Rotating around anchor point in SceneKit - ios

Question

Objective: There is a SCNCylinder object in my scene. I want to be able to drag one end of the cylinder and rotate the cylinder in any direction, while keeping the other end in the same position.
Currently, I am calling localRotate(by: SCNQuaternion) to first rotate the node, then calculate the position offset needed to move the cylinder so that the other end can go back to its original position.
How can I achieve the objective in one step instead of what I am doing now?

Answer 1

The pivot property is what you're looking for. Or, since modern SceneKit often works better / makes nicer Swift / interoperates easier with ARKit when you use SIMD types, the simdPivot property.
Note this bit in the docs:
Changing the pivot transform alters these behaviors in many useful ways. You can:
Offset the node’s contents relative to its position. For example, by setting the pivot to a translation transform you can position a node containing a sphere geometry relative to where the sphere would rest on a floor instead of relative to its center.
Move the node’s axis of rotation. For example, with a translation transform you can cause a node to revolve around a faraway point instead of rotating around its center, and with a rotation transform you can tilt the axis of rotation.
Similarly, for a cylinder, you can make its pivot a transform matrix that translates the origin by half its height, giving it an "anchor point" (for position and rotation changes) at one end instead of in the center. Something like this (untested):
let cylinder = SCNCylinder(radius: /*...*/, height: /*...*/)
let cylinderNode = SCNNode(geometry: cylinder)
cylinderNode.simdPivot = float4x4(translation: cylinder.height / 2)
extension float4x4 {
init(translation vector: float3) {
self.init(float4(1, 0, 0, 0),
float4(0, 1, 0, 0),
float4(0, 0, 1, 0),
float4(vector.x, vector.y, vector.z, 1))
}
}
More generally, whenever you're using a scene-graph / transform-hierarchy based graphics framework, any time you find yourself doing math depending on one transform (rotation, translation, etc) to affect another, it's always good to check for API that can do that math for you — because doing that kind of math is what transform hierarchy is all about.
And if there's not an API fairly specific to what you need, remember that the hierarchy itself is good for making dependent transforms. For example, if you want one node to follow a circular orbit around another, you don't need to set its position using sines and cosines... just make it the child of another node, and rotate that other node.
In this case, pivot is a convenience equivalent to using the node hierarchy. You could just as well create an intermediate node and move the cylinder within it (something like this):
let cylinder = SCNCylinder(radius: /*...*/, height: /*...*/)
let cylinderNode = SCNNode(geometry: cylinder)
let offsetNode = SCNNode()
offsetNode.addChildNode(cylinderNode)
cylinderNode.simdPosition.y = cylinder.height / 2
offsetNode.position = /*...*/ // set world-space position of end of cylinder
offsetNode.eulerAngles.x = /*...*/ // rotate cylinder around its end