swift - 如何弯曲 SCNShape “line” ？

Question

我正在尝试使用 scenekit 创建一个闪电，我正在关注这个 guide .到目前为止，我在场景中使用 UIBezierPath 和挤压使其成为 3d 的垂直线，但我不确定如何按照链接中的描述在中点弯曲“线”。

func createBolt() {
     let path = UIBezierPath()
     path.move(to: CGPoint(x: 0, y: 0))
     path.addLine(to: CGPoint(x: 0, y: 1))
     path.close()


    let shape = SCNShape(path: path, extrusionDepth 0.2)
    let color = UIColor.red
    shape.firstMaterial?.diffuse.contents = color


    let boltNode = SCNNode(geometry: shape)
    boltNode.position.z = 0
    sceneView.scene.rootNode.addChildNode(boltNode)
}

Answer 1

算法非常简单:
您从 A 到 B 的 1 段列表开始，然后在每一代中，您通过在他的范数上随机偏移的中间点移动将每个段分成 2 个段

struct Segment {
    let start: CGPoint
    let end: CGPoint
}

/// Calculate norm of 2d vector
func norm(_ v: CGPoint) -> CGPoint {
    let d = max(sqrt(v.x * v.x + v.y * v.y), 0.0001)
    return CGPoint(x: v.x / d, y: v.y / -d)
}

/// Splitting segment on 2 segments with middle point be shifted by `offset` on norm
func split(_ segment: Segment, by offset: CGFloat) -> [Segment] {
    var midPoint = (segment.start + segment.end) / 2
    midPoint = norm(segment.end - segment.start) * offset + midPoint
    return [
        Segment(start: segment.start, end: midPoint),
        Segment(start: midPoint, end: segment.end)
    ]
}

/// Generate bolt-like line from `start` to `end` with maximal started frequence of `maxOffset`
/// and `generation` of split loops
func generate(from start: CGPoint, to end: CGPoint, withOffset maxOffset: CGFloat, generations: Int = 6) -> UIBezierPath {
    var segments = [Segment(start: start, end: end)]
    var offset = maxOffset
    for _ in 0 ..< generations {
        segments = segments.flatMap { split($0, by: CGFloat.random(in: -offset...offset)) }
        offset /= 2
    }
    let path = UIBezierPath()
    path.move(to: start)
    segments.forEach { path.addLine(to: $0.end) }
    return path
}

// MARK: - Example
let start = CGPoint(x: 10, y: 10)
let end = CGPoint(x: 90, y: 90)
let path = generate(from: start, to: end, withOffset: 30, generations: 5)

// MARK: - Helpers
func + (lhs: CGPoint, rhs: CGPoint) -> CGPoint {
    return CGPoint(x: lhs.x + rhs.x, y: lhs.y + rhs.y)
}
func - (lhs: CGPoint, rhs: CGPoint) -> CGPoint {
    return CGPoint(x: lhs.x - rhs.x, y: lhs.y - rhs.y)
}
func / (lhs: CGPoint, rhs: CGFloat) -> CGPoint {
    return CGPoint(x: lhs.x / rhs, y: lhs.y / rhs)
}
func * (lhs: CGPoint, rhs: CGFloat) -> CGPoint {
    return CGPoint(x: lhs.x * rhs, y: lhs.y * rhs)
}