ARKit 3.5 – How to export OBJ from new iPad Pro with LiDAR?

问题

How can I export the ARMeshGeometry generated by the new SceneReconstruction API on the latest iPad Pro to an .obj file?

Here's SceneReconstruction documentation.

回答1:

Starting with Apple's Visualising Scene Scemantics sample app, you can retrieve the ARMeshGeometry object from the first anchor in the frame.

The easiest approach to exporting the data is to first convert it to an MDLMesh:

extension ARMeshGeometry {
    func toMDLMesh(device: MTLDevice) -> MDLMesh {
        let allocator = MTKMeshBufferAllocator(device: device);

        let data = Data.init(bytes: vertices.buffer.contents(), count: vertices.stride * vertices.count);
        let vertexBuffer = allocator.newBuffer(with: data, type: .vertex);

        let indexData = Data.init(bytes: faces.buffer.contents(), count: faces.bytesPerIndex * faces.count * faces.indexCountPerPrimitive);
        let indexBuffer = allocator.newBuffer(with: indexData, type: .index);

        let submesh = MDLSubmesh(indexBuffer: indexBuffer,
                                 indexCount: faces.count * faces.indexCountPerPrimitive,
                                 indexType: .uInt32,
                                 geometryType: .triangles,
                                 material: nil);

        let vertexDescriptor = MDLVertexDescriptor();
        vertexDescriptor.attributes[0] = MDLVertexAttribute(name: MDLVertexAttributePosition,
                                                            format: .float3,
                                                            offset: 0,
                                                            bufferIndex: 0);
        vertexDescriptor.layouts[0] = MDLVertexBufferLayout(stride: vertices.stride);

        return MDLMesh(vertexBuffer: vertexBuffer,
                       vertexCount: vertices.count,
                       descriptor: vertexDescriptor,
                       submeshes: [submesh]);
    }
}

Once you have the MDLMesh, exporting to an OBJ file is a breeze:

    @IBAction func exportMesh(_ button: UIButton) {
        let meshAnchors = arView.session.currentFrame?.anchors.compactMap({ $0 as? ARMeshAnchor });

        DispatchQueue.global().async {

            let directory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0];
            let filename = directory.appendingPathComponent("MyFirstMesh.obj");

            guard let device = MTLCreateSystemDefaultDevice() else {
                print("metal device could not be created");
                return;
            };

            let asset = MDLAsset();

            for anchor in meshAnchors! {
                let mdlMesh = anchor.geometry.toMDLMesh(device: device);
                asset.add(mdlMesh);
            }

            do {
                try asset.export(to: filename);
            } catch {
                print("failed to write to file");
            }
        }
    }

回答2:

The answer of the @swiftcoder works great. But in the case of several anchors you need to convert the vertices coordinates to the world coordinate system based on the anchor transform. In the opposite case all meshes will be placed at zero position and you will have a mess.

The updated code looks like this:

extension ARMeshGeometry {
    func toMDLMesh(device: MTLDevice, transform: simd_float4x4) -> MDLMesh {
        let allocator = MTKMeshBufferAllocator(device: device)

        let data = Data.init(bytes: transformedVertexBuffer(transform), count: vertices.stride * vertices.count)
        let vertexBuffer = allocator.newBuffer(with: data, type: .vertex)

        let indexData = Data.init(bytes: faces.buffer.contents(), count: faces.bytesPerIndex * faces.count * faces.indexCountPerPrimitive)
        let indexBuffer = allocator.newBuffer(with: indexData, type: .index)

        let submesh = MDLSubmesh(indexBuffer: indexBuffer,
                                 indexCount: faces.count * faces.indexCountPerPrimitive,
                                 indexType: .uInt32,
                                 geometryType: .triangles,
                                 material: nil)

        let vertexDescriptor = MDLVertexDescriptor()
        vertexDescriptor.attributes[0] = MDLVertexAttribute(name: MDLVertexAttributePosition,
                                                            format: .float3,
                                                            offset: 0,
                                                            bufferIndex: 0)
        vertexDescriptor.layouts[0] = MDLVertexBufferLayout(stride: vertices.stride)

        return MDLMesh(vertexBuffer: vertexBuffer,
                       vertexCount: vertices.count,
                       descriptor: vertexDescriptor,
                       submeshes: [submesh])
    }

    func transformedVertexBuffer(_ transform: simd_float4x4) -> [Float] {
        var result = [Float]()
        for index in 0..<vertices.count {
            let vertexPointer = vertices.buffer.contents().advanced(by: vertices.offset + vertices.stride * index)
            let vertex = vertexPointer.assumingMemoryBound(to: (Float, Float, Float).self).pointee
            var vertextTransform = matrix_identity_float4x4
            vertextTransform.columns.3 = SIMD4<Float>(vertex.0, vertex.1, vertex.2, 1)
            let position = (transform * vertextTransform).position
            result.append(position.x)
            result.append(position.y)
            result.append(position.z)
        }
        return result
    }
}

extension simd_float4x4 {
    var position: SIMD3<Float> {
        return SIMD3<Float>(columns.3.x, columns.3.y, columns.3.z)
    }
}

extension Array where Element == ARMeshAnchor {
    func save(to fileURL: URL, device: MTLDevice) throws {
        let asset = MDLAsset()
        self.forEach {
            let mesh = $0.geometry.toMDLMesh(device: device, transform: $0.transform)
            asset.add(mesh)
        }
        try asset.export(to: fileURL)
    }
}

I am not an expert in ModelIO and maybe there is more simple way to transform vertex buffer :) But this code works for me.

来源：https://stackoverflow.com/questions/61063571/arkit-3-5-how-to-export-obj-from-new-ipad-pro-with-lidar