I'm trying to generate UV map for a mesh using Model/IO. The code runs on the simulator and generates a UV map for the input mesh but when I run it on a device it crashes on
mdlMesh.addUnwrappedTextureCoordinates(forAttributeNamed: MDLVertexAttributeTextureCoordinate)
with this error displayed multiple times on the console:
Can't choose for edge creation.
The fatal error that terminates the app is:
libc++abi: terminating with uncaught exception of type std::out_of_range: unordered_map::at: key not found
The code:
let asset = MDLAsset()
let allocator = MTKMeshBufferAllocator(device: device)
let zoneSize = MemoryLayout<Float>.size * 3 * mesh.vertices.count + MemoryLayout<UInt32>.size * indexCount
let zone = allocator.newZone(zoneSize)
let data = Data.init(bytes: vertexBuffer.contents(), count: MemoryLayout<Float>.size * 3 * mesh.vertices.count)
let vBuffer = allocator.newBuffer(from: zone, data: data, type: .vertex)!
let indexData = Data.init(bytes: indexBuffer.contents(), count: MemoryLayout<UInt32>.size * indexCount)
let iBuffer = allocator.newBuffer(from: zone, data: indexData, type: .index)!
let submesh = MDLSubmesh(indexBuffer: iBuffer,
indexCount: indexCount,
indexType: .uint32,
geometryType: .triangles,
material: nil)
let vDescriptor = MDLVertexDescriptor()
// Vertex Positions
vDescriptor.attributes[0] = MDLVertexAttribute(name: MDLVertexAttributePosition,
format: .float3,
offset: 0,
bufferIndex: 0)
vDescriptor.layouts[0] = MDLVertexBufferLayout(stride: MemoryLayout<Float>.size * 3)
let mdlMesh = MDLMesh(vertexBuffer: vBuffer,
vertexCount: mesh.vertices.count,
descriptor: vDescriptor,
submeshes: [submesh])
mdlMesh.addAttribute(withName: MDLVertexAttributeTextureCoordinate, format: .float2)
mdlMesh.addUnwrappedTextureCoordinates(forAttributeNamed: MDLVertexAttributeTextureCoordinate)
asset.add(mdlMesh)
I'm kind of stuck right now when it comes to applying a specific texture on my 3d obj model.
Easiest solution of all would be to do let test = SCNScene(named: "models.scnassets/modelFolder/ModelName.obj"), but this requires that the mtl file maps the texture file directly inside of it which is not something that's possible with my current workflow.
With my current understanding, this leaves me with the option of using a scattering function to apply textures to a specific semantic, something like such :
if let url = URL(string: obj) {
let asset = MDLAsset(url: url)
guard let object = asset.object(at: 0) as? MDLMesh else {
print("Failed to get mesh from asset.")
self.presentAlert(title: "Warning", message: "Could not fetch the model.", firstBtn: "Ok")
return
}
// Create a material from the various textures with a scatteringFunction
let scatteringFunction = MDLScatteringFunction()
let material = MDLMaterial(name: "material", scatteringFunction: scatteringFunction)
let property = MDLMaterialProperty(name: "texture", semantic: .baseColor, url: URL(string: self.textureURL))
material.setProperty(property)
// Apply the texture to every submesh of the asset
object.submeshes?.forEach {
if let submesh = $0 as? MDLSubmesh {
submesh.material = material
}
}
// Wrap the ModelIO object in a SceneKit object
let node = SCNNode(mdlObject: object)
let scene = SCNScene()
scene.rootNode.addChildNode(node)
// Set up the SceneView
sceneView.scene = scene
...
}
The actual problem is the semantics. The 3d models are made on Unreal and for many models there's a png texture which has 3 semantics inside of it, namely Ambient Occlusion, Roughness and Metallic. Ambient Occlusion would need to be applied on the red channel, Roughness on the greed channel and Metallic on the blue channel.
How could I achieve this? An MdlMaterialSemantic has all of these possible semantics, but metallic, ambient occlusion and roughness are all separate. I tried simply applying the texture on each, but obviously this did not work very well.
Considering that my .png texture has all of those 3 "packaged" in it under a different channel, how can I work with this? I was thinking that maybe I could somehow use a small script to add mapping to the texture in the mtl file on my end in the app directly, but this seems sketchy lol..
What are my other options if there's no way of doing this? I've also been trying to use fbx files with assimpKit, but I couldn't manage to load any textures, just the model in black...
I am open to any suggestion, if more info is needed, please let me know! Thank you very much!
Sorry, I don't have enough rep to comment, but this might be more of a comment than an answer!
Have you tried loading the texture png image separately (as a NS/UI/CGImage) and then splitting it into three channels manually, then applying these channels separately? (Splitting into three separate channels is not as simple as it could be... but you could use this grayscale conversion for guidance, and just do one channel at a time.)
Once you have your objects in SceneKit, it is possibly slightly easier to modify these materials. Once you have a SCNNode with a SCNGeometry with a SCNMaterial you can access any of these materials and set the .contents property to almost anything (including a XXImage).
Edit:
Here's an extension you can try to extract the individual channels from a CGImage using Accelerate. You can get a CGImage from an NSImage/UIImage depending on whether you're on Mac or iOS (and you can load the file directly into one of those image formats).
I've just adapted the code from the link above, I am not very experienced with the Accelerate framework, so use at your own risk! But hopefully this puts you on the right path.
extension CGImage {
enum Channel {
case red, green, blue
}
func getChannel(channel: Channel) -> CGImage? {
// code adapted from https://developer.apple.com/documentation/accelerate/converting_color_images_to_grayscale
guard let format = vImage_CGImageFormat(cgImage: cgImage) else {return nil}
guard var sourceImageBuffer = try? vImage_Buffer(cgImage: cgImage, format: format) else {return nil}
guard var destinationBuffer = try? vImage_Buffer(width: Int(sourceImageBuffer.width), height: Int(sourceImageBuffer.height), bitsPerPixel: 8) else {return nil}
defer {
sourceImageBuffer.free()
destinationBuffer.free()
}
let redCoefficient: Float = channel == .red ? 1 : 0
let greenCoefficient: Float = channel == .green ? 1 : 0
let blueCoefficient: Float = channel == .blue ? 1 : 0
let divisor: Int32 = 0x1000
let fDivisor = Float(divisor)
var coefficientsMatrix = [
Int16(redCoefficient * fDivisor),
Int16(greenCoefficient * fDivisor),
Int16(blueCoefficient * fDivisor)
]
let preBias: [Int16] = [0, 0, 0, 0]
let postBias: Int32 = 0
vImageMatrixMultiply_ARGB8888ToPlanar8(&sourceImageBuffer,
&destinationBuffer,
&coefficientsMatrix,
divisor,
preBias,
postBias,
vImage_Flags(kvImageNoFlags))
guard let monoFormat = vImage_CGImageFormat(
bitsPerComponent: 8,
bitsPerPixel: 8,
colorSpace: CGColorSpaceCreateDeviceGray(),
bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue),
renderingIntent: .defaultIntent) else {return nil}
guard let result = try? destinationBuffer.createCGImage(format: monoFormat) else {return nil}
return result
}
}
In an iOS ARKit app, I've been trying to save the ARFaceGeometry data to an OBJ file. I followed the explanation here: How to make a 3D model from AVDepthData?. However, the OBJ isn't created correctly. Here's what I have:
func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
guard let faceAnchor = anchor as? ARFaceAnchor else { return }
currentFaceAnchor = faceAnchor
// If this is the first time with this anchor, get the controller to create content.
// Otherwise (switching content), will change content when setting `selectedVirtualContent`.
if node.childNodes.isEmpty, let contentNode = selectedContentController.renderer(renderer, nodeFor: faceAnchor) {
node.addChildNode(contentNode)
}
// https://stackoverflow.com/questions/52953590/how-to-make-a-3d-model-from-avdepthdata
let geometry = faceAnchor.geometry
let allocator = MDLMeshBufferDataAllocator()
let vertices = allocator.newBuffer(with: Data(fromArray: geometry.vertices), type: .vertex)
let textureCoordinates = allocator.newBuffer(with: Data(fromArray: geometry.textureCoordinates), type: .vertex)
let triangleIndices = allocator.newBuffer(with: Data(fromArray: geometry.triangleIndices), type: .index)
let submesh = MDLSubmesh(indexBuffer: triangleIndices, indexCount: geometry.triangleIndices.count, indexType: .uInt16, geometryType: .triangles, material: MDLMaterial(name: "mat1", scatteringFunction: MDLPhysicallyPlausibleScatteringFunction()))
let vertexDescriptor = MDLVertexDescriptor()
// Attributes
vertexDescriptor.addOrReplaceAttribute(MDLVertexAttribute(name: MDLVertexAttributePosition, format: .float3, offset: 0, bufferIndex: 0))
vertexDescriptor.addOrReplaceAttribute(MDLVertexAttribute(name: MDLVertexAttributeNormal, format: .float3, offset: MemoryLayout<float3>.stride, bufferIndex: 0))
vertexDescriptor.addOrReplaceAttribute(MDLVertexAttribute(name: MDLVertexAttributeTextureCoordinate, format: .float2, offset: MemoryLayout<float3>.stride + MemoryLayout<float3>.stride, bufferIndex: 0))
// Layouts
vertexDescriptor.layouts.add(MDLVertexBufferLayout(stride: MemoryLayout<float3>.stride + MemoryLayout<float3>.stride + MemoryLayout<float2>.stride))
let mdlMesh = MDLMesh(vertexBuffers: [vertices, textureCoordinates], vertexCount: geometry.vertices.count, descriptor: vertexDescriptor, submeshes: [submesh])
mdlMesh.addNormals(withAttributeNamed: MDLVertexAttributeNormal, creaseThreshold: 0.5)
let asset = MDLAsset(bufferAllocator: allocator)
asset.add(mdlMesh)
let documentsPath = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
let exportUrl = documentsPath.appendingPathComponent("face.obj")
try! asset.export(to: exportUrl)
}
The resulting OBJ file looks like this:
# Apple ModelIO OBJ File: face
mtllib face.mtl
g
v -0.000128156 -0.0277879 0.0575149
vn 0 0 0
vt -9.36008e-05 -0.0242016
usemtl material_1
f 1/1/1 1/1/1 1/1/1
f 1/1/1 1/1/1 1/1/1
f 1/1/1 1/1/1 1/1/1
... and many more lines
I would expect many more vertices, and the index values look wrong.
The core issue is that your vertex data isn't described correctly. When you provide a vertex descriptor to Model I/O while constructing a mesh, it represents the layout the data actually has, not your desired layout. You're supplying two vertex buffers, but your vertex descriptor describes an interleaved data layout with only one vertex buffer.
The easiest way to remedy this is to fix the vertex descriptor to reflect the data you're providing:
let vertexDescriptor = MDLVertexDescriptor()
// Attributes
vertexDescriptor.attributes[0] = MDLVertexAttribute(name: MDLVertexAttributePosition,
format: .float3,
offset: 0,
bufferIndex: 0)
vertexDescriptor.attributes[1] = MDLVertexAttribute(name: MDLVertexAttributeTextureCoordinate,
format: .float2,
offset: 0,
bufferIndex: 1)
// Layouts
vertexDescriptor.layouts[0] = MDLVertexBufferLayout(stride: MemoryLayout<float3>.stride)
vertexDescriptor.layouts[1] = MDLVertexBufferLayout(stride: MemoryLayout<float2>.stride)
When you later call addNormals(...), Model I/O will allocate the necessary space and update the vertex descriptor to reflect the new data. Since you're not rendering from the data and are instead immediately exporting it, the internal layout it chooses for the normals isn't important.
How does one extract the SceneKit depth buffer? I make an AR based app that is running Metal and I'm really struggling to find any info on how to extract a 2D depth buffer so I can render out fancy 3D photos of my scenes. Any help greatly appreciated.
Your question is unclear but I'll try to answer.
Depth pass from VR view
If you need to render a Depth pass from SceneKit's 3D environment then you should use, for instance, a SCNGeometrySource.Semantic structure. There are vertex, normal, texcoord, color and tangent type properties. Let's see what a vertex type property is:
static let vertex: SCNGeometrySource.Semantic
This semantic identifies data containing the positions of each vertex in the geometry. For a custom shader program, you use this semantic to bind SceneKit’s vertex position data to an input attribute of the shader. Vertex position data is typically an array of three- or four-component vectors.
Here's a code's excerpt from iOS Depth Sample project.
UPDATED: Using this code you can get a position for every point in SCNScene and assign a color for these points (this is what a zDepth channel really is):
import SceneKit
struct PointCloudVertex {
var x: Float, y: Float, z: Float
var r: Float, g: Float, b: Float
}
#objc class PointCloud: NSObject {
var pointCloud : [SCNVector3] = []
var colors: [UInt8] = []
public func pointCloudNode() -> SCNNode {
let points = self.pointCloud
var vertices = Array(repeating: PointCloudVertex(x: 0,
y: 0,
z: 0,
r: 0,
g: 0,
b: 0),
count: points.count)
for i in 0...(points.count-1) {
let p = points[i]
vertices[i].x = Float(p.x)
vertices[i].y = Float(p.y)
vertices[i].z = Float(p.z)
vertices[i].r = Float(colors[i * 4]) / 255.0
vertices[i].g = Float(colors[i * 4 + 1]) / 255.0
vertices[i].b = Float(colors[i * 4 + 2]) / 255.0
}
let node = buildNode(points: vertices)
return node
}
private func buildNode(points: [PointCloudVertex]) -> SCNNode {
let vertexData = NSData(
bytes: points,
length: MemoryLayout<PointCloudVertex>.size * points.count
)
let positionSource = SCNGeometrySource(
data: vertexData as Data,
semantic: SCNGeometrySource.Semantic.vertex,
vectorCount: points.count,
usesFloatComponents: true,
componentsPerVector: 3,
bytesPerComponent: MemoryLayout<Float>.size,
dataOffset: 0,
dataStride: MemoryLayout<PointCloudVertex>.size
)
let colorSource = SCNGeometrySource(
data: vertexData as Data,
semantic: SCNGeometrySource.Semantic.color,
vectorCount: points.count,
usesFloatComponents: true,
componentsPerVector: 3,
bytesPerComponent: MemoryLayout<Float>.size,
dataOffset: MemoryLayout<Float>.size * 3,
dataStride: MemoryLayout<PointCloudVertex>.size
)
let element = SCNGeometryElement(
data: nil,
primitiveType: .point,
primitiveCount: points.count,
bytesPerIndex: MemoryLayout<Int>.size
)
element.pointSize = 1
element.minimumPointScreenSpaceRadius = 1
element.maximumPointScreenSpaceRadius = 5
let pointsGeometry = SCNGeometry(sources: [positionSource, colorSource], elements: [element])
return SCNNode(geometry: pointsGeometry)
}
}
Depth pass from AR view
If you need to render a Depth pass from ARSCNView it is possible only in case you're using ARFaceTrackingConfiguration for the front-facing camera. If so, then you can employ capturedDepthData instance property that brings you a depth map, captured along with the video frame.
var capturedDepthData: AVDepthData? { get }
But this depth map image is only 15 fps and of lower resolution than corresponding RGB image at 60 fps.
Face-based AR uses the front-facing, depth-sensing camera on compatible devices. When running such a configuration, frames vended by the session contain a depth map captured by the depth camera in addition to the color pixel buffer (see capturedImage) captured by the color camera. This property’s value is always nil when running other AR configurations.
And a real code could be like this:
extension ViewController: ARSCNViewDelegate {
func renderer(_ renderer: SCNSceneRenderer, updateAtTime time: TimeInterval) {
DispatchQueue.global().async {
guard let frame = self.sceneView.session.currentFrame else {
return
}
if let depthImage = frame.capturedDepthData {
self.depthImage = (depthImage as! CVImageBuffer)
}
}
}
}
Depth pass from Video view
Also, you can extract a true Depth pass using 2 back-facing cameras and AVFoundation framework.
Look at Image Depth Map tutorial where Disparity concept will be introduced to you.
I have a Swift app using SceneKit for iOS 8. I load a scene from a .dae file that contains a mesh controlled by a skeleton.
At runtime, I need to modify the texture coordinates. Using a transform is not an option -- I need to compute a different, completely new UV for each vertex in the mesh.
I know geometry is immutable in SceneKit, and I've read that the suggested approach is to make a copy manually. I'm trying to do that, but I always end up crashing when trying to re-create the SCNSkinner in code. The crash is an EXC_BAD_ACCESS inside C3DSourceAccessorGetMutableValuePtrAtIndex. Unfortunately, there is no source code for this, so I'm not sure why exactly it's crashing. I've narrowed it down to the SCNSkinner object attached to the mesh node. If I do not set that, I don't get a crash and things appear to be working.
EDIT: Here is a more complete call stack of the crash:
C3DSourceAccessorGetMutableValuePtrAtIndex
C3DSkinPrepareMeshForGPUIfNeeded
C3DSkinnerMakeCurrentMesh
C3DSkinnerUpdateCurrentMesh
__CFSetApplyFunction_block_invoke
CFBasicHashApply
CFSetApplyFunction
C3DAppleEngineRenderScene
...
I've not found any documentation or example code about how to create an SCNSkinner object manually. Since I'm just creating it based on a previously working mesh, it shouldn't be too difficult. I'm creating the SCNSkinner according to the Swift documentation, passing all of the correct things into the init. However, there is a skeleton property in the SCNSkinner that I'm not sure how to set. I set it to the skeleton that was on the original SCNSkinner of the mesh I'm copying, which I think
should work... but it doesn't. When setting the skeleton property, it does not appear to be assigning. Checking it immediately after the assignment shows that it is still nil. As a test, I tried to set the original mesh's skeleton property to something else, and after the assignment it was left untouched as well.
Can anyone shed any light on what is happening? Or how to correctly create and set up an SCNSkinner object manually?
Here is the code I'm using to manually clone a mesh and replace it with a new one (I have not modified any of the source data here -- I'm simply trying to make sure I can create a copy at this point):
// This is at the start of the app, just so you can see how the scene is set up.
// I add the .dae contents into its own node in the scene. This seems to be the
// standard way to put multiple .dae models into the same scene. This doesn't seem to
// have any impact on the problem I'm having -- I've tried without this indirection
// and the same problem exists.
let scene = SCNScene()
let modelNode = SCNNode()
modelNode.name = "ModelNode"
scene.rootNode.addChildNode(modelNode)
let modelScene = SCNScene(named: "model.dae")
if modelScene != nil {
if let childNodes = modelScene?.rootNode.childNodes {
for childNode in childNodes {
modelNode.addChildNode(childNode as SCNNode)
}
}
}
// This happens later in the app after a tap from the user.
let modelNode = scnView.scene!.rootNode.childNodeWithName("ModelNode", recursively: true)
let modelMesh = modelNode?.childNodeWithName("MeshName", recursively: true)
let verts = modelMesh?.geometry!.geometrySourcesForSemantic(SCNGeometrySourceSemanticVertex)
let normals = modelMesh?.geometry!.geometrySourcesForSemantic(SCNGeometrySourceSemanticNormal)
let texcoords = modelMesh?.geometry!.geometrySourcesForSemantic(SCNGeometrySourceSemanticTexcoord)
let boneWeights = modelMesh?.geometry!.geometrySourcesForSemantic(SCNGeometrySourceSemanticBoneWeights)
let boneIndices = modelMesh?.geometry!.geometrySourcesForSemantic(SCNGeometrySourceSemanticBoneIndices)
let geometry = modelMesh?.geometry!.geometryElementAtIndex(0)
// Note: the vertex and normal data is shared.
let vertsData = NSData(data: verts![0].data)
let texcoordsData = NSData(data: texcoords![0].data)
let boneWeightsData = NSData(data: boneWeights![0].data)
let boneIndicesData = NSData(data: boneIndices![0].data)
let geometryData = NSData(data: geometry!.data!)
let newVerts = SCNGeometrySource(data: vertsData, semantic: SCNGeometrySourceSemanticVertex, vectorCount: verts![0].vectorCount, floatComponents: verts![0].floatComponents, componentsPerVector: verts![0].componentsPerVector, bytesPerComponent: verts![0].bytesPerComponent, dataOffset: verts![0].dataOffset, dataStride: verts![0].dataStride)
let newNormals = SCNGeometrySource(data: vertsData, semantic: SCNGeometrySourceSemanticNormal, vectorCount: normals![0].vectorCount, floatComponents: normals![0].floatComponents, componentsPerVector: normals![0].componentsPerVector, bytesPerComponent: normals![0].bytesPerComponent, dataOffset: normals![0].dataOffset, dataStride: normals![0].dataStride)
let newTexcoords = SCNGeometrySource(data: texcoordsData, semantic: SCNGeometrySourceSemanticTexcoord, vectorCount: texcoords![0].vectorCount, floatComponents: texcoords![0].floatComponents, componentsPerVector: texcoords![0].componentsPerVector, bytesPerComponent: texcoords![0].bytesPerComponent, dataOffset: texcoords![0].dataOffset, dataStride: texcoords![0].dataStride)
let newBoneWeights = SCNGeometrySource(data: boneWeightsData, semantic: SCNGeometrySourceSemanticBoneWeights, vectorCount: boneWeights![0].vectorCount, floatComponents: boneWeights![0].floatComponents, componentsPerVector: boneWeights![0].componentsPerVector, bytesPerComponent: boneWeights![0].bytesPerComponent, dataOffset: boneWeights![0].dataOffset, dataStride: boneWeights![0].dataStride)
let newBoneIndices = SCNGeometrySource(data: boneIndicesData, semantic: SCNGeometrySourceSemanticBoneIndices, vectorCount: boneIndices![0].vectorCount, floatComponents: boneIndices![0].floatComponents, componentsPerVector: boneIndices![0].componentsPerVector, bytesPerComponent: boneIndices![0].bytesPerComponent, dataOffset: boneIndices![0].dataOffset, dataStride: boneIndices![0].dataStride)
let newGeometry = SCNGeometryElement(data: geometryData, primitiveType: geometry!.primitiveType, primitiveCount: geometry!.primitiveCount, bytesPerIndex: geometry!.bytesPerIndex)
let newMeshGeometry = SCNGeometry(sources: [newVerts, newNormals, newTexcoords, newBoneWeights, newBoneIndices], elements: [newGeometry])
newMeshGeometry.firstMaterial = modelMesh?.geometry!.firstMaterial
let newModelMesh = SCNNode(geometry: newMeshGeometry)
let bones = modelMesh?.skinner?.bones
let boneInverseBindTransforms = modelMesh?.skinner?.boneInverseBindTransforms
let skeleton = modelMesh!.skinner!.skeleton!
let baseGeometryBindTransform = modelMesh!.skinner!.baseGeometryBindTransform
newModelMesh.skinner = SCNSkinner(baseGeometry: newMeshGeometry, bones: bones, boneInverseBindTransforms: boneInverseBindTransforms, boneWeights: newBoneWeights, boneIndices: newBoneIndices)
newModelMesh.skinner?.baseGeometryBindTransform = baseGeometryBindTransform
// Before this assignment, newModelMesh.skinner?.skeleton is nil.
newModelMesh.skinner?.skeleton = skeleton
// After, it is still nil... however, skeleton itself is completely valid.
modelMesh?.removeFromParentNode()
newModelMesh.name = "MeshName"
let meshParentNode = modelNode?.childNodeWithName("MeshParentNode", recursively: true)
meshParentNode?.addChildNode(newModelMesh)
This three methods may help you to find the solution:
SCNNode *hero = [SCNScene sceneNamed:#"Hero"].rootNode;
SCNNode *hat = [SCNScene sceneNamed:#"FancyFedora"].rootNode;
hat.skinner.skeleton = hero.skinner.skeleton;
[Export ("initWithFrame:")]
public UIView (System.Drawing.RectangleF frame) : base (NSObjectFlag.Empty)
{
// Invoke the init method now.
var initWithFrame = new Selector ("initWithFrame:").Handle;
if (IsDirectBinding)
Handle = ObjCRuntime.Messaging.IntPtr_objc_msgSend_RectangleF (this.Handle, initWithFrame, frame);
else
Handle = ObjCRuntime.Messaging.IntPtr_objc_msgSendSuper_RectangleF (this.SuperHandle, initWithFrame, frame);
}
See this link as well.
I don't specifically know what causes your code to crash but here is a way of generating a mesh, bones, and skinning that mesh -- all from code. Swift4 and iOS 12.
In the example, there is mesh representing the concatenation of two cylinders, with one of the cylinders branching off at a 45 degree angle, like so:
\
|
The cylinders are just extruded triangles, i.e., radialSegmentCount = 3. (Note that there are 12 vertices, not 9, since the two cylinders aren't really conjoined. The triangles are ordered like this:
v5
^
v3 /__|__\ v1
| | |
| v4 |
v2 |/___\| v0
There are 3 bones, corresponding to the heads and feet of the cylinders, where the middle bone corresponds to the head of the bottom cylinder and simultaneously the foot of the top cylinder. So for example, vertices v0, v2, and v4 correspond to bone0; v1, v3, v5 correspond to bone1, and so forth. That explains why boneIndices (see below) has the value that it does.
The resting positions of the bones corresponds to the resting positions of the cylinders in the geometry (bone2 sprouts off at a 45 degree angle from bone1, just like the cylinder geometry).
With that as context, the following code creates everything needed to skin the geometry:
let vertices = [float3(0.17841241, 0.0, 0.0), float3(0.17841241, 1.0, 0.0), float3(-0.089206174, 0.0, 0.1545097), float3(-0.089206174, 1.0, 0.1545097), float3(-0.089206256, 0.0, -0.15450965), float3(-0.089206256, 1.0, -0.15450965), float3(0.12615661, 1.1261566, 0.0), float3(-0.58094996, 1.8332633, 0.0), float3(-0.063078284, 0.9369217, 0.1545097), float3(-0.7701849, 1.6440284, 0.1545097), float3(-0.063078344, 0.93692166, -0.15450965), float3(-0.77018493, 1.6440284, -0.15450965)]
let indices: [UInt8] = [0, 1, 2, 3, 4, 5, 0, 1, 1, 6, 6, 7, 8, 9, 10, 11, 6, 7]
let geometrySource = SCNGeometrySource(vertices: vertices.map { SCNVector3($0) })
let geometryElement = SCNGeometryElement(indices: indices, primitiveType: .triangleStrip)
let geometry = SCNGeometry(sources: [geometrySource], elements: [geometryElement])
let bone0 = SCNNode()
bone0.simdPosition = float3(0,0,0)
let bone1 = SCNNode()
bone1.simdPosition = float3(0,1,0)
let bone2 = SCNNode()
bone2.simdPosition = float3(0,1,0) + normalize(float3(-1,1,0))
let bones = [bone0, bone1, bone2]
let boneInverseBindTransforms: [NSValue]? = bones.map { NSValue(scnMatrix4: SCNMatrix4Invert($0.transform)) }
var boneWeights: [Float] = vertices.map { _ in 1.0 }
var boneIndices: [UInt8] = [
0, 1, 0, 1, 0, 1,
1, 2, 1, 2, 1, 2,
]
let boneWeightsData = Data(bytesNoCopy: &boneWeights, count: boneWeights.count * MemoryLayout<Float>.size, deallocator: .none)
let boneIndicesData = Data(bytesNoCopy: &boneIndices, count: boneWeights.count * MemoryLayout<UInt8>.size, deallocator: .none)
let boneWeightsGeometrySource = SCNGeometrySource(data: boneWeightsData, semantic: .boneWeights, vectorCount: boneWeights.count, usesFloatComponents: true, componentsPerVector: 1, bytesPerComponent: MemoryLayout<Float>.size, dataOffset: 0, dataStride: MemoryLayout<Float>.size)
let boneIndicesGeometrySource = SCNGeometrySource(data: boneIndicesData, semantic: .boneIndices, vectorCount: boneIndices.count, usesFloatComponents: false, componentsPerVector: 1, bytesPerComponent: MemoryLayout<UInt8>.size, dataOffset: 0, dataStride: MemoryLayout<UInt8>.size)
let skinner = SCNSkinner(baseGeometry: geometry, bones: bones, boneInverseBindTransforms: boneInverseBindTransforms, boneWeights: boneWeightsGeometrySource, boneIndices: boneIndicesGeometrySource)
let node = SCNNode(geometry: geometry)
node.skinner = skinner
Note: In most cases, you should use UInt16 not UInt8.