Why does MDLMesh have unnamed default attributes? - metal

Question

I am discovering Model I/O and I would like to use it to generate vertices to create a sphere, so I created a MDLMesh object using this class method :
let sphere = MDLMesh.newEllipsoidWithRadii(vector_float3(1, 1, 1), radialSegments: 300, verticalSegments: 300, geometryType: MDLGeometryType.KindTriangles, inwardNormals: false, hemisphere: false, allocator: MTKMeshBufferAllocator(device: device))
Then, to understand how it works, I want to inspect the attributes of my this MDLMesh so I am reading them this way :
for attribute in sphere.vertexDescriptor.attributes {
if let vertexAttribute = attribute as? MDLVertexAttribute {
print("Attribute named : \(vertexAttribute.name), of format \(vertexAttribute.format.rawValue)")
}
}
Here is the output :
Attribute named : position, of format 786435
Attribute named : normal, of format 786435
Attribute named : textureCoordinate, of format 786434
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
Attribute named : , of format 0 Attribute named : , of format 0
I believe position and normals were automatically generated but why is there a texture coordinate ? What rules were used to generate those ? Most of all, why is there all of those unnamed attributes of invalid format ?
Thank you
EDIT : It would seem that there is a fixed number of attributes (31 in my case) because even if I add new, custom attributes (for instance by generating normals), the attributes that were invalid are replaced with those new attributes and the total number of attributes is still 31.

Answer 1

It seems Apple configured the default vertexDescriptor to be created with those first 3 attributes ready for you to use. They happen to also be the most common attributes one would need to use. You have 28 more attribute "holders" in case you need to use more than those 3. Btw, they are not of invalid format, they just occupy 0 bytes currently because they were not yet created and named. In total, there are 15 types of vertex attributes you can create, and you can also have more of the same type if needed. For more information about the name and format, you can read the MDLVertexAttribute Class Reference document.