暂停时实时AVAssetWriter同步音频和视频。

Question

我正在尝试使用iPhone的前置摄像头录制一段有声视频。由于我需要同时支持暂停、恢复功能，我需要使用 AVAssetWriter. 我在网上找到了一个用Objective-C编写的例子，它几乎实现了预期的功能(http:/www.gdcl.co.uk20130220iPhone-Pause.html)

不幸的是，在将这个例子转换为Swift后，我注意到，如果我暂停useresume，在每个 "部分 "结束时，会有一小段明显的时间，在这段时间里，视频只是一个静止的框架，而音频正在播放。所以，似乎当 isPaused 触发，录制的音频轨道比录制的视频轨道长。

对不起，如果它可能看起来像一个新手的问题，但我不是一个伟大的专家在 AVFoundation 和一些帮助将被感激!

下面我贴出我对 didOutput sampleBuffer.

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
    var isVideo = true
    if videoConntection != connection {
        isVideo = false
    }
    if (!isCapturing || isPaused) {
        return
    }

    if (encoder == nil) {
        if isVideo {
            return
        }
        if let fmt = CMSampleBufferGetFormatDescription(sampleBuffer) {
            let desc = CMAudioFormatDescriptionGetStreamBasicDescription(fmt as CMAudioFormatDescription)
            if let chan = desc?.pointee.mChannelsPerFrame, let rate = desc?.pointee.mSampleRate {
                let path = tempPath()!
                encoder = VideoEncoder(path: path, height: Int(cameraSize.height), width: Int(cameraSize.width), channels: chan, rate: rate)
            }
        }
    }
    if discont {
        if isVideo {
            return
        }
        discont = false
        var pts = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)
        let last = lastAudio
        if last.flags.contains(CMTimeFlags.valid) {
            if cmOffset.flags.contains(CMTimeFlags.valid) {
                pts = CMTimeSubtract(pts, cmOffset)
            }
            let off = CMTimeSubtract(pts, last)
            print("setting offset from \(isVideo ? "video":"audio")")
            print("adding \(CMTimeGetSeconds(off)) to \(CMTimeGetSeconds(cmOffset)) (pts \(CMTimeGetSeconds(cmOffset)))")
            if cmOffset.value == 0 {
                cmOffset = off
            }
            else {
                cmOffset = CMTimeAdd(cmOffset, off)
            }
        }
        lastVideo.flags = []
        lastAudio.flags = []
        return
    }
    var out:CMSampleBuffer?
    if cmOffset.value > 0 {
        var count:CMItemCount = CMSampleBufferGetNumSamples(sampleBuffer)
        let pInfo = UnsafeMutablePointer<CMSampleTimingInfo>.allocate(capacity: count)
        CMSampleBufferGetSampleTimingInfoArray(sampleBuffer, entryCount: count, arrayToFill: pInfo, entriesNeededOut: &count)
        var i = 0
        while i<count {
            pInfo[i].decodeTimeStamp = CMTimeSubtract(pInfo[i].decodeTimeStamp, cmOffset)
            pInfo[i].presentationTimeStamp = CMTimeSubtract(pInfo[i].presentationTimeStamp, cmOffset)
            i+=1
        }
        CMSampleBufferCreateCopyWithNewTiming(allocator: nil, sampleBuffer: sampleBuffer, sampleTimingEntryCount: count, sampleTimingArray: pInfo, sampleBufferOut: &out)
    }
    else {
        out = sampleBuffer
    }
    var pts = CMSampleBufferGetPresentationTimeStamp(out!)
    let dur = CMSampleBufferGetDuration(out!)
    if (dur.value > 0)
    {
        pts = CMTimeAdd(pts, dur);
    }
    if (isVideo) {
        lastVideo = pts;
    }
    else {
        lastAudio = pts;
    }
    encoder?.encodeFrame(sampleBuffer: out!, isVideo: isVideo)
}

这是我的 VideoEncoder 类。

final class VideoEncoder {
    var writer:AVAssetWriter
    var videoInput:AVAssetWriterInput
    var audioInput:AVAssetWriterInput
    var path:String

    init(path:String, height:Int, width:Int, channels:UInt32, rate:Float64) {
        self.path = path
        if FileManager.default.fileExists(atPath:path) {
            try? FileManager.default.removeItem(atPath: path)
        }
        let url = URL(fileURLWithPath: path)
        writer = try! AVAssetWriter(outputURL: url, fileType: .mp4)
        videoInput = AVAssetWriterInput(mediaType: .video, outputSettings: [
            AVVideoCodecKey: AVVideoCodecType.h264,
            AVVideoWidthKey:height,
            AVVideoHeightKey:width
        ])
        videoInput.expectsMediaDataInRealTime = true
        writer.add(videoInput)

        audioInput = AVAssetWriterInput(mediaType: .audio, outputSettings: [
            AVFormatIDKey:kAudioFormatMPEG4AAC,
            AVNumberOfChannelsKey:channels,
            AVSampleRateKey:rate
        ])
        audioInput.expectsMediaDataInRealTime = true
        writer.add(audioInput)
    }

    func finish(with completionHandler:@escaping ()->Void) {
        writer.finishWriting(completionHandler: completionHandler)
    }

    func encodeFrame(sampleBuffer:CMSampleBuffer, isVideo:Bool) -> Bool {
        if CMSampleBufferDataIsReady(sampleBuffer) {
            if writer.status == .unknown {
                writer.startWriting()
                writer.startSession(atSourceTime: CMSampleBufferGetPresentationTimeStamp(sampleBuffer))
            }
            if writer.status == .failed {
                QFLogger.shared.addLog(format: "[ERROR initiating AVAssetWriter]", args: [], error: writer.error)
                return false
            }
            if isVideo {
                if videoInput.isReadyForMoreMediaData {
                    videoInput.append(sampleBuffer)
                    return true
                }
            }
            else {
                if audioInput.isReadyForMoreMediaData {
                    audioInput.append(sampleBuffer)
                    return true
                }
            }
        }
        return false
    }
}

其余的代码应该是很明显的 但为了让它更完整，以下是我的暂停功能。

isPaused = true
discont = true

这里是简历。

isPaused = false

如果有人能帮我理解如何在现场录制过程中对齐视频和音频轨道，那就太好了！

Answer 1

好吧，原来我提供的代码没有错。我所遇到的问题是由视频平滑化引起的，而视频平滑化被打开了:) 。我想它需要额外的帧来平滑视频，这就是为什么视频输出在最后短时间内冻结的原因。