Cannot Display Sprites Above SK3DNode - ios

Question

I'm displaying some basic 3D geometry within my SpriteKit scene using an instance of SK3DNode to display the contents of a SceneKit scene, as explained in Apple's article here.
I have been able to position the node and 3D contents as I want using SceneKit node transforms and the position/viewport size of the SK3DNode.
Next, I want to display some other sprites in my SpriteKit scene overlaid on top of the 3D content, but I am unable to do so: The contents of the SK3DNode are always drawn on top.
I have tried specifying the zPosition property of both the SK3DNode and the SKSpriteNode, to no avail.
From Apple's documentation on SK3DNode:
Use SK3DNode objects to incorporate 3D SceneKit content into a
SpriteKit-based game. When SpriteKit renders the node, the SceneKit
scene is animated and rendered first. Then this rendered image is
composited into the SpriteKit scene. Use the scnScene property to
specify the SceneKit scene to be rendered.
(emphasis mine)
It is a bit ambiguous withv regard to z-order (it only seems to mention the temporal order in which rendering takes place).
I have put together a minimal demo project on GitHub; the relevant code is:
1. SceneKit Scene
import SceneKit
class SceneKitScene: SCNScene {
override init() {
super.init()
let box = SCNBox(width: 10, height: 10, length: 10, chamferRadius: 0)
let material = SCNMaterial()
material.diffuse.contents = UIColor.green
box.materials = [material]
let boxNode = SCNNode(geometry: box)
boxNode.transform = SCNMatrix4MakeRotation(.pi/2, 1, 1, 1)
self.rootNode.addChildNode(boxNode)
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
2. SpriteKit Scene
import SpriteKit
class SpriteKitScene: SKScene {
override init(size: CGSize) {
super.init(size: size)
// Scene Background
self.backgroundColor = .red
// 3D Node
let objectNode = SK3DNode(viewportSize: size)
objectNode.scnScene = SceneKitScene()
addChild(objectNode)
objectNode.position = CGPoint(x: size.width/2, y: size.height/2)
let camera = SCNCamera()
let cameraNode = SCNNode()
cameraNode.camera = camera
objectNode.pointOfView = cameraNode
objectNode.pointOfView?.position = SCNVector3(x: 0, y: 0, z: 60)
objectNode.zPosition = -100
// 2D Sprite
let sprite = SKSpriteNode(color: .yellow, size: CGSize(width: 250, height: 60))
sprite.position = objectNode.position
sprite.zPosition = +100
addChild(sprite)
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
...And the rendered result is:
(I want the yellow rectangle above the green box)

Answer 1

I made a Technical Support Incident with Apple about this and they just got back to me. The solution is actually very very simple.
If you want 2D sprites to render on top of SK3DNodes, you need to stop the contents of the SK3DNodes from writing to the depth buffer.
To do this, you just need to set writesToDepthBuffer to false on the SCNMaterial.
...
let material = SCNMaterial()
material.diffuse.contents = UIColor.green
material.writesToDepthBuffer = false
...
Boom. Works.

Answer 2

Please note that this is just something I stumbled upon. I have no idea why it works and I probably wouldn't trust it without further understanding, but maybe it'll help find an explanation or a real solution.
It seems that having an SKShapeNode (with a fill) alongside an SK3DNode (either as a sibling, part of a sibling tree, or child), draws it in proper order. The SKShapeNode doesn't seem to need to intersect with the SK3DNode either.
The fill is important, as having a transparent fill makes this not work. Stroke doesn't seem to have any effect.
An SKShapeNode of extremely small size and almost zero alpha fill works too.
Here's my playground:
import PlaygroundSupport
import SceneKit
import SpriteKit
let viewSize = CGSize(width: 300, height: 150)
let viewportSize: CGFloat = viewSize.height * 0.75
func skview(color: UIColor, index: Int) -> SKView {
let scene = SKScene(size: viewSize)
scene.backgroundColor = color
let view = SKView(
frame: CGRect(
origin: CGPoint(x: 0, y: CGFloat(index) * viewSize.height),
size: viewSize
)
)
view.presentScene(scene)
view.showsDrawCount = true
// Draw the box of the 3d node view port
let viewport = SKSpriteNode(color: .orange, size: CGSize(width: viewportSize, height: viewportSize))
viewport.position = CGPoint(x: viewSize.width / 2, y: viewSize.height / 2)
scene.addChild(viewport)
return view
}
func cube() -> SK3DNode {
let mat = SCNMaterial()
mat.diffuse.contents = UIColor.green
let box = SCNBox(width: viewportSize, height: viewportSize, length: viewportSize, chamferRadius: 0)
box.firstMaterial = mat
let boxNode3d = SCNNode(geometry: box)
boxNode3d.runAction(.repeatForever(.rotateBy(x: 10, y: 10, z: 10, duration: 10)))
let scene = SCNScene()
scene.rootNode.addChildNode(boxNode3d)
let boxNode2d = SK3DNode(viewportSize: CGSize(width: viewportSize, height: viewportSize))
boxNode2d.position = CGPoint(x: viewSize.width / 2, y: viewSize.height / 2)
boxNode2d.scnScene = scene
return boxNode2d
}
func shape() -> SKShapeNode {
let shape = SKShapeNode(rectOf: CGSize(width: viewSize.height / 4, height: viewSize.height / 4))
shape.strokeColor = .clear
shape.fillColor = .purple
return shape
}
func rect(_ color: UIColor) -> SKSpriteNode {
let sp = SKSpriteNode(texture: nil, color: color, size: CGSize(width: 200, height: viewSize.height / 4))
sp.position = CGPoint(x: viewSize.width / 2, y: viewSize.height / 2)
return sp
}
// The original issue, untouched.
func v1() -> SKView {
let v = skview(color: .red, index: 0)
v.scene?.addChild(cube())
v.scene?.addChild(rect(.yellow))
return v
}
// Shape added as sibling after the 3d node. Notice that it doesn't overlap the SK3DNode.
func v2() -> SKView {
let v = skview(color: .blue, index: 1)
v.scene?.addChild(cube())
v.scene?.addChild(shape())
v.scene?.addChild(rect(.yellow))
return v
}
// Shape added to the 3d node.
func v3() -> SKView {
let v = skview(color: .magenta, index: 2)
let box = cube()
box.addChild(shape())
v.scene?.addChild(box)
v.scene?.addChild(rect(.yellow))
return v
}
// 3d node added after, but zPos set to -1.
func v4() -> SKView {
let v = skview(color: .cyan, index: 3)
v.scene?.addChild(shape())
v.scene?.addChild(rect(.yellow))
let box = cube()
box.zPosition = -1
v.scene?.addChild(box)
return v
}
// Shape added after the 3d node, but not as a sibling.
func v5() -> SKView {
let v = skview(color: .green, index: 4)
let parent = SKNode()
parent.addChild(cube())
parent.addChild(rect(.yellow))
v.scene?.addChild(parent)
v.scene?.addChild(shape())
return v
}
let container = UIView(frame: CGRect(origin: .zero, size: CGSize(width: viewSize.width, height: viewSize.height * 5)))
container.addSubview(v1())
container.addSubview(v2())
container.addSubview(v3())
container.addSubview(v4())
container.addSubview(v5())
PlaygroundPage.current.liveView = container
TL;DR
In your code, try:
...
let shape = SKShapeNode(rectOf: CGSize(width: 0.01, height: 0.01))
shape.strokeColor = .clear
shape.fillColor = UIColor.black.withAlphaComponent(0.01)
// 3D Node
let objectNode = SK3DNode(viewportSize: size)
objectNode.addChild(shape)
...