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目前我有一些物体的 3D 场景。我想将其渲染为 2D 纹理（例如 MTLTexture）。之后，我想在屏幕上实时显示这个渲染纹理（例如在 MTKView 或 SKScene 中），并同时将其写入电影中。

现在我有 SCNRenderer 和 offscreenTexture 用于渲染：

func setupMetal() {
    device = MTLCreateSystemDefaultDevice()
    commandQueue = device.makeCommandQueue()
    renderer = SCNRenderer(device: device, options: nil)
}

func setupTexture() {
    var rawData0 = [UInt8](repeating: 0, count: Int(textureSizeX) * Int(textureSizeY) * 4)

    let bytesPerRow = 4 * Int(textureSizeX)
    let bitmapInfo = CGBitmapInfo.byteOrder32Big.rawValue | CGImageAlphaInfo.premultipliedLast.rawValue

    let context = CGContext(data: &rawData0, width: Int(textureSizeX), height: Int(textureSizeY), bitsPerComponent: bitsPerComponent, bytesPerRow: bytesPerRow, space: rgbColorSpace, bitmapInfo: bitmapInfo)!
    context.setFillColor(UIColor.black.cgColor)
    context.fill(CGRect(x: 0, y: 0, width: CGFloat(textureSizeX), height: CGFloat(textureSizeY)))

    let textureDescriptor = MTLTextureDescriptor.texture2DDescriptor(pixelFormat: MTLPixelFormat.rgba8Unorm, width: Int(textureSizeX), height: Int(textureSizeY), mipmapped: false)

    textureDescriptor.usage = MTLTextureUsage(rawValue: MTLTextureUsage.renderTarget.rawValue | MTLTextureUsage.shaderRead.rawValue)

    let textureA = device.makeTexture(descriptor: textureDescriptor)

    let region = MTLRegionMake2D(0, 0, Int(textureSizeX), Int(textureSizeY))
    textureA?.replace(region: region, mipmapLevel: 0, withBytes: &rawData0, bytesPerRow: Int(bytesPerRow))

    offscreenTexture = textureA
}

private func initializeRenderPipelineState() {
    guard let device = device, let library = device.makeDefaultLibrary() else { return }

    let pipelineDescriptor = MTLRenderPipelineDescriptor()
    pipelineDescriptor.sampleCount = 1
    pipelineDescriptor.colorAttachments[0].pixelFormat = .bgr10_xr_srgb
    pipelineDescriptor.depthAttachmentPixelFormat = .invalid

    pipelineDescriptor.vertexFunction = library.makeFunction(name: "mapTexture")
    pipelineDescriptor.fragmentFunction = library.makeFunction(name: "displayTexture")

    do {
        renderPipelineState = try device.makeRenderPipelineState(descriptor: pipelineDescriptor)
    }
    catch {
        assertionFailure("Failed creating a render state pipeline. Can't render the texture without one.")
        return
    }
}

和 MTKView 在屏幕上呈现它：

func initializeMetalView() {
    metalView = MTKView(frame: bounds, device: device)
    metalView.delegate = self
    metalView.framebufferOnly = true
    metalView.colorPixelFormat = .bgr10_xr_srgb
    metalView.contentScaleFactor = UIScreen.main.scale
    metalView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
    insertSubview(metalView, at: 0)
}

在每一帧上，我都试图将场景绘制到纹理并将其呈现在 MTKView

func draw(in view: MTKView) {
    let commandBuffer = commandQueue.makeCommandBuffer()

    let viewport = CGRect(x: 0, y: 0, width: CGFloat(textureSizeX), height: CGFloat(textureSizeY))

    //write to offscreenTexture
    let renderPassDescriptor = MTLRenderPassDescriptor()
    renderPassDescriptor.colorAttachments[0].texture = offscreenTexture
    renderPassDescriptor.colorAttachments[0].loadAction = .clear
    renderPassDescriptor.colorAttachments[0].clearColor = MTLClearColorMake(0, 0, 0, 0.0)
    renderPassDescriptor.colorAttachments[0].storeAction = .store

    renderer.scene = scene
    renderer.pointOfView = scene.rootNode.childNodes[0]
    renderer.render(atTime: 0, viewport: viewport, commandBuffer: commandBuffer!, passDescriptor: renderPassDescriptor)

    guard let currentRenderPassDescriptor = metalView.currentRenderPassDescriptor, let currentDrawable = metalView.currentDrawable, let renderPipelineState = renderPipelineState else {
            return
    }

    let encoder = commandBuffer?.makeRenderCommandEncoder(descriptor: currentRenderPassDescriptor)
    encoder?.pushDebugGroup("RenderFrame")
    encoder?.setRenderPipelineState(renderPipelineState)
    encoder?.setFragmentTexture(offscreenTexture, index: 0)
    encoder?.drawPrimitives(type: .triangleStrip, vertexStart: 0, vertexCount: 4, instanceCount: 1)
    encoder?.popDebugGroup()
    encoder?.endEncoding()

    commandBuffer?.present(currentDrawable)
    commandBuffer?.commit()
}

这种技术甚至可以工作，但太慢了。我设法用正确的 fps 和另一个场景视图来做到这一点。我创建了一个平面并将 offscreenTexture 作为材质附加到这个平面上。但我想在 2D 引擎中进行（因为我认为它应该更简单）（并且因为我希望能够像 2D 图像一样在屏幕上移动它）。

PS如果您对SCNView有所了解，请看我的另一个问题。也许您会提出更好的解决方案。基本上我想要 1）向用户呈现全屏背景和 3D 对象（使用透视投影）（简单） 2）我希望能够将 SCNView 的一部分渲染到电影中。（我知道如何用全屏纹理来做，但我没有找到如何裁剪它的快速解决方案）

更新 当我看到这个电话的主要问题时：

guard let currentDrawable = metalView.currentDrawable else { return }

当我的应用程序大约需要“0.013198”秒，但一段时间后只需要“5.388259e-05”秒并且一切正常。

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