AVAssetWriter,AVAssetExportSession,视频时差

Question

我有一个Swift类,它可以拍摄图像并创建该图像的10秒视频.

---

我使用创建视频AVAssetWriter.它输出一个十秒钟的视频.这一切都按预期工作.

 func setup(){

    //Setup video size
    self.videoSize = self.getVideoSizeForImage(self.videoImage!)

    //Setup temp video path
    self.tempVideoPath = self.getTempVideoPath()

    //Setup video writer
    var videoWriter = AVAssetWriter(
        URL: NSURL(fileURLWithPath: self.tempVideoPath!),
        fileType: AVFileTypeMPEG4,
        error: nil)

    //Setup video writer input
    let videoSettings = [
        AVVideoCodecKey: AVVideoCodecH264,
        AVVideoWidthKey: self.videoSize!.width,
        AVVideoHeightKey: self.videoSize!.height
    ]
    let videoWriterInput = AVAssetWriterInput(
        mediaType: AVMediaTypeVideo,
        outputSettings: videoSettings as [NSObject : AnyObject]
    )
    videoWriterInput.expectsMediaDataInRealTime = true

    //Setup video writer adaptor
    let adaptor = AVAssetWriterInputPixelBufferAdaptor(
        assetWriterInput: videoWriterInput,
        sourcePixelBufferAttributes: [
            kCVPixelBufferPixelFormatTypeKey : kCVPixelFormatType_32ARGB,
        ]
    )
    videoWriter.addInput(videoWriterInput)

    //Setup frame time
    self.currentFrameTime = CMTimeMake(0, 10000)

    //Start video writing session
    videoWriter.startWriting()
    videoWriter.startSessionAtSourceTime(self.currentFrameTime!)

    self.currentVideoWriter = videoWriter
    self.currentVideoWriterAdaptor = adaptor
}


  func addImageToVideoWriter(image:UIImage, duration:CGFloat){

    //Get image pixel buffer
    let buffer = self.pixelBufferFromImage(image)

    var frameTime = self.currentFrameTime!

    //Add pixel buffer to video
    let adaptor = self.currentVideoWriterAdaptor!
    while(!adaptor.assetWriterInput.readyForMoreMediaData){}
    if adaptor.assetWriterInput.readyForMoreMediaData{

        adaptor.appendPixelBuffer(
            buffer,
            withPresentationTime: frameTime
        )

        let seconds = CGFloat(CMTimeGetSeconds(frameTime))+duration
        var timescale = frameTime.timescale
        var value = CGFloat(timescale) * seconds

        frameTime.value = Int64(value)
        frameTime.timescale = Int32(timescale)

        self.currentFrameTime = frameTime
        self.lastImage = image
    }

}

   func pixelBufferFromImage(image:UIImage) -> CVPixelBufferRef{

    let size = image.size

    var pixelBuffer: Unmanaged<CVPixelBuffer>?
    let bufferOptions:CFDictionary = [
        "kCVPixelBufferCGBitmapContextCompatibilityKey": NSNumber(bool: true),
        "kCVPixelBufferCGImageCompatibilityKey": NSNumber(bool: true)
    ]
    let status:CVReturn = CVPixelBufferCreate(
        nil,
        Int(size.width),
        Int(size.height),
        OSType(kCVPixelFormatType_32ARGB),
        bufferOptions,
        &pixelBuffer
    )

    let managedPixelBuffer = pixelBuffer!.takeRetainedValue()

    let lockStatus = CVPixelBufferLockBaseAddress(managedPixelBuffer, 0)

    let pixelData = CVPixelBufferGetBaseAddress(managedPixelBuffer)
    let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.PremultipliedFirst.rawValue)
    let rgbColorSpace = CGColorSpaceCreateDeviceRGB()
    let context = CGBitmapContextCreate(
        pixelData,
        Int(size.width),
        Int(size.height),
        8,
        Int(4 * size.width),
        rgbColorSpace,
        bitmapInfo
    )
    CGContextDrawImage(context, CGRectMake(0, 0, size.width, size.height), image.CGImage)

    CVPixelBufferUnlockBaseAddress(managedPixelBuffer, 0)

    return managedPixelBuffer
}


  func saveVideoWriterToDisk(){

    self.addImageToVideoWriter(self.lastImage!, duration: 0)

    self.currentVideoWriter?.endSessionAtSourceTime(self.currentFrameTime!)
    self.currentVideoWriterAdaptor?.assetWriterInput.markAsFinished()

    //Write video to disk
    let semaphore = dispatch_semaphore_create(0)
    self.currentVideoWriter?.finishWritingWithCompletionHandler({
        dispatch_semaphore_signal(semaphore)
    })
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)
}

然后我使用AVAssetExportSession并AVMutableComposition为该视频添加音乐.这大部分时间都有效.

func exportVideo(sourceFilePath:String, destinationFilePath:String) -> Bool{
    let fileManager = NSFileManager()
    var success = false

    //Compile audio and video together
    let composition = AVMutableComposition()
    /*
    //Setup audio track
    let trackAudio:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(
        AVMediaTypeAudio,
        preferredTrackID: CMPersistentTrackID()
    )

    //Add audio file to audio track
    let audioFileAsset = AVURLAsset(URL:self.audioFileURL, options:nil)
    let audioFileAssetTrack = audioFileAsset.tracksWithMediaType(AVMediaTypeAudio).last as! AVAssetTrack
    trackAudio.insertTimeRange(
        audioFileAssetTrack.timeRange,
        ofTrack: audioFileAssetTrack,
        atTime: kCMTimeZero,
        error: nil
    )*/

    //Setup video track
    let trackVideo:AVMutableCompositionTrack = composition.addMutableTrackWithMediaType(
        AVMediaTypeVideo,
        preferredTrackID: CMPersistentTrackID()
    )

    //Add video file to video track
    let videoFileAsset = AVURLAsset(URL: NSURL(fileURLWithPath: sourceFilePath), options: nil)
    let videoFileAssetTracks = videoFileAsset.tracksWithMediaType(AVMediaTypeVideo)
    if videoFileAssetTracks.count > 0{
        let videoFileAssetTrack:AVAssetTrack = videoFileAssetTracks[0] as! AVAssetTrack
        trackVideo.insertTimeRange(
            videoFileAssetTrack.timeRange,
            ofTrack: videoFileAssetTrack,
            atTime: kCMTimeZero,
            error: nil
        )
    }

    //Export compiled video to disk
    if fileManager.fileExistsAtPath(destinationFilePath){
        fileManager.removeItemAtPath(destinationFilePath, error: nil)
    }
    let exporter = AVAssetExportSession(
        asset: composition,
        presetName: AVAssetExportPresetHighestQuality
    )
    exporter.outputFileType = AVFileTypeMPEG4
    exporter.outputURL = NSURL(fileURLWithPath: destinationFilePath)
    let semaphore = dispatch_semaphore_create(0)
    exporter.exportAsynchronouslyWithCompletionHandler({
        success = true
        dispatch_semaphore_signal(semaphore)
    })
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)

    //Delete file at source path
    fileManager.removeItemAtPath(sourceFilePath, error: nil)

    return success
}

然而.

当图像非常大(就分辨率而言,即1600*1600和更大)时,使用AVAssetExportSession创建的视频为8秒而不是10秒.仅在使用大图像时才会出现时差.

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这不是一个大问题.由于人们极不可能将这个类用于大型图像.但是,我仍然想知道发生了什么以及如何解决它.