AVMutableComposition Video Black at Start - ios

Question

I'm using AVMutableComposition and AVAssetExportSession to trim a video down. Randomly, and I mean randomly (I cannot consistently reproduce) users' videos have a few black frames at the start of the trimmed video. The audio is unaffected. I can confirm 100% that the videos being trimmed don't have anything to do with it, as this happens for a wide variety of videos from all different sources.
Any insight into why these videos are being exported with black frames in the start would be very very welcome. Thanks!
Some relevant code (sorry for the length):
// 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

The solution to this for us was to not try to crop and trim the video in the same operation. I still don't have an answer as to why this was happening, but we were able to resolve it by first trimming the video for time, then after we had the video with the proper duration performing the crop operation on it.
Unfortunately I believe this is just a bug in framework, but at least in our case we were able to solve it by doing less in each operation and just stringing operations together.