How to export arbitrary segment of video? “Operation Stopped” error when exporting last X seconds of video

Question

The goal is to export an arbitrary segment of some video (eg, middle third, last half), but AVAssetExportSession only succeeds if the starting point is the start of the video.

If cmStartTime is not 0, AVAssetExportSession fails with this error:

Failed: Optional(Error Domain=AVFoundationErrorDomain Code=-11841 "Operation Stopped" UserInfo=0x175872d00 {NSLocalizedDescription=Operation Stopped, NSLocalizedFailureReason=The video could not be composed.}).

    // Create main composition & its tracks
    let mainComposition = AVMutableComposition()
    let compositionVideoTrack = mainComposition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))
    let compositionAudioTrack = mainComposition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: CMPersistentTrackID(kCMPersistentTrackID_Invalid))

    // Get source video & audio tracks
    let videoURL = NSURL(fileURLWithPath: fileURL)
    let videoAsset = AVURLAsset(URL: videoURL, options: nil)
    let sourceVideoTrack = videoAsset.tracksWithMediaType(AVMediaTypeVideo)[0]
    let sourceAudioTrack = videoAsset.tracksWithMediaType(AVMediaTypeAudio)[0]

    // Define time values for video
    let timescale = Int32(600)
    let cmStartTime = CMTimeMake(Int64(CGFloat(0.5) * CGFloat(timescale)), timescale)
    let cmEndTime = CMTimeMake(10, 1)
    let timeRange = CMTimeRangeMake(cmStartTime, cmEndTime)

    // Add source tracks to composition
    do {
        try compositionVideoTrack.insertTimeRange(timeRange, ofTrack: sourceVideoTrack, atTime: cmStartTime)
        try compositionAudioTrack.insertTimeRange(timeRange, ofTrack: sourceAudioTrack, atTime: cmStartTime)
    } catch {
        printError("Error with insertTimeRange while exporting video: \(error)")
    }

    // Create video composition
    let renderSize = compositionVideoTrack.naturalSize
    let videoComposition = AVMutableVideoComposition()
    videoComposition.renderSize = renderSize
    videoComposition.frameDuration = CMTimeMake(Int64(1), Int32(frameRate))

    // Add layer instruction to video composition
    ...

    // Apply effects to video
    ...

    // Define export URL
    let exportPath = getUniqueTempPath(gMP4File)
    let exportURL = NSURL(fileURLWithPath: exportPath)

    // Create exporter
    let exporter = AVAssetExportSession(asset: mainComposition, presetName: AVAssetExportPresetHighestQuality)!
    exporter.videoComposition = videoComposition
    exporter.outputFileType = AVFileTypeMPEG4
    exporter.outputURL = exportURL
    exporter.shouldOptimizeForNetworkUse = true
    exporters.append(exporter)

    // Export video
    exporter.exportAsynchronouslyWithCompletionHandler() {
        // Finish stuff
    }

Answer 1

The problem arose from not understanding CMTimeRangeMake and insertTimeRange .

The second value of CMTimeRangeMake should be the clip duration, not the end time. So if your start time is the 5 second mark, and the clip lasts 10 seconds, the second value should be 10, not 15.

The atTime parameter of insertTimeRange should be kCMTimeZero since the goal is to create a new clip. In other words, this value says where in the new track to insert the clip from the source track.