How to programmatically export 3D mesh as USDZ using ModelIO? - swift

Question

Is it possible to programmatically export 3D mesh as .usdz file format using ModelIO and MetalKit frameworks?
Here's a code:
import ARKit
import RealityKit
import MetalKit
import ModelIO
let asset = MDLAsset(bufferAllocator: allocator)
asset.add(mesh)
let filePath = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask).first!
let usdz: URL = filePath.appendingPathComponent("model.usdz")
do {
try asset.export(to: usdz)
let controller = UIActivityViewController(activityItems: [usdz],
applicationActivities: nil)
controller.popoverPresentationController?.sourceView = sender
self.present(controller, animated: true, completion: nil)
} catch let error {
fatalError(error.localizedDescription)
}
When I press a Save button I get an error.

Answer 1

The Andy Jazz answer is correct, but needs modification in order to work in a SwiftUI Sandboxed app:
First, the SCNScene needs to be rendered in order to export correctly. You can't create a bunch of nodes, stuff them into the scene's root node and call write() and get a correctly rendered usdz. It must first be put on screen in a SwiftUI SceneView, which causes all the assets to load, etc. I suppose you could instantiate a SCNRenderer and call prepare() on the root node, but that has some extra complications.
Second, the Sandbox prevents a direct export to a URL provided by .fileExporter(). This is because Scene.write() works in two steps: it first creates a .usdc export, and zips the resulting files into a single .usdz. The intermediate files don't have the write privileges the URL provided by .fileExporter() does (assuming you've set the Sandbox "User Selected File" privilege to "Read/Write"), so Scene.write() fails, even if the target URL is writeable, if the target directory is outside the Sandbox.
My solution was to write a custom FileWrapper, which I return if the WriteConfiguration UTType is .usdz:
public class USDZExportFileWrapper: FileWrapper {
var exportScene: SCNScene
public init(scene: SCNScene) {
exportScene = scene
super.init(regularFileWithContents: Data())
}
required init?(coder inCoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override public func write(to url: URL,
options: FileWrapper.WritingOptions = [],
originalContentsURL: URL?) throws {
let tempFilePath = NSTemporaryDirectory() + UUID().uuidString + ".usdz"
let tempURL = URL(fileURLWithPath: tempFilePath)
exportScene.write(to: tempURL, delegate: nil)
try FileManager.default.moveItem(at: tempURL, to: url)
}
}
Usage in a ReferenceFileDocument:
public func fileWrapper(snapshot: Data, configuration: WriteConfiguration) throws -> FileWrapper {
if configuration.contentType == .usdz {
return USDZExportFileWrapper(scene: scene)
}
return .init(regularFileWithContents: snapshot)
}

Answer 2

08th January 2023
At the moment iOS developers still can export only .usd, .usda and .usdc files; you can check this using canExportFileExtension(_:) type method:
let usd = MDLAsset.canExportFileExtension("usd")
let usda = MDLAsset.canExportFileExtension("usda")
let usdc = MDLAsset.canExportFileExtension("usdc")
let usdz = MDLAsset.canExportFileExtension("usdz")
print(usd, usda, usdc, usdz)
It prints:
true true true false
However, you can easily export SceneKit's scenes as .usdz files using instance method called: write(to:options:delegate:progressHandler:).
let path = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask)[0]
.appendingPathComponent("file.usdz")
sceneKitScene.write(to: path,
options: nil,
delegate: nil,
progressHandler: nil)