SceneKit get ancestor-relative (and scene-relative) position and rotation

_LICENSE

MIT License

Copyright (c) 2023 Logan Moore

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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SOFTWARE.

_README.md

README

Note that this uses another extension on CGFloat (also included here) to wrap values greater than CGFloat.pi.

Usage

Get the current node's position and rotation within the scene

  let (scenePosition, sceneRotation) = self.getPositionAndRotation(within: self.scene!)

Compare the relative rotation of two children within the coordinate space of a common ancestor.

Consider the following node tree somewhere within the scene:

commonAncestor
|> grandParent1
   |> parent1
      |> child1
|> parent2
   |> child2

let child1Rotation = child1.getRotation(within: commonAncestor)
let child2Rotation = child2.getRotation(within: commonAncestor)
let rotationDifference = child2Rotation - child1Rotation

SKNodeGetPosition.swift

import Foundation
import SpriteKit

extension SKNode {
  func getPosition(within ancestor: SKNode) -> CGPoint {
    let (position, _) = getPositionAndRotation(within: ancestor)

    return position
  }

  private func getPositionAndRotation(within ancestor: SKNode) -> (CGPoint, CGFloat) {
    var node = self
    var childPosition = CGPoint.zero
    var childRotation = CGFloat.zero

    while let nodeParent = node.parent, node != ancestor {
      let cosZ = cos(node.zRotation)
      let sinZ = sin(node.zRotation)

      childPosition = CGPoint(
        x: node.position.x
           + cosZ * childPosition.x
           - sinZ * childPosition.y,
        y: node.position.y
           + sinZ * childPosition.x
           + cosZ * childPosition.y)
      childRotation = (childRotation + node.zRotation).wrap(around: CGFloat.pi)

      node = nodeParent
    }

    return (childPosition, childRotation)
  }

  func getRotation(within ancestor: SKNode) -> CGFloat {
    let (_, rotation) = getPositionAndRotation(within: ancestor)

    return rotation;
  }
}

extension CGFloat {
  /// Reduces a value into its smallest representation in the range `[-boundary,boundary]`, modulo `boundary * 2`. This is mainly useful for pulling things back into their closest-to-zero representation within the unit circle with `boundary: CGFloat.pi`.
  func wrap(around boundary: CGFloat) -> CGFloat {
    let doubleBoundary = boundary * 2
    let remainder = truncatingRemainder(dividingBy: doubleBoundary)
    
    let remainderAbs = abs(remainder)
    
    // Aside: it feels like there should be a way to write this without a
    // check here but that requires too much thinking.
    if (remainderAbs > boundary) {
      // We got back to a value in the range [-boundary*2, boundary*2], which
      //  needs to be pulled back into the   [-boundary,   boundary  ] range.
      return remainder < 0
        ?  doubleBoundary + remainder
        : -doubleBoundary + remainder
    }

    return remainder
  }
}