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skills/charleswiltgen/axiom/axiom-scenekit

axiom-scenekit

SKILL.md

SceneKit Development Guide

Purpose: Maintain existing SceneKit code safely and plan migration to RealityKit iOS Version: iOS 8+ (SceneKit), deprecated iOS 26+ Xcode: Xcode 15+

When to Use This Skill

Use this skill when:

  • Maintaining existing SceneKit code
  • Building a SceneKit prototype (with awareness of deprecation)
  • Planning migration from SceneKit to RealityKit
  • Debugging SceneKit rendering, physics, or animation issues
  • Integrating SceneKit content with SwiftUI
  • Loading 3D models via Model I/O or SCNSceneSource

Do NOT use this skill for:

  • New 3D projects (use axiom-realitykit)
  • AR experiences (use axiom-realitykit)
  • visionOS development (use axiom-realitykit)
  • SpriteKit 2D games (axiom-spritekit)
  • Metal shader programming (axiom-metal-migration-ref)

Deprecation Context

SceneKit is soft-deprecated as of iOS 26 (WWDC 2025). This means:

  • Existing apps continue to work
  • No new features or general bug fixes
  • Only critical security patches
  • SceneView (SwiftUI) is formally deprecated in iOS 26

Apple's forward path is RealityKit. All new 3D projects should use RealityKit. SceneKit knowledge remains valuable for maintaining legacy code and understanding concepts during migration.

In RealityKit: ECS architecture replaces scene graph. See axiom-scenekit-ref for the complete concept mapping table.

1. Mental Model

Scene Graph Architecture

SceneKit uses a tree of nodes (SCNNode) attached to a root node in an SCNScene. Each node has a transform (position, rotation, scale) relative to its parent.

SCNScene
└── rootNode
    ├── cameraNode (SCNCamera)
    ├── lightNode (SCNLight)
    ├── playerNode (SCNGeometry + SCNPhysicsBody)
    │   ├── weaponNode
    │   └── particleNode (SCNParticleSystem)
    └── environmentNode
        ├── groundNode
        └── wallNodes

In RealityKit: Entities replace nodes. Components replace node properties. The hierarchy concept persists, but behavior is driven by Systems rather than node callbacks.

Coordinate System

SceneKit uses a right-handed Y-up coordinate system:

     +Y (up)
      |
      |
      +──── +X (right)
     /
    /
  +Z (toward viewer)

This matches RealityKit's coordinate system, so spatial concepts transfer directly during migration.

Transform Hierarchy

Transforms cascade parent → child. A child's world transform = parent's world transform × child's local transform.

let parent = SCNNode()
parent.position = SCNVector3(10, 0, 0)

let child = SCNNode()
child.position = SCNVector3(0, 5, 0)
parent.addChildNode(child)

// child.worldPosition = (10, 5, 0)
// child.position (local) = (0, 5, 0)

In RealityKit: Same concept. entity.position is local, entity.position(relativeTo: nil) gives world position.

2. Scene Setup and Rendering

SCNView (UIKit)

let sceneView = SCNView(frame: view.bounds)
sceneView.scene = SCNScene(named: "scene.scn")
sceneView.allowsCameraControl = true
sceneView.showsStatistics = true
sceneView.backgroundColor = .black
view.addSubview(sceneView)

SceneView (SwiftUI) — Deprecated iOS 26

// Still works but deprecated. Use SCNViewRepresentable for new code.
import SceneKit

SceneView(
    scene: scene,
    pointOfView: cameraNode,
    options: [.allowsCameraControl, .autoenablesDefaultLighting]
)

SCNViewRepresentable (SwiftUI replacement)

struct SceneKitView: UIViewRepresentable {
    let scene: SCNScene

    func makeUIView(context: Context) -> SCNView {
        let view = SCNView()
        view.scene = scene
        view.allowsCameraControl = true
        view.autoenablesDefaultLighting = true
        return view
    }

    func updateUIView(_ view: SCNView, context: Context) {}
}

In RealityKit: Use RealityView in SwiftUI — no UIViewRepresentable needed.

3. Geometry and Materials

Built-in Geometries

let box = SCNBox(width: 1, height: 1, length: 1, chamferRadius: 0.1)
let sphere = SCNSphere(radius: 0.5)
let cylinder = SCNCylinder(radius: 0.3, height: 1)
let plane = SCNPlane(width: 2, height: 2)
let torus = SCNTorus(ringRadius: 1, pipeRadius: 0.3)
let capsule = SCNCapsule(capRadius: 0.3, height: 1)
let cone = SCNCone(topRadius: 0, bottomRadius: 0.5, height: 1)
let tube = SCNTube(innerRadius: 0.3, outerRadius: 0.5, height: 1)
let text = SCNText(string: "Hello", extrusionDepth: 0.2)

PBR Materials

let material = SCNMaterial()
material.lightingModel = .physicallyBased
material.diffuse.contents = UIColor.red          // or UIImage
material.metalness.contents = 0.8
material.roughness.contents = 0.2
material.normal.contents = UIImage(named: "normal_map")
material.ambientOcclusion.contents = UIImage(named: "ao_map")

let node = SCNNode(geometry: sphere)
node.geometry?.firstMaterial = material

In RealityKit: Use PhysicallyBasedMaterial with similar properties but different API surface. See axiom-scenekit-ref Part 1 for the mapping.

Shader Modifiers

SceneKit supports GLSL/Metal shader snippets injected at specific entry points:

// Fragment modifier — custom effect on surface
material.shaderModifiers = [
    .fragment: """
    float stripe = sin(_surface.position.x * 20.0);
    _output.color.rgb *= step(0.0, stripe);
    """
]

Entry points: .geometry, .surface, .lightingModel, .fragment

In RealityKit: Use ShaderGraphMaterial with Reality Composer Pro, or CustomMaterial with Metal functions.

4. Lighting

Light Types

Type Description Shadows
.omni Point light, radiates in all directions No
.directional Parallel rays (sun) Yes
.spot Cone-shaped beam Yes
.area Rectangle emitter (soft shadows) Yes
.IES Real-world light profile Yes
.ambient Uniform, no direction No
.probe Environment lighting from cubemap No 
let light = SCNLight()
light.type = .directional
light.intensity = 1000
light.castsShadow = true
light.shadowRadius = 3
light.shadowSampleCount = 8

let lightNode = SCNNode()
lightNode.light = light
lightNode.eulerAngles = SCNVector3(-Float.pi / 4, 0, 0)
scene.rootNode.addChildNode(lightNode)

In RealityKit: Use DirectionalLightComponent, PointLightComponent, SpotLightComponent as components on entities. Image-based lighting via EnvironmentResource.

5. Animation

SCNAction (Declarative)

let moveUp = SCNAction.moveBy(x: 0, y: 2, z: 0, duration: 1)
let fadeOut = SCNAction.fadeOut(duration: 0.5)
let sequence = SCNAction.sequence([moveUp, fadeOut])
let forever = SCNAction.repeatForever(moveUp.reversed())
node.runAction(sequence)

Implicit Animation (SCNTransaction)

SCNTransaction.begin()
SCNTransaction.animationDuration = 0.5
node.position = SCNVector3(0, 5, 0)
node.opacity = 0.5
SCNTransaction.commit()

Explicit Animation (CAAnimation bridge)

let animation = CABasicAnimation(keyPath: "rotation")
animation.toValue = NSValue(scnVector4: SCNVector4(0, 1, 0, Float.pi * 2))
animation.duration = 2
animation.repeatCount = .infinity
node.addAnimation(animation, forKey: "spin")

Loading Animations from Files

let scene = SCNScene(named: "character.dae")!
let animationPlayer = scene.rootNode
    .childNode(withName: "mixamorig:Hips", recursively: true)!
    .animationPlayer(forKey: nil)!

characterNode.addAnimationPlayer(animationPlayer, forKey: "walk")
animationPlayer.play()

In RealityKit: Use entity.playAnimation() with animations loaded from USD files. Transform animations via entity.move(to:relativeTo:duration:).

6. Physics

Physics Bodies

// Dynamic — simulation controls position
node.physicsBody = SCNPhysicsBody(type: .dynamic,
    shape: SCNPhysicsShape(geometry: node.geometry!, options: nil))

// Static — immovable collision surface
ground.physicsBody = SCNPhysicsBody(type: .static, shape: nil)

// Kinematic — code controls position, participates in collisions
platform.physicsBody = SCNPhysicsBody(type: .kinematic, shape: nil)

Collision Categories

struct PhysicsCategory {
    static let player:    Int = 1 << 0   // 1
    static let enemy:     Int = 1 << 1   // 2
    static let projectile: Int = 1 << 2  // 4
    static let wall:      Int = 1 << 3   // 8
}

playerNode.physicsBody?.categoryBitMask = PhysicsCategory.player
playerNode.physicsBody?.collisionBitMask = PhysicsCategory.wall | PhysicsCategory.enemy
playerNode.physicsBody?.contactTestBitMask = PhysicsCategory.enemy | PhysicsCategory.projectile

Contact Delegate

class GameScene: SCNScene, SCNPhysicsContactDelegate {
    func setupPhysics() {
        physicsWorld.contactDelegate = self
    }

    func physicsWorld(_ world: SCNPhysicsWorld,
                      didBegin contact: SCNPhysicsContact) {
        let nodeA = contact.nodeA
        let nodeB = contact.nodeB
        // Handle collision
    }
}

In RealityKit: Use PhysicsBodyComponent, CollisionComponent, and collision event subscriptions via scene.subscribe(to: CollisionEvents.Began.self).

7. Hit Testing and Interaction

// In SCNView tap handler
let results = sceneView.hitTest(tapLocation, options: [
    .searchMode: SCNHitTestSearchMode.closest.rawValue,
    .boundingBoxOnly: false
])

if let hit = results.first {
    let tappedNode = hit.node
    let worldPosition = hit.worldCoordinates
}

In RealityKit: Use ManipulationComponent for drag/rotate/scale gestures, or collision-based hit testing.

8. Asset Pipeline

Supported Formats

Format Extension Notes
USD/USDZ .usdz, .usda, .usdc Preferred format, works in both SceneKit and RealityKit
Collada .dae Legacy, still supported
SceneKit Archive .scn Xcode-specific, not portable to RealityKit
Wavefront OBJ .obj Geometry only, no animations
Alembic .abc Animation baking

Loading Models

// From bundle
let scene = SCNScene(named: "model.usdz")!

// From URL
let scene = try SCNScene(url: modelURL, options: nil)

// Via Model I/O (for format conversion)
let asset = MDLAsset(url: modelURL)
let scene = SCNScene(mdlAsset: asset)

Migration tip: Convert .scn files to .usdz using xcrun scntool --convert file.scn --format usdz before migrating to RealityKit.

9. ARKit Integration (Legacy)

// ARSCNView — SceneKit + ARKit (legacy approach)
let arView = ARSCNView(frame: view.bounds)
arView.delegate = self
arView.session.run(ARWorldTrackingConfiguration())

// Adding virtual content at anchors
func renderer(_ renderer: SCNSceneRenderer,
              didAdd node: SCNNode, for anchor: ARAnchor) {
    let box = SCNBox(width: 0.1, height: 0.1, length: 0.1, chamferRadius: 0)
    node.addChildNode(SCNNode(geometry: box))
}

In RealityKit: Use RealityView with AnchorEntity types. ARSCNView is legacy — all new AR development should use RealityKit.

10. Anti-Patterns

Anti-Pattern 1: Starting New Projects in SceneKit

Time cost: Weeks of rework when you eventually must migrate

SceneKit is deprecated. New projects should use RealityKit from the start, even if the learning curve is steeper initially.

Anti-Pattern 2: Using .scn Files Without USDZ Conversion

Time cost: Hours when migration begins

.scn files are SceneKit-specific and cannot be loaded in RealityKit. Convert early:

xcrun scntool --convert model.scn --format usdz --output model.usdz

Anti-Pattern 3: Deep Shader Modifier Customization

Time cost: Complete rewrite during migration

SceneKit shader modifiers use a proprietary entry-point system. Heavy investment here has zero portability to RealityKit's ShaderGraphMaterial.

Anti-Pattern 4: Relying on SCNRenderer for Custom Pipelines

Time cost: Architecture redesign during migration

If you need custom render pipelines, build on Metal directly or use RealityRenderer (RealityKit's Metal-level API).

Anti-Pattern 5: Ignoring Deprecation Warnings

Time cost: Surprise breakage when Apple removes APIs

Track SceneView deprecation warnings and plan UIViewRepresentable fallback or RealityKit migration.

Anti-Pattern 6: Creating Hundreds of Nodes in a Loop

Time cost: 2-4 hours debugging frame drops, often misdiagnosed as GPU issue

// ❌ WRONG: Each SCNNode has overhead (transform, bounding box, hit test)
for i in 0..<500 {
    let node = SCNNode(geometry: SCNSphere(radius: 0.05))
    node.position = randomPosition()
    scene.rootNode.addChildNode(node)  // 500 nodes = terrible frame rate
}

// ✅ RIGHT: Use SCNParticleSystem for particle-like effects
let particles = SCNParticleSystem()
particles.birthRate = 500
particles.particleSize = 0.05
particles.emitterShape = SCNBox(width: 5, height: 5, length: 5, chamferRadius: 0)
particleNode.addParticleSystem(particles)

// ✅ RIGHT: Use geometry instancing for identical objects
let source = SCNGeometrySource(/* instance transforms */)
geometry.levelsOfDetail = [SCNLevelOfDetail(geometry: lowPoly, screenSpaceRadius: 20)]

Rule: If >50 identical objects, use SCNParticleSystem or flatten geometry. If different objects, use SCNNode.flattenedClone() to reduce draw calls.

11. Migration Decision Tree

Should you migrate to RealityKit?
│
├─ Is this a new project?
│   └─ YES → Use RealityKit from the start. No question.
│
├─ Does the app need AR features?
│   └─ YES → Migrate. ARSCNView is legacy, RealityKit is the only forward path.
│
├─ Does the app target visionOS?
│   └─ YES → Must migrate. SceneKit doesn't support visionOS spatial features.
│
├─ Is the codebase heavily invested in SceneKit?
│   ├─ YES, and app is stable → Maintain in SceneKit for now, plan phased migration.
│   └─ YES, but needs new features → Migrate incrementally (new features in RealityKit).
│
├─ Is performance a concern?
│   └─ YES → RealityKit is optimized for Apple Silicon with Metal-first rendering.
│
└─ Is the app in maintenance mode?
    └─ YES → Keep SceneKit until critical. Security patches will continue.

12. Pressure Scenarios

Scenario 1: "Just Use SceneKit, It Works Fine"

Pressure: Team familiarity with SceneKit, deadline to ship

Wrong approach: Start new project in SceneKit because the team knows it.

Correct approach: Invest in RealityKit learning. SceneKit will receive no new features. The longer you wait, the larger the migration debt.

Push-back template: "SceneKit is deprecated as of iOS 26. Starting new work in it creates migration debt that grows with every feature we add. RealityKit's ECS model is different but learnable — let's invest the time now."

Scenario 2: "We Don't Have Time to Learn RealityKit"

Pressure: Tight deadline, team unfamiliar with ECS

Wrong approach: Build everything in SceneKit to meet the deadline.

Correct approach: Build the prototype in SceneKit if necessary, but document every SceneKit dependency and plan the migration. Use USDZ assets from the start so they're portable.

Push-back template: "Let's use USDZ assets and keep the SceneKit layer thin. When we migrate, the assets transfer directly and only the code layer changes."

Scenario 3: "Port Everything At Once"

Pressure: Desire for a clean migration

Wrong approach: Attempt to rewrite the entire SceneKit codebase in RealityKit at once.

Correct approach: Migrate incrementally. New features in RealityKit. Existing SceneKit code stays until it needs changes. Modularize with Swift packages (per Apple's migration guide).

Push-back template: "Apple's own migration guide recommends modularizing into Swift packages and migrating system by system. A big-bang rewrite risks introducing new bugs across the entire app."

Code Review Checklist

  • No new SceneKit code in projects targeting iOS 26+ without migration plan
  • Assets in USDZ format (not .scn) for portability
  • No deep shader modifier customization without RealityKit equivalent identified
  • SCNTransaction used for implicit animations (not direct property changes without animation context)
  • Physics categoryBitMask explicitly set (not relying on defaults)
  • Contact delegate set and protocol conformance added
  • [weak self] in completion handlers and closures
  • Debug overlays enabled during development (showsStatistics = true)

Resources

WWDC: 2014-609, 2014-610, 2017-604, 2019-612

Docs: /scenekit, /scenekit/scnscene, /scenekit/scnnode, /scenekit/scnmaterial, /scenekit/scnphysicsbody

Skills: axiom-scenekit-ref, axiom-realitykit, axiom-realitykit-ref

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