Lossless Video Cut using AVAssetWriter

SwiftLosslessCut.swift

let source = URL(fileURLWithPath: "input.mov")
let destination = URL(fileURLWithPath: "output.mov")
try? FileManager.default.removeItem(at: destination)

let asset = AVAsset(url: source)
let reader = try! AVAssetReader(asset: asset)
let writer = try! AVAssetWriter(outputURL: destination, fileType: .mov)

let videoTrack = await asset.getFirstTrack(withMediaType: .video)

let videoOutput = AVAssetReaderTrackOutput(track: videoTrack!, outputSettings: nil)
let videoInput = AVAssetWriterInput(mediaType: .video, outputSettings: nil)

reader.add(videoOutput)
writer.add(videoInput)

videoInput.transform = videoTrack!.fixedPreferredTransform

// Time Range and Duration
let timeScale = videoTrack!.naturalTimeScale
let startTime: CMTime = CMTime(seconds: 3, preferredTimescale: timeScale)
let endTime: CMTime = CMTime(seconds: 5, preferredTimescale: timeScale) // asset.duration.seconds
reader.timeRange = CMTimeRange(start: startTime, end: endTime)

writer.shouldOptimizeForNetworkUse = true

reader.startReading()
writer.startWriting()
// writer.startSession(atSourceTime: .zero)
writer.startSession(atSourceTime: startTime)

let semathore = DispatchSemaphore(value: 0)
let videoQueue = DispatchQueue(label: "LosslessVideoCutQueue")
videoInput.requestMediaDataWhenReady(on: videoQueue) {
    while videoInput.isReadyForMoreMediaData {
        if let sampleBuffer = videoOutput.copyNextSampleBuffer() {
            // If starting session from .zero is required in your case (`writer.startSession(atSourceTime: .zero)`) use
            /*let presentationTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
            let newPresentationTime = CMTimeSubtract(presentationTime, startTime)

            let status = CMSampleBufferSetOutputPresentationTimeStamp(sampleBuffer, newValue: newPresentationTime)
            if status == noErr {
                videoInput.append(sampleBuffer)
            }*/
            // instead of:
            videoInput.append(sampleBuffer)
        } else {
            videoInput.markAsFinished()

            writer.finishWriting {
                if writer.status == .completed {
                    semathore.signal()
                } else {
                    fatalError(writer.error?.localizedDescription ?? "Unknown error")
                }
            }
        }
    }
}

semathore.wait()