swift – 在顶点着色器之前将计算/内核函数应用于顶点缓冲区

我想使用计算着色器来修改我的顶点,然后将它们传递给顶点着色器.我找不到任何这样的例子或解释,除了它似乎在这里提到： Metal emulate geometry shaders using compute shaders.这对我没有帮助,因为它没有解释它的cpu部分.

我已经看过许多在计算着色器中读取和写入纹理缓冲区的示例,但我需要读取和修改顶点缓冲区,其中包含带法线的自定义顶点结构,并由MDLMesh创建.我将永远感谢一些示例代码！

背景

我真正想要实现的是能够修改GPU上的顶点法线.另一种选择是,如果我可以从顶点着色器访问整个三角形,就像在链接的答案中一样.出于某种原因,我只能使用stage_in属性访问单个顶点.在这种特殊情况下,使用整个缓冲区对我来说不起作用,这可能与使用Model I / O和MDLMesh提供的网格有关.当我手动创建顶点时,我能够访问顶点缓冲区数组.话虽如此,使用该解决方案,我将不得不为每个三角形计算新的顶点法线向量三次,这似乎是浪费,并且无论如何我希望能够将计算着色器应用于顶点缓冲区！

解决方法

感谢Ken Thomases的评论,我设法找到了解决方案.他让我意识到这很简单：

我正在使用如下所示的顶点结构：

// Metal side
struct Vertex {
    float4 position;
    float4 normal;
    float4 color;
};

// Swift side
struct Vertex {
    var position: float4
    var normal: float4
    var color: float4
}

在我通常创建顶点缓冲区,索引缓冲区和渲染管道状态的设置期间,我现在还创建一个计算管道状态：

// Vertex buffer
let dataSize = vertexData.count*MemoryLayout<Vertex>.stride
vertexBuffer = device.makeBuffer(bytes: vertexData,length: dataSize,options: [])!

// Index buffer
indexCount = indices.count
let indexSize = indexCount*MemoryLayout<UInt16>.stride
indexBuffer = device.makeBuffer(bytes: indices,length: indexSize,options: [])!

// Compute pipeline state
let adjustmentFunction = library.makeFunction(name: "adjustment_func")!
cps = try! device.makeComputePipelinestate(function: adjustmentFunction)

// Render pipeline state
let rpld = MTLRenderPipelineDescriptor()
rpld.vertexFunction = library.makeFunction(name: "vertex_func")
rpld.fragmentFunction = library.makeFunction(name: "fragment_func")
rpld.colorAttachments[0].pixelFormat = .bgra8Unorm
rps = try! device.makeRenderPipelinestate(descriptor: rpld)

commandQueue = device.makeCommandQueue()!

然后我的渲染函数如下所示：

let black = MTLClearColor(red: 0,green: 0,blue: 0,alpha: 1)
rpd.colorAttachments[0].texture = drawable.texture
rpd.colorAttachments[0].clearColor = black
rpd.colorAttachments[0].loadAction = .clear

let commandBuffer = commandQueue.makeCommandBuffer()!

let computeCommandEncoder = commandBuffer.makeComputeCommandEncoder()!
computeCommandEncoder.setComputePipelinestate(cps)
computeCommandEncoder.setBuffer(vertexBuffer,offset: 0,index: 0)
computeCommandEncoder.dispatchThreadgroups(MTLSize(width: meshSize*meshSize,height: 1,depth: 1),threadsPerThreadgroup: MTLSize(width: 4,depth: 1))
computeCommandEncoder.endEncoding()

let renderCommandEncoder = commandBuffer.makeRenderCommandEncoder(descriptor: rpd)!
renderCommandEncoder.setRenderPipelinestate(rps)
renderCommandEncoder.setFrontFacing(.counterClockwise)
renderCommandEncoder.setCullMode(.back)

updateUniforms(aspect: Float(size.width/size.height))
renderCommandEncoder.setVertexBuffer(vertexBuffer,index: 0)
renderCommandEncoder.setVertexBuffer(uniformBuffer,index: 1)
renderCommandEncoder.setFragmentBuffer(uniformBuffer,index: 1)
renderCommandEncoder.drawIndexedPrimitives(type: .triangle,indexCount: indexCount,indexType: .uint16,indexBuffer: indexBuffer,indexBufferOffset: 0)
renderCommandEncoder.endEncoding()

commandBuffer.present(drawable)
commandBuffer.commit()

最后我的计算着色器看起来像这样：

kernel void adjustment_func(const device Vertex *vertices [[buffer(0)]],uint2 gid [[thread_position_in_grid]]) {
    vertices[gid.x].position = function(pos.xyz);
}

这是我的顶点函数的签名：

vertex VertexOut vertex_func(const device Vertex *vertices [[buffer(0)]],uint i [[vertex_id]],constant Uniforms &uniforms [[buffer(1)]])