ios - 以多边形为原始类型的 SCNGeometry

Question

试图弄清楚我如何创建一个多边形作为原始类型的 SCNGeometry，我的目标是添加多边形节点作为球体节点的子节点，并使其看起来像 map 套件的 MKPolygon，like in this example .

我当前的代码是:

//Take an arbitrary array of vectors
let vertices: [SCNVector3] = [
SCNVector3Make(-0.1304485, 0.551937, 0.8236193),
SCNVector3Make(0.01393811, 0.601815, 0.7985139),
SCNVector3Make(0.2971005, 0.5591929, 0.7739732),
SCNVector3Make(0.4516893, 0.5150381, 0.7285002),
SCNVector3Make(0.4629132, 0.4383712, 0.7704169),
SCNVector3Make(0.1333823, 0.5224985, 0.8421428),
SCNVector3Make(-0.1684743, 0.4694716, 0.8667254)]

//Does polygon shape require indices?
let indices: [Int] = [0,1,2,3,4,5,6]

let vertexSource = SCNGeometrySource(vertices: vertices)
let indexData = Data(bytes: indices, count: indices.count * MemoryLayout<Int>.size)

//Note!!! I get compiler error if primitiveCount is greater than 0
let element = SCNGeometryElement(data: indexData, primitiveType: .polygon, primitiveCount: 0, bytesPerIndex: MemoryLayout<Int>.size)
let geometry = SCNGeometry(sources: [vertexSource], elements: [element])

let material = SCNMaterial()
material.diffuse.contents = UIColor.purple.withAlphaComponent(0.75)
material.isDoubleSided = true
geometry.firstMaterial = material

let node = SCNNode(geometry: geometry)

像这样使用 SCNGeometryElement 时，我得到一个空节点。

Answer 1

你有两个问题:

1. SceneKit(和 Metal)仅支持 32 位整数作为索引 ( source )。因此，您的索引数组的类型需要是 [Int32]。

2. SceneKit 需要两条多边形信息:多边形中的点数和顶点数组中点的索引。来自 Apple 关于 SCNGeometryPrimitiveTypePolygon 的文档(仅存在于 Objective-C 中):

The element’s data property holds two sequences of values.

• The first sequence has a number of values equal to the geometry element’s primitiveCount value. Each value in this sequence specifies the number of vertices in a polygon primitive. For example, if the first sequence is [5, 3], the geometry element contains a pentagon followed by a triangle.
• The rest of the data is a sequence of vertex indices. Each entry in the first sequence specifies a corresponding number of entries in the second sequence. For example, if the first sequence includes the values [5, 3], the second sequence includes five indices for the pentagon, followed by three indices for the triangle.

您需要将索引数组更改为:

let indices: [Int32] = [7, /* We have a polygon with seven points */,
                        0,1,2,3,4,5,6 /* The seven indices for our polygon */
                       ]

然后，将 primitiveCount 设置为 1(我们要绘制一个多边形)并更改缓冲区的大小:

let indexData = Data(bytes: indices, 
                     count: indices.count * MemoryLayout<Int32>.size)

// Now without runtime error
let element = SCNGeometryElement(data: indexData, 
                                 primitiveType: .polygon,
                                 primitiveCount: 1, 
                                 bytesPerIndex: MemoryLayout<Int32>.size)