A RealityKit ModelEntity extension that replaces its model with a new model with reversed triangle indices

MeshEntity+GenerateSolidInteriorModel.swift

//
//  MeshEntity+GenerateSolidInteriorModel.swift
//
//  Created by Drew Olbrich on 12/16/23.
//

import RealityKit

extension ModelEntity {
    
    /// Replaces the model entity's `model` property with a new model suitable for
    /// drawing a solid interior on visionOS.
    ///
    /// In a better world, we'd simply enable front face culling for the solid interior
    /// geometry, but on visionOS, RealityKit doesn't provide a `faceCulling` option for
    /// `ShaderGraphMaterial`, like it does for `CustomMaterial` on iOS.
    ///
    /// To work around that limitation, this method reverses the order of the triangle
    /// indices in the model entity's meshes. RealityKit's back face culling will then
    /// effectively perform front face culling.
    func generateSolidInteriorModel() async {
        guard let model else {
            assertionFailure("model is undefined")
            return
        }
        
        var updatedContents = MeshResource.Contents()
        
        let contents = model.mesh.contents
        
        updatedContents.instances = contents.instances
        updatedContents.skeletons = contents.skeletons
        
        var updatedModels = MeshModelCollection()
        
        for model in contents.models {
            var updatedParts: [MeshResource.Part] = []
            
            for part in model.parts {
                guard let triangleIndices = part.triangleIndices else {
                    assertionFailure("triangleIndices is undefined")
                    updatedParts.append(part)
                    return
                }
                
                let reversedTriangleIndices = MeshBuffers.TriangleIndices(triangleIndices.elements.reversed())
                
                var updatedPart = MeshResource.Part(id: part.id, materialIndex: part.materialIndex)
                
                updatedPart.triangleIndices = reversedTriangleIndices
                
                updatedPart.positions = part.positions
                
                // We don't need the texture coordinates or normals for the solid interior
                // geometry, but if we remove the following lines, our regular lit exterior
                // geometry mysteriously disappears.
                updatedPart.textureCoordinates = part.textureCoordinates
                updatedPart.normals = part.normals
                updatedPart.tangents = part.tangents
                updatedPart.bitangents = part.bitangents
                updatedPart.skeletonID = part.skeletonID
                updatedPart.jointInfluences = part.jointInfluences
                
                updatedParts.append(updatedPart)
            }
            
            let updatedModel = MeshResource.Model(id: model.id, parts: updatedParts)
            updatedModels.insert(updatedModel)
        }
        
        updatedContents.models = updatedModels
        
        guard let updatedMesh = try? await MeshResource(from: updatedContents) else {
            assertionFailure("updatedMesh is undefined")
            return
        }
        
        let updatedModel = ModelComponent(mesh: updatedMesh, materials: model.materials)
        
        self.model = updatedModel
    }
    
}