3D artwork that is exported as USDZ has different colors

Question

I want to start by saying that I am new to SceneKit and playing with AR environments on iOS.

I have a 3D model object, a van, as part of a Blender file(.blend) --> it can be found here.

I am exporting it as a .glb file from Blender --> it can be found here.

Then I open the .glb file in Reality Converter app, and export it as a .usdz file --> it can be found here.

The issue is when I preview the object in XCode and also when I place it in AR, the object colours are wrong.

Is there a problem with the Blender object? If so, what should I change to it so that the colours are exported correctly?

Answer 1

About Blender, Maya and Xcode

Unfortunately, the issue you mentioned here is a great old issue in Blender. Indeed, Blender outputs wrong (a.k.a. inconsistent) shaders' colors, due to the fact that Blender operates in linear color space (not sRGB) of shader colors. There are 2 possible solutions in this case. The first one is quite radical - to use a real PRO software, like Autodesk Maya 2023 with USD plugin (because it takes a decent amount of time to learn Maya), in which exported shaders' colors are reproduced very accurately in correct color space – you must reassign all the materials from scratch. The second one – fix colors in Xcode's inspector manually, or programmatically.

From Maya you can export your model as binary or ASCII .usd file format (make sure that MayaUSD plugin is loaded). Then use Reality Converter, to convert the resulting model in .usdz archive.

Xcode solution

Load your van.usdz model into Xcode. Rename model's nodes in hierarchy for convenience.

Very important!

Select Body_Orange node and go to Attributes Inspector tab. Delete Colors attribute.

Colors attribute keeps .usdz colour schema.

Then in Material Inspector assign a needed Diffuse color of physicallyBased shader.

Also, you can do it programmatically. Here's a SwiftUI sample code for SceneKit:

import SwiftUI
import SceneKit
...

func makeUIView(context: Context) -> SCNView {

    let sceneView = SCNView(frame: .zero)
    sceneView.scene = SCNScene(named: "van.usdz")
    
    let orangeBody = sceneView.scene?.rootNode.childNodes[0].childNodes[0]
        .childNodes[0].childNode(withName: "Body_Orange", recursively: true)

    orangeBody?.geometry?.firstMaterial?.diffuse.contents = 
          UIColor(red: 218/255, green: 154/255, blue: 93/255, alpha: 255/255)

    let blueBody = sceneView.scene?.rootNode.childNodes[0].childNodes[0]
        .childNodes[0].childNode(withName: "Body_Blue", recursively: true)

    blueBody?.geometry?.firstMaterial?.diffuse.contents = 
          UIColor(red: 58/255, green: 150/255, blue: 207/255, alpha: 255/255)

    sceneView.backgroundColor = .black
    sceneView.autoenablesDefaultLighting = true
    sceneView.allowsCameraControl = true
    sceneView.showsStatistics = true

    return sceneView
}

And here's a SwiftUI sample code for RealityKit version:

import SwiftUI
import RealityKit
...

func makeUIView(context: Context) -> ARView {
    
    let arView = ARView(frame: .zero)
    arView.backgroundColor = .black
    
    let vanModel = try! ModelEntity.load(named: "van.usdz")
     
    let orangeBodyPart = vanModel.children[0].children[0].children[0]
                                 .children[0].children[0] as! ModelEntity
    let colorOrange = UIColor(red: 218/255, green: 154/255, 
                             blue: 93/255, alpha: 255/255)
    orangeBodyPart.model?.materials = [SimpleMaterial(color: colorOrange, 
                                                 isMetallic: false)]
    
    let blueBodyPart = vanModel.children[0].children[0].children[0]
                               .children[0].children[1] as! ModelEntity
    let colorBlue = UIColor(red: 58/255, green: 150/255, 
                           blue: 207/255, alpha: 255/255)
    blueBodyPart.model?.materials = [SimpleMaterial(color: colorBlue, 
                                               isMetallic: false)]

    let anchor = AnchorEntity(world: [0, 0,-5])
    vanModel.setParent(anchor)
    arView.scene.anchors.append(anchor)        
    return arView
}

A few words on poly count and retopology in Maya

I would like to make one remark about the polygons' count in your model. Such a simple model can be made having 2K to 5K polygons. But your model has over 50,000 polygons, which is a tremendous number. Your hi-poly model will consume a lot of resources (CPU, GPU, memory), thus, drop frame is possible (58 fps, as you can see in the picture).

So, what can you do about it? Ideally, it's a 3D modeler's job to conceive and create low-poly models for games and mobile platforms – it's really a precious skill. Modifying an already finished mesh (saved to distribution formats like .fbx or .usdz) is often a bad idea. However, Autodesk Maya for example, has a retopology tool (Mesh –> Retopologize) that helps you significantly reduce the number of polygons using advanced algorithm. Watch the official video (Time - 00:50).