Convert VNRectangleObservation points to other coordinate system

Question

I need to convert the VNRectangleObservation received CGPoints (bottomLeft, bottomRight, topLeft, topRight) to another coordinate system (e.g. a view's coordinate on screen).

I define a request:

    // Rectangle Request
    let rectangleDetectionRequest = VNDetectRectanglesRequest(completionHandler: handleRectangles)
    rectangleDetectionRequest.minimumSize = 0.5
    rectangleDetectionRequest.maximumObservations = 1

I get the sampleBuffer from camera in delegate call, and perform a detection request:

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {

    guard let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else {return}
    var requestOptions:[VNImageOption:Any] = [:]
    if let cameraIntrinsicData = CMGetAttachment(sampleBuffer, kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix, nil) {
        requestOptions = [.cameraIntrinsics:cameraIntrinsicData]
    }
    let imageRequestHandler = VNImageRequestHandler(cvPixelBuffer: pixelBuffer, orientation: CGImagePropertyOrientation(rawValue:6)!, options: requestOptions)
    do {
        try imageRequestHandler.perform(self.requests)
    } catch {
        print(error)
    }

}

Later in completionHandler I receive the results:

func handleRectangles (request:VNRequest, error:Error?) {

     guard let results = request.results as? [VNRectangleObservation] else { return }

     let flipTransform = CGAffineTransform(scaleX: 1, y: -1).translatedBy(x: 0, y: -self.previewView.frame.height)
     let scaleTransform = CGAffineTransform.identity.scaledBy(x: self.previewView.frame.width, y: self.previewView.frame.height)

     for rectangle in results {
        let rectangleBounds = rectangle.boundingBox.applying(scaleTransform).applying(flipTransform)
        // convertedTopLeft = conversion(rectangle.topLeft) 
        // convertedTopRight = conversion(rectangle.topRight) 
        // convertedBottomLeft = conversion(rectangle.bottomLeft) 
        // convertedBottomRight = conversion(rectangle.bottomRight) 
    }
}

This works for boundingBox which is CGRect, but I need to transform the CGPoints instead to a coordinate system of another view. The problem is that I don't know how to get the transformation from the sampleBuffer: CMSampleBuffer's coordinate system to the previewView coordinate system.

Thanks!

Answer 1

That was simply a matter of applying the transform to the CGPoint itself where size is the CGSize of the destination view for which I need transpose the four points.

    let transform = CGAffineTransform.identity
        .scaledBy(x: 1, y: -1)
        .translatedBy(x: 0, y: -size.height)
        .scaledBy(x: size.width, y: size.height)

    let convertedTopLeft = rectangle.topLeft.applying(transform)
    let convertedTopRight = rectangle.topRight.applying(transform)
    let convertedBottomLeft = rectangle.bottomLeft.applying(transform)
    let convertedBottomRight = rectangle.bottomRight.applying(transform)

Answer 2

@mihaicris answer works, but only in portrait mode. In landscape, we need to do it a little different.

if UIApplication.shared.statusBarOrientation.isLandscape {
    transform = CGAffineTransform.identity
        .scaledBy(x: -1, y: 1)
        .translatedBy(x: -size.width, y: 0)
        .scaledBy(x: size.width, y: size.height)
} else {
    transform = CGAffineTransform.identity
        .scaledBy(x: 1, y: -1)
        .translatedBy(x: 0, y: -size.height)
        .scaledBy(x: size.width, y: size.height)
}

let convertedTopLeft = rectangle.topLeft.applying(transform)
let convertedTopRight = rectangle.topRight.applying(transform)
let convertedBottomLeft = rectangle.bottomLeft.applying(transform)
let convertedBottomRight = rectangle.bottomRight.applying(transform)