如何计算 SCNPlane 的法线向量？

Question

目标：定义由SCNPlane 表示的平面的法线向量。

正如 Apple 的文档 Working with Vectors - Calculate the Normal of a Triangle 中所述，我可以根据平面上的 3 个点计算法线向量。这里的问题是我不知道我如何才能得到 3 分，他们构成了正确的三角形。我注意到SCNPlane 有一个属性boundingBox，它可以代表两个三角形顶点（min 和max）。如何找到位于SCNPlane 上的第三个顶点？我不能使用SCNPlane 的中心，因为它会创建一条线以及来自boundingBox 的min 和max 点。

还有其他方法可以帮助我检索SCNPlane 的法线向量吗？

Answer 1

从documentation 我们了解到

表面是单面的。它的表面法向量指向其局部坐标空间的正z轴方向，因此默认情况下只能从该方向可见。

SCNPlane 的法线在本地空间中始终为(0, 0, 1)，并且无法更改。

当它附加到一个节点时，该节点的方向决定了另一个坐标系中的法线。您可以使用simdConvertVector:toNode:在坐标空间之间进行转换：

// normal expressed in world space
let normal = simd_normalize(node.simdConvertVector(simd_float3(0, 0, 1), to: nil))

Answer 2

要添加到已接受的答案，为了检索定义平面的点，您可以查询平面的几何源。

let plane = SCNPlane(width: 100, height: 20)
print("Sources for normal: \(vertices(sources: plane.sources(for: .normal)))")
print("Sources for vertex: \(vertices(sources: plane.sources(for: .vertex)))")

extension UnsafeRawPointer {
    func loadUnaligned<T>(as: T.Type, count: Int) -> [T] {
        assert(_isPOD(T.self)) // relies on the type being POD (no refcounting or other management)
        let buffer = UnsafeMutablePointer<T>.allocate(capacity: count)
        defer { buffer.deallocate() }
        memcpy(buffer, self, MemoryLayout<T>.size * count)
        return (0..<count).map({ index in buffer.advanced(by: index).pointee })
    }
}

func vertices(sources: [SCNGeometrySource]) -> [[SCNVector3]] {

    var result = [[SCNVector3]]()
    result.reserveCapacity(sources.count)
    for source in sources {
        precondition(source.usesFloatComponents == true, "SCNVector3 can handle only three-component vectors whose components are floating-point values, i.e., floats or doubles")
        precondition(source.componentsPerVector == 3, "SCNVector3 can only be used for three components per vector")
        
        let shouldUseFloatNotDouble: Bool
        if source.bytesPerComponent == 4 {
            shouldUseFloatNotDouble = true
        }
        else if source.bytesPerComponent == 8 {
            shouldUseFloatNotDouble = false
        }
        else {
            assert(false, "The SCNGeometrySource has reported an unexpected byte size for its vector components, not 4 bytes (float) or 8 bytes (double) but \(source.bytesPerComponent). I am not equipped for this so I am going to use floats and hope for the best. This will probably not work. Sorry.")
            shouldUseFloatNotDouble = true
        }
        
        let vectors = source.data.withUnsafeBytes {
            (p: UnsafeRawBufferPointer) -> [SCNVector3] in
            if (shouldUseFloatNotDouble) {
                let simdArray = (p.baseAddress! + source.dataOffset).loadUnaligned(as: SIMD3<Float>.self, count: source.vectorCount)
                return simdArray.map { simd in SCNVector3(simd)}
            } else {
                let simdArray = (p.baseAddress! + source.dataOffset).loadUnaligned(as: SIMD3<Double>.self, count: source.vectorCount)
                return simdArray.map { simd in SCNVector3(simd)}
            }
        }
        result.append(vectors)
    }
    return result
}

输出：

Sources for normal: [[__C.SCNVector3(x: 0.0, y: 0.0, z: 1.0), __C.SCNVector3(x: 1.0, y: 0.5, z: -0.5), __C.SCNVector3(x: 0.0, y: 0.0, z: 1.0), __C.SCNVector3(x: 1.0, y: -0.5, z: 0.5)]]
Sources for vertex: [[__C.SCNVector3(x: -0.5, y: -0.5, z: 0.0), __C.SCNVector3(x: 0.0, y: 1.0, z: 0.0), __C.SCNVector3(x: 0.5, y: -0.5, z: 0.0), __C.SCNVector3(x: 0.0, y: 1.0, z: 1.0)]]