When making an AVFoundation video copy, how do you only add particular ranges of the original video for which there exist trajectories

I've been looking through Apple's sample code Building a Feature-Rich App for Sports Analysis and its associated WWDC video to learn to reason about AVFoundation and VNDetectTrajectoriesRequest. My goal is to allow the user to import videos (this part I have working, the user sees a UIDocumentBrowserViewController, picks a video file, and then a copy is made), but I only want segments of the original video copied where trajectories are detected from a ball moving.

I've tried as best I can to grasp the two parts, at the very least finding where the video copy is made and where the trajectory request is made.

The full video copy happens in CameraViewController.swift (I'm starting with just imported video for now and not reading live from the device's video camera), line 160:

Code Block 
func startReadingAsset(_ asset: AVAsset) {
		videoRenderView = VideoRenderView(frame: view.bounds)
		setupVideoOutputView(videoRenderView)
		
		let displayLink = CADisplayLink(target: self, selector: #selector(handleDisplayLink(_:)))
		displayLink.preferredFramesPerSecond = 0
		displayLink.isPaused = true
		displayLink.add(to: RunLoop.current, forMode: .default)
		
		guard let track = asset.tracks(withMediaType: .video).first else {
				AppError.display(AppError.videoReadingError(reason: "No video tracks found in AVAsset."), inViewController: self)
				return
		}
		
		let playerItem = AVPlayerItem(asset: asset)
		let player = AVPlayer(playerItem: playerItem)
		let settings = [
				String(kCVPixelBufferPixelFormatTypeKey): kCVPixelFormatType_420YpCbCr8BiPlanarFullRange
		]
		let output = AVPlayerItemVideoOutput(pixelBufferAttributes: settings)
		playerItem.add(output)
		player.actionAtItemEnd = .pause
		player.play()

self.displayLink = displayLink
self.playerItemOutput = output
self.videoRenderView.player = player

Code Block 
		let affineTransform = track.preferredTransform.inverted()
		let angleInDegrees = atan2(affineTransform.b, affineTransform.a) * CGFloat(180) / CGFloat.pi
		var orientation: UInt32 = 1
		switch angleInDegrees {
		case 0:
				orientation = 1 // Recording button is on the right
		case 180, -180:
				orientation = 3 // abs(180) degree rotation recording button is on the right
		case 90:
				orientation = 8 // 90 degree CW rotation recording button is on the top
		case -90:
				orientation = 6 // 90 degree CCW rotation recording button is on the bottom
		default:
				orientation = 1
		}
		videoFileBufferOrientation = CGImagePropertyOrientation(rawValue: orientation)!
		videoFileFrameDuration = track.minFrameDuration
		displayLink.isPaused = false
}
@objc
private func handleDisplayLink(_ displayLink: CADisplayLink) {
		guard let output = playerItemOutput else {
				return
		}
		
		videoFileReadingQueue.async {
				let nextTimeStamp = displayLink.timestamp + displayLink.duration
				let itemTime = output.itemTime(forHostTime: nextTimeStamp)
				guard output.hasNewPixelBuffer(forItemTime: itemTime) else {
						return
				}
				guard let pixelBuffer = output.copyPixelBuffer(forItemTime: itemTime, itemTimeForDisplay: nil) else {
						return
				}
				// Create sample buffer from pixel buffer
				var sampleBuffer: CMSampleBuffer?
				var formatDescription: CMVideoFormatDescription?
				CMVideoFormatDescriptionCreateForImageBuffer(allocator: nil, imageBuffer: pixelBuffer, formatDescriptionOut: &formatDescription)
				let duration = self.videoFileFrameDuration
				var timingInfo = CMSampleTimingInfo(duration: duration, presentationTimeStamp: itemTime, decodeTimeStamp: itemTime)
				CMSampleBufferCreateForImageBuffer(allocator: nil,
																					 imageBuffer: pixelBuffer,
																					 dataReady: true,
																					 makeDataReadyCallback: nil,
																					 refcon: nil,
																					 formatDescription: formatDescription!,
																					 sampleTiming: &timingInfo,
																					 sampleBufferOut: &sampleBuffer)
				if let sampleBuffer = sampleBuffer {
						self.outputDelegate?.cameraViewController(self, didReceiveBuffer: sampleBuffer, orientation: self.videoFileBufferOrientation)
						DispatchQueue.main.async {
								let stateMachine = self.gameManager.stateMachine
								if stateMachine.currentState is GameManager.SetupCameraState {
										// Once we received first buffer we are ready to proceed to the next state
										stateMachine.enter(GameManager.DetectingBoardState.self)
								}
						}
				}
		}
}

Line 139 self.outputDelegate?.cameraViewController(self, didReceiveBuffer: sampleBuffer, orientation: self.videoFileBufferOrientation) is where the video sample buffer is passed to the Vision framework subsystem for analyzing trajectories, the second part. This delegate callback is implemented in GameViewController.swift on line 335:

Code Block 
				// Perform the trajectory request in a separate dispatch queue.
				trajectoryQueue.async {
						do {
								try visionHandler.perform([self.detectTrajectoryRequest])
								if let results = self.detectTrajectoryRequest.results {
										DispatchQueue.main.async {
												self.processTrajectoryObservations(controller, results)
										}
								}
						} catch {
								AppError.display(error, inViewController: self)
						}
				}

Trajectories found are drawn over the video in self.processTrajectoryObservations(controller, results).

Where I'm stuck now is modifying this so that instead of drawing the trajectories, the new video only copies parts of the original video to it where trajectories were detected in the frame.