Metal/Swift - First element of vertex buffer has one wrong value

Question

I'm just trying to render a red square using metal, and I'm creating a vertex buffer from an array of Vertex structures that look like this:

struct Vertex {
    var position: SIMD3<Float>
    var color: SIMD4<Float>
}

This is where I'm rendering the square:

var vertices: [Vertex] = [
    Vertex(position: [-0.5, -0.5, 0], color: [1, 0, 0, 1]),
    Vertex(position: [-0.5, 0.5, 0], color: [1, 0, 0, 1]),
    Vertex(position: [0.5, -0.5, 0], color: [1, 0, 0, 1]),
    Vertex(position: [0.5, 0.5, 0], color: [1, 0, 0, 1])
]

var vertexBuffer: MTLBuffer?

func render(using renderCommandEncoder: MTLRenderCommandEncoder) {
    if self.vertexBuffer == nil {
        self.vertexBuffer = self.device.makeBuffer(
            bytes: self.vertices,
            length: MemoryLayout<Vertex>.stride * self.vertices.count,
            options: []
        )
    }

    if let vertexBuffer = self.vertexBuffer {
        renderCommandEncoder.setRenderPipelineState(RenderPipelineStates.defaultState)
        renderCommandEncoder.setVertexBuffer(vertexBuffer, offset: 0, index: 0)
        renderCommandEncoder.drawPrimitives(type: .triangleStrip, vertexStart: 0, vertexCount: vertexBuffer.length / MemoryLayout<Vertex>.stride)
    }
}

This is what my render pipeline state looks like:

let library = device.makeDefaultLibrary()!
let vertexShader = library.makeFunction(name: "basicVertexShader")
let fragmentShader = library.makeFunction(name: "basicFragmentShader")

let renderPipelineDescriptor = MTLRenderPipelineDescriptor()
renderPipelineDescriptor.colorAttachments[0].pixelFormat = .bgra8Unorm
renderPipelineDescriptor.vertexFunction = vertexShader
renderPipelineDescriptor.fragmentFunction = fragmentShader
renderPipelineDescriptor.sampleCount = 4

let vertexDescriptor = MTLVertexDescriptor()

vertexDescriptor.attributes[0].format = .float3
vertexDescriptor.attributes[0].bufferIndex = 0 // Position
vertexDescriptor.attributes[0].offset = 0

vertexDescriptor.attributes[1].format = .float4
vertexDescriptor.attributes[1].bufferIndex = 0 // Color
vertexDescriptor.attributes[1].offset = MemoryLayout<SIMD3<Float>>.stride

vertexDescriptor.layouts[0].stride = MemoryLayout<Vertex>.stride
renderPipelineDescriptor.vertexDescriptor = vertexDescriptor

self.defaultState = try! device.makeRenderPipelineState(descriptor: renderPipelineDescriptor)

The vertex and fragment shaders just pass through the position and color. For some reason, when this is rendered the first float of the color of the first vertex comes into the vertex shader as an extremely small value, effectively showing black. It only happens for the red value of the first vertex in the array.

Red square with one black vertex

I can see from debugging the GPU frame that the first vertex has a red color component of 5E-41 (essentially 0).

I have no idea why this is the case, it happens some time when the vertices are added to the vertex buffer. I'm guessing it has something to do with my render pipeline vertex descriptor, but I haven't been able to figure out what's wrong. Thanks for any help!

Answer 1

This is, with high likelihood, a duplicate of this question. I'd encourage you to consider the workarounds there, and also to file your own feedback to raise visibility of this bug. - warrenm

Correct, this appears to be a driver bug of some sorts. I fixed it by adding the cpuCacheModeWriteCombined option to makeBuffer and have filed feedback.

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