swift - 如何在 spritekit swift 中制作一个旋转角度的正弦？

Question

我希望在我的游戏中屏幕上有某种声波。我可以制作直线的正弦，但是如何制作旋转 20 度的正弦呢？例如，从左下角到右上角？

        let pi = CGFloat.pi
        let wave = Waves[0]
        let x = path.currentPoint.x
        let c = (2 * pi) / (wave.Wavelength * 30)
        let a = 30 * wave.Amplitude

        let y = a * sin(c * x)

        path.addLine(to: CGPoint(x: x + 2, y: y ))

Answer 1

您可以使用 2 Bézier curves 制作一个非常准确地近似正弦波的 UIBezierPath :

/// Make a path that approximates a peak-to-peak sine wave
func sinePath(size: CGSize) -> UIBezierPath {
  let path = UIBezierPath()

  // path starts at 0,0
  path.move(to: CGPoint.zero)

  let halfWidth = size.width / 2.0

  // base x-values for control points
  // baseX approximates the best fit Bézier curve for a sine wave
  let baseX:  = CGFloat(0.3642124232)
  let firstX  = halfWidth * baseX
  let secondX = halfWidth * (1.0 - baseX)

  // curve from start peak to trough
  path.addCurve(to: CGPoint(x: halfWidth, y: size.height),
                controlPoint1: CGPoint(x: firstX , y: 0),
                controlPoint2: CGPoint(x: secondX, y: size.height))

  // curve from trough to end peak
  path.addCurve(to: CGPoint(x: size.width, y:0),
                controlPoint1: CGPoint(x: size.width - secondX, y: size.height),
                controlPoint2: CGPoint(x: size.width - firstX , y: 0))
  return path
}

然后您可以使用 CGAffineTransform 旋转它，如下所示:

/// Convert degrees to radians
func degreeToRadian(_ angle: CGFloat) -> CGFloat {
  return angle * CGFloat.pi / 180.0
}

let path = sinePath(size: CGSize(width: 200.0, height: 100.0))
path.apply(CGAffineTransform(rotationAngle: degreeToRadian(20)))

我从这个问题中得到了baseX常量:

How to approximate a half-cosine curve with bezier paths in SVG?