ios - 使用 AVAssetWriter 录制视频 : first frames are black

Question

我正在用 AVAssetWriter 录制视频(用户也可以只切换到音频)。我在应用程序启动时开始录制。 但是第一帧是黑色(或非常暗)。当我从音频切换到视频时也会发生这种情况。 感觉 AVAssetWriter 和/或 AVAssetWriterInput 还没有准备好记录。我怎样才能避免这种情况？

我不知道这是否有用，但我也使用 GLKView 来显示视频。

func start_new_record(){
    do{
        try self.file_writer=AVAssetWriter(url: self.file_url!, fileType: AVFileTypeMPEG4)
        if video_on{
            if file_writer.canAdd(video_writer){
                file_writer.add(video_writer)
            }
        }
        if file_writer.canAdd(audio_writer){
            file_writer.add(audio_writer)
        }
    }catch let e as NSError{
        print(e)
    }
}

func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, from connection: AVCaptureConnection!){
    guard is_recording else{
        return
    }

    guard CMSampleBufferDataIsReady(sampleBuffer) else{
        print("data not ready")
        return
    }

    guard let w=file_writer else{
        print("video writer nil")
        return
    }

    if w.status == .unknown && start_recording_time==nil{
        if (video_on && captureOutput==video_output) || (!video_on && captureOutput==audio_output){
            print("START RECORDING")
            file_writer?.startWriting()
            start_recording_time=CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
            file_writer?.startSession(atSourceTime: start_recording_time!)
        }else{
            return
        }
    }

    if w.status == .failed{
        print("failed /", w.error ?? "")
        return
    }

    if captureOutput==audio_output{
        if audio_writer.isReadyForMoreMediaData{
            if !video_on || (video_on && video_written){
                audio_writer.append(sampleBuffer)
                //print("write audio")
            }
        }else{
            print("audio writer not ready")
        }
    }else if video_output != nil && captureOutput==video_output{
        if video_writer.isReadyForMoreMediaData{
            video_writer.append(sampleBuffer)
            if !video_written{
                print("added 1st video frame")
                video_written=true
            }
        }else{
            print("video writer not ready")
        }
    }
}

Answer 1

SWIFT 4

解决方案 #1:

我通过在启动应用程序时尽快调用 file_writer?.startWriting() 解决了这个问题。然后，当您想要开始录制时，执行 file_writer?.startSession(atSourceTime:...)。

当您完成录制并调用 finishRecording 时，当您收到表明完成的回调时，再次设置新的写入 session 。

解决方案#2:

我通过在调用 AVAssetWriter.startSession 时将开始时间增加半秒来解决这个问题，如下所示:

start_recording_time = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
let startingTimeDelay = CMTimeMakeWithSeconds(0.5, 1000000000)
let startTimeToUse = CMTimeAdd(start_recording_time!, startingTimeDelay)

file_writer?.startSession(atSourceTime: startTimeToUse)

解决方案 #3:

这里更好的解决方案是记录您收到并决定写入的第一帧的时间戳，然后以此开始您的 session 。那么你不需要任何延迟:

//Initialization, elsewhere:

var is_session_started = false
var videoStartingTimestamp = CMTime.invalid

// In code where you receive frames that you plan to write:

if (!is_session_started) {
    // Start writing at the timestamp of our earliest sample
    videoStartingTimestamp = currentTimestamp
    print ("First video sample received: Starting avAssetWriter Session: \(videoStartingTimestamp)")
    avAssetWriter?.startSession(atSourceTime: videoStartingTimestamp)

    is_session_started = true
}

// add the current frame
pixelBufferAdapter?.append(myPixelBuffer, withPresentationTime: currentTimestamp)