEntity()
anchor.addChild(fiat)
arView.scene.anchors.append(anchor)
return arView
}
func updateUIView(_ view: ARView, context: Context) {
DispatchQueue.main.asyncAfter(deadline: .now() + 0.75) {
let wheels = fiat.findEntity(named: "Rodas_Material_001_0")?
.children[0] as? ModelEntity
wheels?.model?.materials[0] = UnlitMaterial(color: .green)
DispatchQueue.main.asyncAfter(deadline: .now() + 0.75) {
let body = fiat.findEntity(named: "Fiat_UNO_Material_001_0")?
.children[0] as? ModelEntity
body?.model?.materials[0] = UnlitMaterial(color: .blue)
}
}
}
}
struct ContentView : View {
var body: some View {
ARViewContainer().ignoresSafeArea()
}
}