SceneKit – Rotate and animate a SCNNode

Question

I'm trying to display a pyramid that points following the z axis and then rotates on itself around z too. As my camera is on the z axis, I'm expecting to see the pyramid from above. I managed to rotate the pyramid to see it this way but when I add the animation it seems to rotate on multiple axis.

Here is my code:

// The following create the pyramid and place it how I want
let pyramid = SCNPyramid(width: 1.0, height: 1.0, length: 1.0)
let pyramidNode = SCNNode(geometry: pyramid)
pyramidNode.position = SCNVector3(x: 0, y: 0, z: 0)
pyramidNode.rotation = SCNVector4(x: 1, y: 0, z: 0, w: Float(M_PI / 2))
scene.rootNode.addChildNode(pyramidNode)

// But the animation seems to rotate aroun 2 axis and not just z
var spin = CABasicAnimation(keyPath: "rotation")
spin.byValue = NSValue(SCNVector4: SCNVector4(x: 0, y: 0, z: 1, w: 2*Float(M_PI)))
spin.duration = 3
spin.repeatCount = HUGE
pyramidNode.addAnimation(spin, forKey: "spin around")

Answer 1

Trying to both manually set and animate the same property can cause issues. Using a byValue animation makes the problem worse -- that concatenates to the current transform, so it's harder to keep track of whether the current transform is what the animation expects to start with.

Instead, separate the fixed orientation of the pyramid (its apex is in the -z direction) from the animation (it spins around the axis it points in). There's two good ways to do this:

1. Make pyramidNode the child of another node that gets the one-time rotation (π/2 around x-axis), and apply the spin animation directly to pyramidNode. (In this case, the apex of the pyramid will still point in the +y direction of its local space, so you'll want to spin around that axis instead of the z-axis.)

2. Use the pivot property to transform the local space of pyramidNode's contents, and animate pyramidNode relative to its containing space.

Here's some code to show the second approach:

let pyramid = SCNPyramid(width: 1.0, height: 1.0, length: 1.0)
let pyramidNode = SCNNode(geometry: pyramid)
pyramidNode.position = SCNVector3(x: 0, y: 0, z: 0)
// Point the pyramid in the -z direction
pyramidNode.pivot = SCNMatrix4MakeRotation(CGFloat(M_PI_2), 1, 0, 0)
scene.rootNode.addChildNode(pyramidNode)

let spin = CABasicAnimation(keyPath: "rotation")
// Use from-to to explicitly make a full rotation around z
spin.fromValue = NSValue(SCNVector4: SCNVector4(x: 0, y: 0, z: 1, w: 0))
spin.toValue = NSValue(SCNVector4: SCNVector4(x: 0, y: 0, z: 1, w: CGFloat(2 * M_PI)))
spin.duration = 3
spin.repeatCount = .infinity
pyramidNode.addAnimation(spin, forKey: "spin around")

Some unrelated changes to improve code quality:

• Use CGFloat when explicit conversion is required to initialize an SCNVector component; using Float or Double specifically will break on 32 or 64 bit architecture.
• Use .infinity instead of the legacy BSD math constant HUGE. This type-infers to whatever the type of spin.repeatCount is, and uses a constant value that's defined for all floating-point types.
• Use M_PI_2 for π/2 to be pedantic about precision.
• Use let instead of var for the animation, since we never assign a different value to spin.

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More on the CGFloat error business: In Swift, numeric literals have no type until the expression they're in needs one. That's why you can do things like spin.duration = 3 -- even though duration is a floating-point value, Swift lets you pass an "integer literal". But if you do let d = 3; spin.duration = d you get an error. Why? Because variables/constants have explicit types, and Swift doesn't do implicit type conversion. The 3 is typeless, but when it gets assigned to d, type inference defaults to choosing Int because you haven't specified anything else.

If you're seeing type conversion errors, you probably have code that mixes literals, constants, and/or values returned from functions. You can probably just make the errors go away by converting everything in the expression to CGFloat (or whatever the type you're passing that expression to is). Of course, that'll make your code unreadable and ugly, so once you get it working you might start removing conversions one at a time until you find the one that does the job.