How to add a watermark to GPUImageVideoCamera

Question

I have a question about GPUImage，I want to record a video with a watermark，I can add to the video filter effect，But the added watermark does not appear。The code is as follows：

    videoCamera = GPUImageVideoCamera(sessionPreset: AVCaptureSessionPresetHigh, cameraPosition: .front)
    videoCamera.outputImageOrientation = UIInterfaceOrientation.portrait
    videoCamera.horizontallyMirrorRearFacingCamera = false
    videoCamera.horizontallyMirrorFrontFacingCamera = false
    filter = GPUImageBrightnessFilter()
    videoCamera.addTarget(filter)

    filterImageView = GPUImageView(frame: view.bounds)
    view.addSubview(filterImageView)
    filter.addTarget(filterImageView)

    // waterMark
    let imageView = UIImageView(frame: CGRect(x: 100, y: 100, width: 100, height: 100))
    imageView.image = UIImage(named: "watermark.png")

    let subView = UIView(frame: CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height))
    subView.backgroundColor = UIColor.red
    imageView.center = subView.center
    subView.addSubview(imageView)


    let uielement = GPUImageUIElement(view: subView)
    let markFilter = GPUImageAlphaBlendFilter()
    markFilter.mix = 1.0

    filter.addTarget(markFilter)
    uielement?.addTarget(filter)
    videoCamera.addTarget(markFilter)
    videoCamera.startCapture()

    uielement?.frameProcessingCompletionBlock = {filter, time in
        uielement?.update()
    }

I tried a lot of ways, can not solve this problem, if you can get the answer here, would be grateful