How to make surface material double-sided for MDLAsset? - ios

Question

I am trying to create an app that allows me to scan a room and then export a 3D file using lidar. I am able to do this with the following code (thanks to ARKit – How to export OBJ from iPhone/iPad with LiDAR?); however, it seems that the surfaces are all one-sided (when you rotate the object around, the backside of every surface is clear). How can I make the surface double-sided in my saveButtonTapped method?
import RealityKit
import ARKit
import MetalKit
import ModelIO
#IBOutlet var arView: ARView!
var saveButton: UIButton!
let rect = CGRect(x: 50, y: 50, width: 100, height: 50)
override func viewDidLoad() {
super.viewDidLoad()
let tui = UIControl.Event.touchUpInside
saveButton = UIButton(frame: rect)
saveButton.setTitle("Save", for: [])
saveButton.addTarget(self, action: #selector(saveButtonTapped), for: tui)
self.view.addSubview(saveButton)
}
#objc func saveButtonTapped(sender: UIButton) {
print("Saving is executing...")
guard let frame = arView.session.currentFrame
else { fatalError("Can't get ARFrame") }
guard let device = MTLCreateSystemDefaultDevice()
else { fatalError("Can't create MTLDevice") }
let allocator = MTKMeshBufferAllocator(device: device)
let asset = MDLAsset(bufferAllocator: allocator)
let meshAnchors = frame.anchors.compactMap { $0 as? ARMeshAnchor }
for ma in meshAnchors {
let geometry = ma.geometry
let vertices = geometry.vertices
let faces = geometry.faces
let vertexPointer = vertices.buffer.contents()
let facePointer = faces.buffer.contents()
for vtxIndex in 0 ..< vertices.count {
let vertex = geometry.vertex(at: UInt32(vtxIndex))
var vertexLocalTransform = matrix_identity_float4x4
vertexLocalTransform.columns.3 = SIMD4<Float>(x: vertex.0,
y: vertex.1,
z: vertex.2,
w: 1.0)
let vertexWorldTransform = (ma.transform * vertexLocalTransform).position
let vertexOffset = vertices.offset + vertices.stride * vtxIndex
let componentStride = vertices.stride / 3
vertexPointer.storeBytes(of: vertexWorldTransform.x,
toByteOffset: vertexOffset,
as: Float.self)
vertexPointer.storeBytes(of: vertexWorldTransform.y,
toByteOffset: vertexOffset + componentStride,
as: Float.self)
vertexPointer.storeBytes(of: vertexWorldTransform.z,
toByteOffset: vertexOffset + (2 * componentStride),
as: Float.self)
}
let byteCountVertices = vertices.count * vertices.stride
let byteCountFaces = faces.count * faces.indexCountPerPrimitive * faces.bytesPerIndex
let vertexBuffer = allocator.newBuffer(with: Data(bytesNoCopy: vertexPointer,
count: byteCountVertices,
deallocator: .none), type: .vertex)
let indexBuffer = allocator.newBuffer(with: Data(bytesNoCopy: facePointer,
count: byteCountFaces,
deallocator: .none), type: .index)
let indexCount = faces.count * faces.indexCountPerPrimitive
let material = MDLMaterial(name: "material",
scatteringFunction: MDLPhysicallyPlausibleScatteringFunction())
let submesh = MDLSubmesh(indexBuffer: indexBuffer,
indexCount: indexCount,
indexType: .uInt32,
geometryType: .triangles,
material: material)
let vertexFormat = MTKModelIOVertexFormatFromMetal(vertices.format)
let vertexDescriptor = MDLVertexDescriptor()
vertexDescriptor.attributes[0] = MDLVertexAttribute(name: MDLVertexAttributePosition,
format: vertexFormat,
offset: 0,
bufferIndex: 0)
vertexDescriptor.layouts[0] = MDLVertexBufferLayout(stride: ma.geometry.vertices.stride)
let mesh = MDLMesh(vertexBuffer: vertexBuffer,
vertexCount: ma.geometry.vertices.count,
descriptor: vertexDescriptor,
submeshes: [submesh])
asset.add(mesh)
}
let filePath = FileManager.default.urls(for: .documentDirectory,
in: .userDomainMask).first!
let usd: URL = filePath.appendingPathComponent("model.usd")
if MDLAsset.canExportFileExtension("usd") {
do {
try asset.export(to: usd)
let controller = UIActivityViewController(activityItems: [usd],
applicationActivities: nil)
controller.popoverPresentationController?.sourceView = sender
self.present(controller, animated: true, completion: nil)
} catch let error {
fatalError(error.localizedDescription)
}
} else {
fatalError("Can't export USD")
}
}