SceneKit: orient one node toward another in 3Dspace - rotation

Question

I need to orient one node to point its Z-axis at another node in 3D. Yeah, the perfect job for the LookAtConstraint. And for most of my work LookAt is fine. But when I apply LookAt to a particular node, I can no longer animate that node's translation with SCNAction. Picture a hydrogen atom leaving a molecule as it ionizes. The orientation is needed to properly rotate the bond (a cylinder) bewteen the hydrogen and an oxygen atom on the molecule.
I can orient the bond FROM the oxygen TO the hydrogen and animate. But this disorients most of the other bonds which were getting by just fine with LookAt's.
I gave this a mighty try before realizing it answers a somewhat different question:
Calculate rotations to look at a 3D point?

Answer 1

I had a similar issue with a project. What I eventually realized was that I need to use multiple constraints. One for translation (movement) and the other using the look at constraint.
I would move the object and then apply the look at constraint; in this case, it was a camera following an objects being moved using actions. Code snippet follows:
let targetNodeConstraint = SCNLookAtConstraint(target: someObject)
targetNodeConstraint.gimbalLockEnabled = true
let followObjectConstraint = SCNTransformConstraint(inWorldSpace: true, withBlock: { (node, matrix) -> SCNMatrix4 in
let transformMatrix = SCNMatrix4MakeTranslation(
self.someObject.position.x - 1.0,
self.someObject.position.y, self.someObject.position.z + 1.0)
return transformMatrix
})
// Position the object behind the other object & rotate it to
roadCamera.constraints = [followObjectConstraint, targetNodeConstraint]
The important thing to note is the order in which the constraints are added to the object using an array. In the code above, I am ignoring the current matrix before I apply a transform matrix (I should re-write this code someday)
The complete source code of this "experiment" is on GitHub as I try things out.
https://github.com/ManjitBedi/CubeTrip
Hopefully, this is helpful.

Answer 2

My solution here. Deal with situation that node continuously translate in space and should always toward a position.
#discardableResult
func yew(_ node:SCNNode, toPosition position:SCNVector3) -> Float
{
var eularAngle = SCNVector3Zero
let tranform = node.transform
var forward = GLKVector3Make(tranform.m31, tranform.m32, tranform.m33)
var toWard = GLKVector3Make(position.x - node.position.x, position.y - node.position.y, position.z - node.position.z)
forward = GLKVector3Normalize(GLKVector3Make(forward.x, 0, forward.z))
toWard = GLKVector3Normalize(GLKVector3Make(toWard.x, 0, toWard.z))
var dotProduct = GLKVector3DotProduct(forward,toWard)
dotProduct = (dotProduct > 1) ? 1 : ((dotProduct < -1) ? -1 : dotProduct)
var yew = acos(dotProduct)
if yew < 0 {
assert(false)
}
//toward is clockwise of forward
let isCW = GLKVector3CrossProduct(forward, toWard).y < 0
if isCW {
yew = -yew
}
eularAngle.y = yew
node.eulerAngles = SCNVector3Make(eularAngle.x + wrapperNode.eulerAngles.x,
eularAngle.y + wrapperNode.eulerAngles.y,
eularAngle.z + wrapperNode.eulerAngles.z)
return yew
}
#discardableResult
func pitch(_ node:SCNNode, toPosition position:SCNVector3) -> Float{
var eularAngle = SCNVector3Zero
let tranform = node.transform
var toWard = GLKVector3Make(position.x - node.position.x, position.y - node.position.y, position.z - node.position.z)
var forward = GLKVector3Make(tranform.m31, tranform.m32, tranform.m33)
forward = GLKVector3Normalize(forward)
toWard = GLKVector3Normalize(toWard)
var dotProduct = GLKVector3DotProduct(forward,toWard)
dotProduct = (dotProduct > 1) ? 1 : ((dotProduct < -1) ? -1 : dotProduct)
var pitch = acos(dotProduct)
//toward is clockwise of forward, if right vector of model and crossProfuct.x has same direction
let crossProduct = GLKVector3CrossProduct(forward, toWard)
let isCW = (crossProduct.x <= 0) != (tranform.m11 <= 0)
if isCW {
pitch = -pitch
}
eularAngle.x = pitch
node.eulerAngles = SCNVector3Make(eularAngle.x + node.eulerAngles.x,
eularAngle.y + node.eulerAngles.y,
eularAngle.z + node.eulerAngles.z)
return pitch
}
func orient(_ node:SCNNode, toPosition position:SCNVector3) {
self.yew(node, toPosition: position)
self.pitch(node, toPosition: position)
}