Rendering Quad from Vertex Buffer in Metal

I'm trying to render a pixel aligned 2D quad using Metal, but can't seem to get it right.

My vertex buffer's contents (as logged from the CPU side, on creation) are:

Vertex(position: float4(0.0, 0.0, 0.5, 1.0), textureCoordinate: float2(0.0, 0.0))
Vertex(position: float4(0.0, 64.0, 0.5, 1.0), textureCoordinate: float2(0.0, 1.0))
Vertex(position: float4(64.0, 0.0, 0.5, 1.0), textureCoordinate: float2(1.0, 0.0))
Vertex(position: float4(64.0, 64.0, 0.5, 1.0), textureCoordinate: float2(1.0, 1.0))

The index buffer to draw two triangles contains the following indices:

0, 1, 2, 2, 1, 3

The texture I'm using is:

...but I get something like this:

When capturing the frame and inspecting the vertex buffer, I get this:

Clearly, position and texture coordinates are mixed up.

This is the code I use to create the geometry:

import Metal
import simd

struct Vertex {
    var position = float4(x: 0, y: 0, z: 0, w: 1)
    var textureCoordinate = float2(x: 0, y: 0)
}

class Quad {
    let vertexBuffer: MTLBuffer
    let indexBuffer: MTLBuffer
    let indexCount: Int
    let indexType: MTLIndexType
    let primitiveType: MTLPrimitiveType

    init(sideLength: Float, device: MTLDevice) {
        self.primitiveType = .triangle

        var vertexData = [Vertex]()

        var topLeft = Vertex()
        topLeft.position.x = 0
        topLeft.position.y = 0
        topLeft.position.z = 0.5
        topLeft.textureCoordinate.x = 0
        topLeft.textureCoordinate.y = 0
        vertexData.append(topLeft)

        var bottomLeft = Vertex()
        bottomLeft.position.x = 0
        bottomLeft.position.y = sideLength
        bottomLeft.position.z = 0.5
        bottomLeft.textureCoordinate.x = 0
        bottomLeft.textureCoordinate.y = 1
        vertexData.append(bottomLeft)

        var topRight = Vertex()
        topRight.position.x = sideLength
        topRight.position.y = 0
        topRight.position.z = 0.5
        topRight.textureCoordinate.x = 1
        topRight.textureCoordinate.y = 0
        vertexData.append(topRight)

        var bottomRight = Vertex()
        bottomRight.position.x = sideLength
        bottomRight.position.y = sideLength
        bottomRight.position.z = 0.5
        bottomRight.textureCoordinate.x = 1
        bottomRight.textureCoordinate.y = 1
        vertexData.append(bottomRight)

        for vertex in vertexData {
            Swift.print(vertex) // logs the structs posted above
        }

        let vertexBufferSize = vertexData.count * MemoryLayout<Vertex>.stride
        self.vertexBuffer = device.makeBuffer(bytes: vertexData, length: vertexBufferSize, options: [])

        var indexData = [UInt32]()

        // First triangle: Top left, bottom left, top right (CCW)
        indexData.append(0)
        indexData.append(1)
        indexData.append(2)

        // Second triangle: top right, bottom left, bottom right (CCW)
        indexData.append(2)
        indexData.append(1)
        indexData.append(3)

        for index in indexData {
            Swift.print(index) // logs the integers posted before
        }

        self.indexType = .uint32
        self.indexCount = indexData.count

        let indexBufferSize = indexData.count * MemoryLayout<UInt32>.stride
        self.indexBuffer = device.makeBuffer(bytes: indexData, length: indexBufferSize, options: [])
    }
}

...the Shaders:

#include <metal_stdlib>
using namespace metal;

struct Constants {
    float4x4 modelViewProjectionMatrix;
    float4 tintColor;
};
struct VertexIn {
    packed_float4 position [[ attribute(0) ]];
    packed_float2 texCoords [[ attribute(1) ]];
};
struct VertexOut {
    float4 position [[position]];
    float2 texCoords;
};

vertex VertexOut sprite_vertex_transform(device VertexIn *vertices [[buffer(0)]],
                                     constant Constants &uniforms [[buffer(1)]],
                                     uint vertexId [[vertex_id]]) {

    float4 modelPosition = vertices[vertexId].position;

    VertexOut out;

    out.position = uniforms.modelViewProjectionMatrix * modelPosition;
    out.texCoords = vertices[vertexId].texCoords;

    return out;
}

fragment half4 sprite_fragment_textured(
    VertexOut fragmentIn [[stage_in]],
    texture2d<float, access::sample> tex2d [[texture(0)]],
    sampler sampler2d [[sampler(0)]]){

    half4 surfaceColor = half4(tex2d.sample(sampler2d, fragmentIn.texCoords).rgba);

    return surfaceColor;
}

...and this is the code to render it (indexed):

renderEncoder.setVertexBuffer(quad.vertexBuffer, offset: 0, at: 0)
renderEncoder.setVertexBytes(&constants, length: MemoryLayout<Constants>.stride, at: 1)
renderEncoder.setFragmentTexture(texture, at: 0)
renderEncoder.setFragmentSamplerState(sampler, at: 0)

// quad is an instance  of the class above, with sideLength == 64
renderEncoder.drawIndexedPrimitives(
        type: quad.primitiveType,
        indexCount: quad.indexCount,
        indexType: quad.indexType,
        indexBuffer: quad.indexBuffer,
        indexBufferOffset: 0)

Clearly, the data isn't being copied to the vertex buffer appropriately and I'm getting the strides wrong somewhere, but I can't quite figure out where.

The whole project is on GitHub.

Solution

It seems this part was causing the trouble:

struct VertexIn {
    packed_float4 position [[ attribute(0) ]];
    packed_float2 texCoords [[ attribute(1) ]];
};

From the debugger console, on the CPU side:

(lldb) print MemoryLayout<Vertex>.stride
(Int) $R0 = 32
(lldb) print MemoryLayout<Vertex>.size
(Int) $R1 = 24
(lldb)

Removing the packed_ prefix solved it:

struct VertexIn {
    float4 position [[ attribute(0) ]];
    float2 texCoords [[ attribute(1) ]];
};

But I guess now my buffer is not optimal in terms of space usage. I'll have to work out how to get the packed_ version to work...

I really need a refresher on struct alignment and packing...

Note: Originally, I adapted the code from a sample project by Apple where the vertex struct had position and normal, both of type float4. Cleary changing the struct "net" size from 8*sizeof(float) to 6*sizeof(float) is what caused the problem.