iOS Metal低饱和度低透明度颜色渲染异常问题求助

现象细节 #

使用MTKView渲染三角形时，多数场景表现正常，但降低颜色饱和度和透明度后，三角形会完全透明。相同参数创建的SwiftUI Color则显示正常：

• 仅低饱和度颜色触发问题：100%饱和度颜色（如#FF0000）即使仅1%透明度仍可正常渲染
• 以颜色#FF8888测试不同像素格式的表现：
  • bgra8Unorm：需至少55%透明度才能渲染
  • bgra8Unorm_srgb：需至少77%透明度才能渲染，且颜色明显偏浅

排查情况 #

• Swift中颜色存储为[Float]，MSL中转为float4*，顶点和片段函数仅返回输入值，其他颜色渲染正常，排除这部分代码问题
• 修改MTKView的colorPixelFormat会改变结果，但不确定是否仅需调整该参数，也不清楚正确的格式选择

代码配置 #

MTKView基础配置 #

clearColor = MTLClearColor(red: 0, green: 0, blue: 0, alpha: 0)
isOpaque = false
layer.magnificationFilter = .nearest
layer.minificationFilter = .nearest

渲染管线状态设置 #

let pipelineDescriptor = MTLRenderPipelineDescriptor()
pipelineDescriptor.vertexFunction = vertexFunction
pipelineDescriptor.fragmentFunction = fragmentFunction
pipelineDescriptor.colorAttachments[0].pixelFormat = .bgra8Unorm

pipelineState = try? device.makeRenderPipelineState(descriptor: pipelineDescriptor)

绘制逻辑代码 #

guard let vertexBuffer = vertexBuffer,
      let indexBuffer = indexBuffer,
      let indexCount = indexCount,
      let colorBuffer = colorBuffer,
      let pipelineState = pipelineState,
      let discriptor = view.currentRenderPassDescriptor,
      let commandBuffer = commandQueue.makeCommandBuffer(),
      let commandEncoder = commandBuffer.makeRenderCommandEncoder(
        descriptor: discriptor
      ),
      let drawable = view.currentDrawable else {
  return
}

commandEncoder.setRenderPipelineState(pipelineState)
commandEncoder.setVertexBuffer(vertexBuffer, offset: 0, index: 0)
commandEncoder.setVertexBuffer(colorBuffer, offset: 0, index: 1)
commandEncoder.drawIndexedPrimitives(
  type: .triangle,
  indexCount: indexCount,
  indexType: .uint32,
  indexBuffer: indexBuffer,
  indexBufferOffset: 0
)

commandEncoder.endEncoding()
commandBuffer.present(drawable)
commandBuffer.commit()

内容的提问来源于stack exchange，提问作者Son Nguyen