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?

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Answer 1

DTS Engineer OP

Apple

3w

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:

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