Trouble with MDLMesh.newBox()

Question

ronm OP

Created May ’25

Replies 1

Boosts 0

Participants 2

I'm trying to build an MDLMesh then add normals

let mdlMesh = MDLMesh.newBox(withDimensions: SIMD3<Float>(1, 1, 1),
                                 segments: SIMD3<UInt32>(2, 2, 2),
                                 geometryType: MDLGeometryType.triangles,
                                 inwardNormals:false,
                                 allocator: allocator)

mdlMesh.addNormals(withAttributeNamed: MDLVertexAttributeNormal, creaseThreshold: 0)

When I render the mesh, some normals are (0,0,0). I don't know if the problem is in the mesh, or in the conversion to MTKMesh. Is there a way to examine an MDLMesh with the geometry viewer? When I look at the variable values for my mdlMesh I get this:

Screenshot 2025-05-16 at 10.25.34 PM.png

Not too useful. I don't know how to track down the normals. What's the best way to find out where the normals getting broken?

Answered by DTS Engineer in 840340022

To check the normals you could export to a .obj and then open the result in an editing tool to inspect them.

For example, the following:

let device = MTLCreateSystemDefaultDevice()
let allocator = MTKMeshBufferAllocator(device: device!)
let mdlMesh = MDLMesh.newBox(withDimensions: SIMD3<Float>(1, 1, 1),
                                 segments: SIMD3<UInt32>(2, 2, 2),
                                 geometryType: MDLGeometryType.triangles,
                                 inwardNormals:false,
                                 allocator: allocator)
mdlMesh.addNormals(withAttributeNamed: MDLVertexAttributeNormal, creaseThreshold: 0)

let asset = MDLAsset()
asset.add(mdlMesh)
/* substitute your username in the path below */
let url = URL(fileURLWithPath: "/Users/dtsengineer42/Desktop/exported_mesh.obj") // Choose your desired path and format
do {
    try asset.export(to: url)
    print("MDLMesh saved successfully to \(url)")
} catch {
    print("Error exporting MDLMesh: \(error)")
}

exports the mesh as a file named exported_mesh.obj on the desktop. Importing that file into a Blender scene and displaying the normals produces the following result:

Boost

Answer 1

DTS Engineer OP

Apple

May ’25

Recommended

To check the normals you could export to a .obj and then open the result in an editing tool to inspect them.

For example, the following:

let device = MTLCreateSystemDefaultDevice()
let allocator = MTKMeshBufferAllocator(device: device!)
let mdlMesh = MDLMesh.newBox(withDimensions: SIMD3<Float>(1, 1, 1),
                                 segments: SIMD3<UInt32>(2, 2, 2),
                                 geometryType: MDLGeometryType.triangles,
                                 inwardNormals:false,
                                 allocator: allocator)
mdlMesh.addNormals(withAttributeNamed: MDLVertexAttributeNormal, creaseThreshold: 0)

let asset = MDLAsset()
asset.add(mdlMesh)
/* substitute your username in the path below */
let url = URL(fileURLWithPath: "/Users/dtsengineer42/Desktop/exported_mesh.obj") // Choose your desired path and format
do {
    try asset.export(to: url)
    print("MDLMesh saved successfully to \(url)")
} catch {
    print("Error exporting MDLMesh: \(error)")
}

exports the mesh as a file named exported_mesh.obj on the desktop. Importing that file into a Blender scene and displaying the normals produces the following result:

0