ios - 如何导出任意一段视频？导出最后 X 秒视频时出现 "Operation Stopped"错误

Question

目标是导出某个视频的任意片段(例如，中间三分之一、后半部分)，但 AVAssetExportSession 只有在起点是视频的开头时才会成功。

如果 cmStartTime 不为 0，AVAssetExportSession 将失败并出现此错误:

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

问题是由于不理解 CMTimeRangeMake 和 insertTimeRange 引起的。

CMTimeRangeMake 的第二个值应该是剪辑持续时间，而不是结束时间。因此，如果您的开始时间是 5 秒标记，而剪辑持续 10 秒，则第二个值应该是 10，而不是 15。

insertTimeRange 的atTime 参数应该是kCMTimeZero，因为目标是创建一个新剪辑。换句话说，此值表示在新轨道中的何处插入来自源轨道的剪辑。