Import USDZ and manipulate materials for each surface separately at runtime

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.

Solution

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()
    }
}