AVMutableComposition视频黑启动

Question

我正在使用AVMutableComposition和AVAssetExportSession来修剪一个视频。随机的，我的意思是随机的(我不能始终如一地复制)用户的视频在修剪视频的开始有一些黑色的帧。音频不受影响。我可以确认100%的视频被裁剪与它没有任何关系，因为这发生在各种不同来源的视频中。

任何洞察为什么这些视频被输出与黑色帧在一开始是非常受欢迎的。谢谢!

一些相关代码(对长度表示抱歉)：

// AVURLAssetPreferPreciseDurationAndTimingKey added in attempt to solve issue
let videoAsset = AVURLAsset(URL: url, options: [AVURLAssetPreferPreciseDurationAndTimingKey: true])
var mixComposition = AVMutableComposition()

let compositionVideoTrack = mixComposition.addMutableTrackWithMediaType(
    AVMediaTypeVideo,
    preferredTrackID: Int32(kCMPersistentTrackID_Invalid)
)
let clipVideoTrack = videoAsset.tracksWithMediaType(AVMediaTypeVideo)[0] as! AVAssetTrack
let videoSize = clipVideoTrack.naturalSize
// startTime and duration are NSTimeInterval types
let start = startTime == 0 ? kCMTimeZero : CMTimeMakeWithSeconds(startTime, videoAsset.duration.timescale)
var dur = CMTimeMakeWithSeconds(duration, videoAsset.duration.timescale)
if dur.value >= videoAsset.duration.value {

    dur = videoAsset.duration

}
compositionVideoTrack.insertTimeRange(
    CMTimeRange(start: start, duration: dur),
    ofTrack:clipVideoTrack,
    atTime: kCMTimeZero,
    error:nil
)

compositionVideoTrack.preferredTransform = videoAsset.tracksWithMediaType(AVMediaTypeVideo)[0].preferredTransform

let compositionAudioTrack = mixComposition.addMutableTrackWithMediaType(AVMediaTypeAudio, preferredTrackID: Int32(kCMPersistentTrackID_Invalid))
let clipAudioTrack = videoAsset.tracksWithMediaType(AVMediaTypeAudio)[0] as! AVAssetTrack
compositionAudioTrack.insertTimeRange(
    CMTimeRange(start: start, duration: dur),
    ofTrack: clipAudioTrack,
    atTime: kCMTimeZero,
    error: nil
)

let parentLayer = CALayer()
parentLayer.backgroundColor = UIColor.blackColor().CGColor
let videoLayer = CALayer()
videoLayer.backgroundColor = UIColor.blackColor().CGColor
var parentFrame = CGRect(
    x: 0,
    y: 0,
    width: videoSize.width,
    height: videoSize.height
)
if parentFrame.width % 2 > 0 {
    parentFrame.size.width = parentFrame.size.width - 1
}
// Fix crop frame height
if parentFrame.size.height % 2 > 0 {
    parentFrame.size.height = parentFrame.size.height - 1
}
parentLayer.frame = parentFrame
videoLayer.frame = CGRect(
    x: 0,
    y: 0,
    width: videoSize.width,
    height: videoSize.height
)
parentLayer.addSublayer(videoLayer)

let videoComp = AVMutableVideoComposition()
videoComp.renderSize = parentLayer.frame.size
videoComp.frameDuration = CMTimeMake(1, Int32(clipVideoTrack.nominalFrameRate))
videoComp.animationTool = AVVideoCompositionCoreAnimationTool(postProcessingAsVideoLayer: videoLayer, inLayer: parentLayer)

let instruction = AVMutableVideoCompositionInstruction()
instruction.timeRange = CMTimeRange(start: kCMTimeZero, duration: mixComposition.duration)
let videoTrack = mixComposition.tracksWithMediaType(AVMediaTypeVideo)[0] as! AVAssetTrack
let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: videoTrack)

layerInstruction.setTransform(CGAffineTransformMakeScale(parentLayer.frame.size.width / videoSize.width, parentLayer.frame.size.height / videoSize.height), atTime: kCMTimeZero)
instruction.layerInstructions = [layerInstruction]
videoComp.instructions = [instruction]

// Export
let exportSession = AVAssetExportSession(
    asset: mixComposition,
    presetName: AVAssetExportPresetHighestQuality
)
exportSession.videoComposition = videoComp
let renderFileName = "video.mp4"
let renderURL = NSURL(fileURLWithPath: NSTemporaryDirectory().stringByAppendingPathComponent(renderFileName))
exportSession.outputURL = renderURL
exportSession.outputFileType = AVFileTypeQuickTimeMovie
exportSession.exportAsynchronouslyWithCompletionHandler { ... }

Answer 1

对我们来说，解决这个问题的方法是不要试图在相同的操作中裁剪视频。我仍然没有答案，为什么会发生这种情况，但我们能够解决它的第一次修剪视频的时间，然后我们得到的视频，适当的时间执行它的作物操作。

不幸的是，我认为这只是框架中的一个bug，但至少在我们的例子中，我们能够通过在每个操作中少做几个操作并将操作串在一起来解决它。