avmutablecomcomposition视频黑色开始

Question

我使用AVMutableComposition和AVAssetExportSession修剪视频下来。随机地，我的意思是随机地(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帧。音频不受影响。我可以100%确认被修剪的视频与此无关，因为来自不同来源的各种视频都会发生这种情况。

如果你能了解一下为什么这些视频一开始都是黑帧的，我将非常非常欢迎。谢谢!

相关代码:

// 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

我们的解决方案是不要尝试在同一操作中裁剪和修剪视频。关于为什么会发生这种情况，我仍然没有答案，但是我们能够通过首先修剪视频来解决这个问题，然后在我们有适当持续时间的视频后对其进行裁剪操作。

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，我们能够通过减少每个操作的工作量并将操作串在一起来解决这个问题。