如何在Swift AVPlayer中获取特定的视频区域？

Question

我正在研究Swift基础MAC OS-X应用程序.用户将选择一个视频文件,NSView Controller将使用AVPlayerView播放视频.用户可以在AVPlayerView上绘制一个矩形,我已经实现了这个功能.如何将矩形中的选定区域视频显示到摄像机右下方的预览图层？

这是输出:

这是代码:

var videoURL = NSURL() // The video file URL by the file chooser
var videoSize = NSSize() // Here the video resolution size will be kept i.e. 720*480
var player:AVPlayerView! // The AVPlayerView to play video

var cameraPreviewLayer:AVPlayerLayer! //The right bottom camera Preview Layer
var rectangleLayer:AVPlayerLayer! // The rectangle that will draw over AVPlayer while mouse dragging

var isClicked = false
var startPoint = NSPoint() // holds the starting point location of mouse pointer while draging

var rect:NSRect!

override func viewDidLoad() {
    super.viewDidLoad()

    self.player = AVPlayerView()

    self.player.frame.origin = CGPoint(x: 0, y: 0)
    self.player.setFrameSize(self.videoSize)

    self.player.player = AVPlayer.playerWithURL(videoURL) as! AVPlayer
    self.view.addSubview(self.player)

    setupRectangle()
    setupCameraPreviewLayer()
}

// intially setup white color rectangle that will draged over AVPlayer
func setupRectangle() {
    self.rectangleLayer = AVPlayerLayer()
    self.rectangleLayer.backgroundColor = NSColor.clearColor().CGColor
    self.rectangleLayer.borderColor = NSColor.whiteColor().CGColor
    self.rectangleLayer.borderWidth = 1.5
    self.rectangleLayer.frame = self.player.bounds
}
// intially setup CameraPreview Layer that will show at right bottom of AVPlayer
func setupCameraPreviewLayer(){
    cameraPreviewLayer = AVPlayerLayer(player: self.player.player)

    //place the cameraPreview layer at right bottom
    cameraPreviewLayer.frame.origin.x = self.player.framesize.width-100
    cameraPreviewLayer.frame.origin.y = self.player.frame.origin.y-100
    cameraPreviewLayer.frame.size.width = 100
    cameraPreviewLayer.frame.size.height = 100
}

override func  mouseDown(theEvent: NSEvent) {
    startPoint = theEvent.locationInWindow
    isClicked = true
    removeCameraPreviewLayer()
}

override func mouseDragged(theEvent: NSEvent) {
    var endPoint = theEvent.locationInWindow

    if( isClicked ){
        rect = NSRect(x: startPoint.x, y: startPoint.y, width: -(startPoint.x-endPoint.x), height: -(startPoint.y-endPoint.y))
        drawCustomRect(rect)
    }
}

override func  mouseUp(theEvent: NSEvent) {
    var endPoint = theEvent.locationInWindow

    if( isClicked ){

// I think we have to do some magic code here
        addCameraPreviewLayer()
    }
    isClicked = false;
}

// redraw the white color rectange over avplayer
func drawCustomRect(rect: NSRect) {
    self.rectangleLayer.frame = rect
    self.player.layer?.addSublayer(self.rectangleLayer)
}

// add Camera PreviewLayer from AVPlayer
func addCameraPreviewLayer() {
    self.player.layer?.addSublayer(self.layer)
}

// remove Camera PreviewLayer from AVPlayer
func removeCameraPreviewLayer() {
    self.cameraPreviewLayer.removeFromSuperlayer()
}

这是我想要的输出图片.

假设视频的大小为720*480,用户绘制一个矩形,其点为(x1,y1)(x2,y1)(x3,y3)(x4,y4).如何在相机预览图层(右下角)中裁剪视频,该视频显示的区域视频与用户选择的矩形相同？

任何人都可以帮助如何实现这一功能？我已经花了很多天在这上面并且筋疲力尽.

注意:我可以通过使用ROI的视频处理在open-CV中完成,但要求是使用原生SWIFT语言.