How to see the vertex descriptor of the MDLMesh helper geometries

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1

Hi all,

In the book chapters, we use the VertexDescriptor.defaultLayout to specify the vertex layout of our meshes and conform them to the render pipelines:

extension MDLVertexDescriptor {
  static var defaultLayout: MDLVertexDescriptor = {
      let vertexDescriptor = MDLVertexDescriptor()
      // fill in the vertexDescriptor
      return vertexDescriptor
  }
}

I am having hard times understanding how do we conform other type of meshes to this?

Or to paraphrase my question, how do I conform a mesh created by:

MDLMesh(
    boxWithExtent extent: vector_float3,
    segments: vector_uint3,
    inwardNormals: Bool,
    geometryType: MDLGeometryType,
    allocator: MDLMeshBufferAllocator?
)

The spec hardly says anything about the vertex layout of this mesh. I can see only one vertex buffer is returned, so the normals, pos, uv, etc must be somehow interleaved. By trial and error I can see that the first 3 floats are positions, but can’t understand how to unpack the rest and fit it into the VertexDescriptor.defaultLayout

Thanks!

2

@nikoloffgeorgi - so sorry - I missed this one.

When you have an MDLMesh, you can print out its vertex descriptor to the console.

let mdlMesh = MDLMesh(
  boxWithExtent: [0.8, 0.8, 0.8],
  segments: [10, 10, 10],
  inwardNormals: false,
  geometryType: .triangles,
  allocator: allocator
)
print(mdlMesh.vertexDescriptor)

This gives the result for your box:

<MDLVertexDescriptor: 0x600000d18000 attributes:(
    "<MDLVertexAttribute: 0x600001810e80 name=position format=Float3 bufferIndex=0 offset=0>",
    "<MDLVertexAttribute: 0x600001810e40 name=normal format=Float3 bufferIndex=0 offset=12>",
    "<MDLVertexAttribute: 0x600001810fc0 name=textureCoordinate format=Float2 bufferIndex=0 offset=24>"
) layouts:{
    0 = "<MDLVertexBufferLayout: 0x600000f28790 stride=32>";
}>

So all the attributes are in buffer[0] with a stride of 32. position and normal are float3 and textureCoordinate is float2.

You can create a new pipeline state for this vertex descriptor instead of using defaultVertexDescriptor. You could also decide to reconfigure defaultVertexDescriptor depending on how you want your models to import.

1 Like
3

I also think that assigning an MDLVertexDescriptor to the MDLMesh will rearrange the buffers when you convert it to an MTKMesh. :crossed_fingers:

This playground appears to work.

VertexDescriptor.playground.zip (13.7 KB)

Here I have used MDLVertexDescriptor.defaultLayout from the lighting chapter, and assigned it to the box mesh. The MDLMesh to MTKMesh conversion appears to take place correctly, according to the new vertex descriptor.

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