UIImagesの配列をビデオに変換する際の黒いフレーム

UIImageの配列をビデオに変換しようとしていますが、結果のファイルに多くの黒いフレームがあります（たとえば、最初に4つの黒いフレーム、その後に3つの良いフレーム、その後に3つの黒いフレームと2つの良いフレームなど）このパターンは、ビデオの最後まで繰り返されます）。

私のコードはこのソリューションに基づいていますが、問題の主な原因はコードのこの部分にあるはずです。

func build(progress: (NSProgress -> Void), success: (NSURL -> Void), failure: (NSError -> Void)) {
    //videosizes and path to temp output file
    let inputSize = CGSize(width: 568, height: 320)
    let outputSize = CGSize(width: 568, height: 320)
    var error: NSError?
    let documentsPath = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as! NSString
    let videoOutputURL = NSURL(fileURLWithPath: documentsPath.stringByAppendingPathComponent("TempVideo.mov"))!
    NSFileManager.defaultManager().removeItemAtURL(videoOutputURL, error: nil)

    videoWriter = AVAssetWriter(URL: videoOutputURL, fileType: AVFileTypeMPEG4, error: &error)

    if let videoWriter = videoWriter {
        let videoSettings: [NSObject : AnyObject] = [
            AVVideoCodecKey  : AVVideoCodecH264,
            AVVideoWidthKey  : outputSize.width,
            AVVideoHeightKey : outputSize.height,
        ]

        let videoWriterInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: videoSettings)
        let pixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(
            assetWriterInput: videoWriterInput,
            sourcePixelBufferAttributes: [
                kCVPixelBufferPixelFormatTypeKey : kCVPixelFormatType_32ARGB,
                kCVPixelBufferWidthKey : inputSize.width,
                kCVPixelBufferHeightKey : inputSize.height,
            ]
        )

        assert(videoWriter.canAddInput(videoWriterInput))
        videoWriter.addInput(videoWriterInput)

        if videoWriter.startWriting() {
            videoWriter.startSessionAtSourceTime(kCMTimeZero)
            assert(pixelBufferAdaptor.pixelBufferPool != nil)

            let media_queue = dispatch_queue_create("mediaInputQueue", nil)

            videoWriterInput.requestMediaDataWhenReadyOnQueue(media_queue, usingBlock: { () -> Void in
                let fps: Int32 = 30
                let frameDuration = CMTimeMake(1, fps)
                let currentProgress = NSProgress(totalUnitCount: Int64(self.photoURLs.count))

                var frameCount: Int64 = 0

                for var i = 0; i < self.photoURLs.count - 1; i++ {

                    var currentFrame = self.photoURLs[i]
                    var lastFrameTime = CMTimeMake(Int64(i), fps)                       
                    var presentationTime = CMTimeAdd(lastFrameTime, frameDuration)

                    //this one is needed because sometimes videoWriter is not ready, and we have to wait for a while
                    while videoWriterInput.readyForMoreMediaData == false {
                        var maxDate = NSDate(timeIntervalSinceNow: 0.5)
                        var currentRunLoop = NSRunLoop()
                        currentRunLoop.runUntilDate(maxDate)

                    }

                    self.appendPixelBufferForImageAtURL(currentFrame, pixelBufferAdaptor: pixelBufferAdaptor, presentationTime: presentationTime)

                    frameCount++
                    currentProgress.completedUnitCount = frameCount
                    progress(currentProgress)

                }

                videoWriterInput.markAsFinished()
                videoWriter.finishWritingWithCompletionHandler { () -> Void in
                    if error == nil {
                        success(videoOutputURL)
                    }
                }
            })
        } else {
            error = NSError(
                domain: kErrorDomain,
                code: kFailedToStartAssetWriterError,
                userInfo: ["description": "AVAssetWriter failed to start writing"]
            )
        }
    }

    if let error = error {
        failure(error)
    }
}

明らかに私は何か間違ったことをしていますが、何ですか？一部の画像は変換に問題がないため、ここにあるはずですが、pixelbufferにはさらに2つの関数があります。

func appendPixelBufferForImageAtURL(image: UIImage, pixelBufferAdaptor: AVAssetWriterInputPixelBufferAdaptor, presentationTime: CMTime) -> Bool {
    var appendSucceeded = true

    autoreleasepool {


                var pixelBuffer: Unmanaged<CVPixelBuffer>?
                let status: CVReturn = CVPixelBufferPoolCreatePixelBuffer(
                    kCFAllocatorDefault,
                    pixelBufferAdaptor.pixelBufferPool,
                    &pixelBuffer
                )

                if let pixelBuffer = pixelBuffer where status == 0 {
                    let managedPixelBuffer = pixelBuffer.takeRetainedValue()

                    fillPixelBufferFromImage(image, pixelBuffer: managedPixelBuffer)

                    appendSucceeded = pixelBufferAdaptor.appendPixelBuffer(
                        managedPixelBuffer,
                        withPresentationTime: presentationTime
                    )
                } else {
                    NSLog("error: Failed to allocate pixel buffer from pool")
                }

    }

    return appendSucceeded
}

func fillPixelBufferFromImage(image: UIImage, pixelBuffer: CVPixelBufferRef) {

    let imageData = CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage))
    let lockStatus:UInt8 = UInt8(CVPixelBufferLockBaseAddress(pixelBuffer, 0))

    let pixelData = CVPixelBufferGetBaseAddress(pixelBuffer)
    let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.PremultipliedFirst.rawValue)
    let rgbColorSpace = CGColorSpaceCreateDeviceRGB()

    let context = CGBitmapContextCreate(
        pixelData,
        Int(568),
        Int(320),
        8,
        Int(8 * 320),
        rgbColorSpace,
        bitmapInfo
    )



    var imageDataProvider = CGDataProviderCreateWithCFData(imageData)
    var imageRef = CGImageCreateWithJPEGDataProvider(imageDataProvider, nil, true, kCGRenderingIntentDefault)

    CGContextDrawImage(context, CGRectMake(0, 0, 568, 320), imageRef)

    CVPixelBufferUnlockBaseAddress(pixelBuffer, 0)
}