将ARFaceGeometry保存到OBJ文件 [英] Save ARFaceGeometry to OBJ file

问题描述

在iOS ARKit应用中，我一直试图将ARFaceGeometry数据保存到OBJ文件中.我按照以下说明进行操作:如何从AVDepthData创建3D模型? /a>.但是，OBJ创建不正确.这就是我所拥有的:

In an iOS ARKit app, I've been trying to save the ARFaceGeometry data to an OBJ file. I followed the explanation here: How to make a 3D model from AVDepthData?. However, the OBJ isn't created correctly. Here's what I have:

    func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
        guard let faceAnchor = anchor as? ARFaceAnchor else { return }
        currentFaceAnchor = faceAnchor

        // If this is the first time with this anchor, get the controller to create content.
        // Otherwise (switching content), will change content when setting `selectedVirtualContent`.
        if node.childNodes.isEmpty, let contentNode = selectedContentController.renderer(renderer, nodeFor: faceAnchor) {
            node.addChildNode(contentNode)
        }

        // https://stackoverflow.com/questions/52953590/how-to-make-a-3d-model-from-avdepthdata
        let geometry = faceAnchor.geometry        
        let allocator = MDLMeshBufferDataAllocator()
        let vertices = allocator.newBuffer(with: Data(fromArray: geometry.vertices), type: .vertex)
        let textureCoordinates = allocator.newBuffer(with: Data(fromArray: geometry.textureCoordinates), type: .vertex)
        let triangleIndices = allocator.newBuffer(with: Data(fromArray: geometry.triangleIndices), type: .index)
        let submesh = MDLSubmesh(indexBuffer: triangleIndices, indexCount: geometry.triangleIndices.count, indexType: .uInt16, geometryType: .triangles, material: MDLMaterial(name: "mat1", scatteringFunction: MDLPhysicallyPlausibleScatteringFunction()))

        let vertexDescriptor = MDLVertexDescriptor()
        // Attributes
        vertexDescriptor.addOrReplaceAttribute(MDLVertexAttribute(name: MDLVertexAttributePosition, format: .float3, offset: 0, bufferIndex: 0))
        vertexDescriptor.addOrReplaceAttribute(MDLVertexAttribute(name: MDLVertexAttributeNormal, format: .float3, offset: MemoryLayout<float3>.stride, bufferIndex: 0))
        vertexDescriptor.addOrReplaceAttribute(MDLVertexAttribute(name: MDLVertexAttributeTextureCoordinate, format: .float2, offset: MemoryLayout<float3>.stride + MemoryLayout<float3>.stride, bufferIndex: 0))
        // Layouts
        vertexDescriptor.layouts.add(MDLVertexBufferLayout(stride: MemoryLayout<float3>.stride + MemoryLayout<float3>.stride + MemoryLayout<float2>.stride))

        let mdlMesh = MDLMesh(vertexBuffers: [vertices, textureCoordinates], vertexCount: geometry.vertices.count, descriptor: vertexDescriptor, submeshes: [submesh])
        mdlMesh.addNormals(withAttributeNamed: MDLVertexAttributeNormal, creaseThreshold: 0.5)
        let asset = MDLAsset(bufferAllocator: allocator)
        asset.add(mdlMesh)

        let documentsPath = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
        let exportUrl = documentsPath.appendingPathComponent("face.obj")
        try! asset.export(to: exportUrl)
    }

生成的OBJ文件如下所示:

The resulting OBJ file looks like this:

# Apple ModelIO OBJ File: face
mtllib face.mtl
g 
v -0.000128156 -0.0277879 0.0575149
vn 0 0 0
vt -9.36008e-05 -0.0242016
usemtl material_1
f 1/1/1 1/1/1 1/1/1
f 1/1/1 1/1/1 1/1/1
f 1/1/1 1/1/1 1/1/1
... and many more lines

我希望有更多的顶点，并且索引值看起来不正确.

I would expect many more vertices, and the index values look wrong.

推荐答案

核心问题是未正确描述您的顶点数据.当您在构造网格时向模型I/O提供顶点描述符时，它表示数据实际具有的布局，而不是所需的布局.您提供了两个顶点缓冲区，但是您的顶点描述符描述了只有一个顶点缓冲区的交错数据布局.

The core issue is that your vertex data isn't described correctly. When you provide a vertex descriptor to Model I/O while constructing a mesh, it represents the layout the data actually has, not your desired layout. You're supplying two vertex buffers, but your vertex descriptor describes an interleaved data layout with only one vertex buffer.

最简单的解决方法是修复顶点描述符以反映您提供的数据:

The easiest way to remedy this is to fix the vertex descriptor to reflect the data you're providing:

let vertexDescriptor = MDLVertexDescriptor()
// Attributes
vertexDescriptor.attributes[0] = MDLVertexAttribute(name: MDLVertexAttributePosition,
                                                    format: .float3,
                                                    offset: 0,
                                                    bufferIndex: 0)
vertexDescriptor.attributes[1] = MDLVertexAttribute(name: MDLVertexAttributeTextureCoordinate,
                                                    format: .float2,
                                                    offset: 0,
                                                    bufferIndex: 1)
// Layouts
vertexDescriptor.layouts[0] = MDLVertexBufferLayout(stride: MemoryLayout<float3>.stride)
vertexDescriptor.layouts[1] = MDLVertexBufferLayout(stride: MemoryLayout<float2>.stride)

稍后调用addNormals(...)时，模型I/O将分配必要的空间并更新顶点描述符以反映新数据.由于您不是从数据进行渲染而是立即导出数据，因此为法线选择的内部布局并不重要.

When you later call addNormals(...), Model I/O will allocate the necessary space and update the vertex descriptor to reflect the new data. Since you're not rendering from the data and are instead immediately exporting it, the internal layout it chooses for the normals isn't important.

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