AVAssetWriterInput - black screen, 46 hours long

Question

I'm trying to take an AVCaptureSession and encode to mp4. It seems like this should be straightforward, and I'm trying to encode a single 960x540 video stream; I'm not worried about audio for the purpose of this issue.

When I run the following code and grab out2.mp4 out of the documents container with Xcode, I get a black screen in quicktime and the duration is 46 hours long. At least the resolution looks right. Here's output from ffmpeg -i out2.mp4

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'out2.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 1
    compatible_brands: mp41mp42isom
    creation_time   : 2015-11-18 01:25:55
  Duration: 46:43:04.21, start: 168178.671667, bitrate: 0 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, smpte170m/bt709/bt709), 960x540, 1860 kb/s, 27.65 fps, 29.97 tbr, 600 tbn, 1200 tbc (default)
    Metadata:
      creation_time   : 2015-11-18 01:25:55
      handler_name    : Core Media Video

Why can't I append sample buffers to the AVAssetWriterInput in this scenario?

var videoInput: AVAssetWriterInput?
var assetWriter: AVAssetWriter?

override func viewDidLoad() {
    super.viewDidLoad()
    self.startStream()
    NSTimer.scheduledTimerWithTimeInterval(5, target: self, selector: "swapSegment", userInfo: nil, repeats: false)
}

func swapSegment() {
    assetWriter?.finishWritingWithCompletionHandler(){
        print("File written")
    }
    videoInput = nil
}

func pathForOutput() -> String {
    let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
    if let documentDirectory: NSURL = urls.first {
        let fileUrl = documentDirectory.URLByAppendingPathComponent("out1.mp4")
        return fileUrl.path!
    }
    return ""
}

func startStream() {
    assetWriter = try! AVAssetWriter(URL: NSURL(fileURLWithPath: self.pathForOutput()), fileType: AVFileTypeMPEG4)

    let videoSettings: [String: AnyObject] = [AVVideoCodecKey: AVVideoCodecH264, AVVideoWidthKey: 960, AVVideoHeightKey: 540]
    videoInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: videoSettings)
    videoInput!.expectsMediaDataInRealTime = true
    assetWriter?.addInput(videoInput!)
    assetWriter!.startWriting()
    assetWriter!.startSessionAtSourceTime(kCMTimeZero)

    let videoHelper = VideoHelper()
    videoHelper.delegate = self
    videoHelper.startSession()
}

func captureOutput(captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBufferRef, fromConnection connection: AVCaptureConnection!) {
    if let videoOutput = captureOutput as? AVCaptureVideoDataOutput {
        videoInput?.appendSampleBuffer(sampleBuffer)
    }
}

score 5 · Accepted Answer · answered Nov 18 '15 at 02:12

5

Maybe your presentation times are not relative to your sourceTime (kCMTimeZero). You could use the first buffer presentation time stamp as the source time.

p.s. maybe 46hrs is approximately your device uptime

answered Nov 18 '15 at 02:12

Rhythmic Fistman

34,352
5
87
159

Even if with if `!sessionStarted { sessionStarted = true assetWriter!.startSessionAtSourceTime(CMSampleBufferGetPresentationTimeStamp(sampleBuffer)) }`, I get a duration of 46 hours. – Stefan Kendall Nov 18 '15 at 14:56
Never mind, it works! Make sure to delete the file you're writing to before trying to write to it. – Stefan Kendall Nov 18 '15 at 15:17

AVAssetWriterInput - black screen, 46 hours long

1 Answers1