MDLMesh based on your own vertex data

Hi,

I used initWithVertex data for a plane mesh that I generated programatically. However I was *not* able to get the normal generation working, I ended up having to generate my normals manually as before so I am not sure how useful it is really, probably not useful at all tbh, the only thing I like about it is it makes the renderer code a bit cleaner because all of my meshes end up being MTKMesh.

I think Model I/O is meant to read from a obj file directly then that way it has full control over how the buffers are created on the device.

If you still want to do it try to follow these steps and let me know if you have any problems.

1. Create the MTLVertexDescriptor object that describes how your programatically generated verticies and normals are laid out in memory

2. Use this object to initialize the MDLVertexDescriptor object via MTKModelIOVertexDescriptorFromMetal

3. Create your position vertex, normal, and index buffers like so:

        MTKMeshBufferAllocator * allocator = [[MTKMeshBufferAllocator alloc] initWithDevice:device];
        NSData * vertexData = [NSData dataWithBytesNoCopy:_positions length:sizeof(vector_float3) * plane.vertexCount];
        MTKMeshBuffer * vertexBuffer = (MTKMeshBuffer *)[allocator newBufferWithData:vertexData type:MDLMeshBufferTypeVertex];
     
        NSData * normalData = [NSData dataWithBytesNoCopy:_normals length:normalBufferLength];
        MTKMeshBuffer * normalBuffer = (MTKMeshBuffer *)[allocator newBufferWithData:normalData type:MDLMeshBufferTypeVertex];
     
        NSData * indexData = [NSData dataWithBytesNoCopy:_triangleStripIndicies length:sizeof(UInt16) * plane.indexCount];
        MTKMeshBuffer * indexBuffer = (MTKMeshBuffer *)[allocator newBufferWithData:indexData type:MDLMeshBufferTypeIndex];

4. Initialize your submesh with the index buffer: [[MDLSubmesh alloc] initWithIndexBuffer:indexBuffer indexCount:plane.indexCount indexType:MDLIndexBitDepthUInt16 geometryType:MDLGeometryTypeTriangleStrips material:nil];

5. Initialize and return your parent mesh object: mesh = [[MDLMesh alloc] initWithVertexBuffers:@[vertexBuffer, normalBuffer] vertexCount:plane.vertexCount descriptor:mdlVertexDescriptor submeshes:@[submesh]];