A rotating disco ball RealityView created from a LowLevelMesh

DiscoBallRealityView.swift

import RealityKit
import SwiftUI

struct DiscoBallRealityView: View {
    @State private var currentEntity: Entity?
    @State private var morphFactor: Float = 0.0
    @State private var frameDuration: TimeInterval = 0.0
    @State private var lastUpdateTime = CACurrentMediaTime()
    
    static let animationFrameDuration: TimeInterval = 1.0 / 120.0
    private let timer = Timer.publish(every: animationFrameDuration, on: .main, in: .common).autoconnect()
    
    var body: some View {
        GeometryReader3D { proxy in
            RealityView { content in
                let size = content.convert(proxy.frame(in: .local), from: .local, to: .scene).extents
                let radius = Float(0.5 * size.x)
                currentEntity = try! createSphereEntity(morphFactor: 0, radius: radius)
                if let entity = currentEntity {
                    content.add(entity)
                }
            } update: { content in
                let size = content.convert(proxy.frame(in: .local), from: .local, to: .scene).extents
                let radius = Float(0.5 * size.x)
                if let modelComponent = try? getModelComponent(morphFactor: morphFactor, radius: radius) {
                    currentEntity?.components.set(modelComponent)
                }
            }
            .onReceive(timer) { _ in
                let currentTime = CACurrentMediaTime()
                frameDuration = currentTime - lastUpdateTime
                lastUpdateTime = currentTime
                let morphAmount = Float(frameDuration)
                morphFactor = morphFactor + morphAmount
            }
        }
    }
    
    func createSphereEntity(morphFactor: Float, radius: Float = 0.5) throws -> Entity {
        let modelComponent = try getModelComponent(morphFactor: morphFactor, radius: radius)
        
        let entity = Entity()
        entity.name = "Sphere"
        entity.components.set(modelComponent)
        entity.scale *= 0.3
        return entity
    }
    
    func getModelComponent(morphFactor: Float, radius: Float = 0.5) throws -> ModelComponent {
        let lowLevelMesh = try sphereMesh(morphFactor: morphFactor, radius: radius)
        let resource = try MeshResource(from: lowLevelMesh)
        
        var material = PhysicallyBasedMaterial()
        material.baseColor.tint = .init(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)
        material.roughness.scale = 0.0
        material.metallic.scale = 1.0
        material.faceCulling = .none

        return ModelComponent(mesh: resource, materials: [material])
    }
    
    func sphereMesh(morphFactor: Float, radius: Float = 0.5) throws -> LowLevelMesh {
        let latitudeBands = 30
        let longitudeBands = 50
        
        let vertexCount = (latitudeBands + 1) * (longitudeBands + 1)
        let indexCount = latitudeBands * longitudeBands * 6
        
        var desc = MyVertex.descriptor
        desc.vertexCapacity = vertexCount
        desc.indexCapacity = indexCount
        
        let mesh = try LowLevelMesh(descriptor: desc)
        
        let rotationMatrix = getRotationMatrix(angle: morphFactor*0.8)
        
        mesh.withUnsafeMutableBytes(bufferIndex: 0) { rawBytes in
            let vertices = rawBytes.bindMemory(to: MyVertex.self)
            var vertexIndex = 0
            
            for latNumber in 0...latitudeBands {
                let theta = Float(latNumber) * Float.pi / Float(latitudeBands)
                let sinTheta = sin(theta)
                let cosTheta = cos(theta)
                
                for longNumber in 0...longitudeBands {
                    let phi = Float(longNumber) * 2 * Float.pi / Float(longitudeBands)
                    let sinPhi = sin(phi)
                    let cosPhi = cos(phi)
                    
                    let x = cosPhi * sinTheta
                    let y = cosTheta
                    let z = sinPhi * sinTheta

                    var position = SIMD3<Float>(x, y, z) * radius
                    position = simd_mul(rotationMatrix, position)
                    
                    let color = 0xFFFFFFFF
                    vertices[vertexIndex] = MyVertex(position: position, color: UInt32(color))
                    vertexIndex += 1
                }
            }
        }
        
        mesh.withUnsafeMutableIndices { rawIndices in
            let indices = rawIndices.bindMemory(to: UInt32.self)
            var index = 0
            
            for latNumber in 0..<latitudeBands {
                for longNumber in 0..<longitudeBands {
                    let first = (latNumber * (longitudeBands + 1)) + longNumber
                    let second = first + longitudeBands + 1
                    
                    indices[index] = UInt32(first)
                    indices[index + 1] = UInt32(second)
                    indices[index + 2] = UInt32(first + 1)
                    indices[index + 3] = UInt32(second)
                    indices[index + 4] = UInt32(second + 1)
                    indices[index + 5] = UInt32(first + 1)
                    
                    index += 6
                }
            }
        }
        
        let meshBounds = BoundingBox(min: [-radius, -radius, -radius], max: [radius, radius, radius])
        mesh.parts.replaceAll([
            LowLevelMesh.Part(
                indexCount: indexCount,
                topology: .triangleStrip,
                bounds: meshBounds
            )
        ])
        
        return mesh
    }
    
    func getRotationMatrix(angle: Float) -> simd_float3x3 {
        let rotationY = simd_float3x3(
            SIMD3<Float>(cos(angle), 0, sin(angle)),
            SIMD3<Float>(0, 1, 0),
            SIMD3<Float>(-sin(angle), 0, cos(angle))
        )
        
        return rotationY
    }
}

struct MyVertex {
    var position: SIMD3<Float> = .zero
    var color: UInt32 = .zero
    
    static var vertexAttributes: [LowLevelMesh.Attribute] = [
        .init(semantic: .position, format: .float3, offset: MemoryLayout<Self>.offset(of: \.position)!),
        .init(semantic: .color, format: .uchar4Normalized_bgra, offset: MemoryLayout<Self>.offset(of: \.color)!)
    ]


    static var vertexLayouts: [LowLevelMesh.Layout] = [
        .init(bufferIndex: 0, bufferStride: MemoryLayout<Self>.stride)
    ]


    static var descriptor: LowLevelMesh.Descriptor {
        var desc = LowLevelMesh.Descriptor()
        desc.vertexAttributes = MyVertex.vertexAttributes
        desc.vertexLayouts = MyVertex.vertexLayouts
        desc.indexType = .uint32
        return desc
    }
}

#Preview {
    DiscoBallRealityView()
}