AVCaptureMultiCamSession で取得したカメラの映像をMetalで合成&表示する

override func viewDidLoad() {
    super.viewDidLoad()

    // マルチカメラは A12X か A12 を搭載している端末でしか使えません
    guard AVCaptureMultiCamSession.isMultiCamSupported else {
      assertionFailure("not supported")
      return
    }

    configure()

    // 設定が終わったら startRunning でスタート
    session.startRunning()
}

func configure() {
    // 設定は beginConfiguration と commitConfiguration の間で行う
    session.beginConfiguration()
    configureBackCamera()
    configureFrontCamera()
    session.commitConfiguration()
}

func configureBackCamera() {
    // デバイスを初期化
    guard let device = AVCaptureDevice.default(.builtInWideAngleCamera, for: .video, position: .back),

    // 入力をセッションに追加
    let input = try? AVCaptureDeviceInput(device: device) else { return }
    if session.canAddInput(input) {
        session.addInputWithNoConnections(input)
    }

    // 出力をセッションに追加
    backCameraVideoDataOutput.videoSettings = [kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA]
    backCameraVideoDataOutput.setSampleBufferDelegate(self, queue: outputQueue)
    if session.canAddOutput(backCameraVideoDataOutput) {
        session.addOutputWithNoConnections(backCameraVideoDataOutput)
    }

    // 入力と出力を接続
    let port = input.ports(for: .video, sourceDeviceType: device.deviceType, sourceDevicePosition: device.position)
    let connection = AVCaptureConnection(inputPorts: port, output: backCameraVideoDataOutput)
    connection.videoOrientation = .portrait

    if session.canAddConnection(connection) {
        session.addConnection(connection)
    }
}

func configureFrontCamera() {
    // configureBackCameraと同様なので省略
}

if output == backCameraVideoDataOutput {
    // リアカメラ
}
else if output == frontCameraVideoDataOutput {
    // フロントカメラ
}

let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)

if output == backCameraVideoDataOutput {
    guard let currentFrontBuffer = currentFrontBuffer else { return }

    // captureOutput(_:didOutput:from:) は指定したスレッドで実行されるので、描画処理はメインスレッドへ
    DispatchQueue.main.async {
        // pixelBuffer と currentFrontBuffer を合成&描画
    }
}
else if output == frontCameraVideoDataOutput {
    currentFrontBuffer = pixelBuffer
}

let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)

func createMetalTexture(from buffer: CVPixelBuffer) -> MTLTexture? {
    guard let textureCache = textureCache else { return nil }

    var cvMetalTexture: CVMetalTexture?
    CVMetalTextureCacheCreateTextureFromImage(kCFAllocatorDefault,
                                              textureCache,
                                              pixelBuffer,
                                              nil,
                                              .bgra8Unorm,
                                              CVPixelBufferGetWidth(pixelBuffer),
                                              CVPixelBufferGetHeight(pixelBuffer),
                                              0,
                                              &cvMetalTexture)

    guard let texture = cvMetalTexture else { return nil }

    return CVMetalTextureGetTexture(texture)
}

kernel void mix(texture2d<half, access::read> mainTexture [[ texture(0) ]],
                texture2d<half, access::sample> subTexture [[ texture(1) ]],
                texture2d<half, access::write> outputTexture [[ texture(2) ]],
                uint2 id [[thread_position_in_grid]]) {
    float scale = 0.25;
    float2 origin = float2(50, 100);
    float2 size = float2(mainTexture.get_width(), mainTexture.get_height()) * scale;

    half4 output;

    if ((id.x >= origin.x && id.x <= origin.x + size.x) &&
        (id.y >= origin.y && id.y <= origin.y + size.y)) {
        constexpr sampler textureSampler (filter::linear, coord::pixel);
        float2 sampleCoord = (float2(id) - origin)/scale;
        output = subTexture.sample(textureSampler, sampleCoord);
    } else {
        output = mainTexture.read(id);
    }

    outputTexture.write(output, id);
}

override func draw(_ rect: CGRect) {
    ・・・

    let currentDrawable = currentDrawable else { return }

    let encoder = commandBuffer?.makeComputeCommandEncoder()
    encoder?.setComputePipelineState(pipelineState)
    encoder?.setTexture(main, index: 0)
    encoder?.setTexture(sub, index: 1)
    encoder?.setTexture(currentDrawable.texture, index: 2)

    ・・・