ios - 如何修改 AVFoundation 视频副本(多次拆分和片段删除)？

Question

我一直在查看 Apple 的示例代码 Building a Feature-Rich App for Sports Analysis及其相关的 WWDC 视频，学习推理 AVFoundation 和 VNDetectTrajectoriesRequest .我的目标是允许用户导入视频(这部分我在工作，用户看到一个 UIDocumentBrowserViewController ，选择一个视频文件，然后制作一个副本)，但我只想要原始视频的片段复制到轨迹从移动的球中检测到。

这两部分我已经尽量掌握了，最起码找到了视频拷贝的位置和轨迹请求的位置。

完整的视频副本发生在 CameraViewController.swift 中(我现在只从导入的视频开始，而不是从设备的摄像机实时读取)，第 160 行:

func startReadingAsset(_ asset: AVAsset) {
    videoRenderView = VideoRenderView(frame: view.bounds)
    setupVideoOutputView(videoRenderView)
    
    // Setup display link
    let displayLink = CADisplayLink(target: self, selector: #selector(handleDisplayLink(_:)))
    displayLink.preferredFramesPerSecond = 0 // Use display's rate
    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

    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)
                }
            }
        }
    }
}

139行 self.outputDelegate?.cameraViewController(self, didReceiveBuffer: sampleBuffer, orientation: self.videoFileBufferOrientation) 是将视频样本缓冲区传递给Vision框架子系统用于分析轨迹的地方，第二个部分。此委托(delegate)回调在第 335 行的 GameViewController.swift 中实现:

        // 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)
            }
        }

找到的轨迹绘制在 self.processTrajectoryObservations(controller, results) 中的视频上。

我现在遇到的问题是修改它，这样新视频就不会绘制轨迹，而是只将原始视频的一部分复制到在帧中检测到轨迹的地方。

Answer 1

如果您知道当前视频的秒数和持续时间，您可以使用 AVAssetExportSession 转码/导出原始视频的部分内容。

这是我几年前用来做类似事情的一段代码。它是 swift 3，所以语法可能与现在略有不同。

let asset = AVURLAsset(url: originalFileURL)
let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetHighestQuality)!

exportSession.outputURL = someOutputURLYouWant
exportSession.outputFileType = AVFileTypeMPEG4 // can choose other types than mp4 if you want

let start = CMTimeMakeWithSeconds(secondsIntoVideoFloat, 600)
let duration = CMTimeMakeWithSeconds(numberOfSecondsFloat, 600)
exportSession.timeRange = CMTimeRange(start, duration)

exportSession.exportAsynchronously {
  switch exportSession.status {
    // handle completed, failed, cancelled states.
  }
}