Question

我正在尝试使用iPhone的前置摄像头录制带有声音的视频。因为我还需要支持暂停/恢复功能，所以我需要使用AVAssetWriter。我在网上找到了一个用Objective-C编写的示例，该示例几乎可以实现所需的功能（http://www.gdcl.co.uk/2013/02/20/iPhone-Pause.html）

不幸的是，在将本示例转换为Swift之后，我注意到如果我暂停/继续播放，则在每个“部分”的末尾都有一个很小但很明显的时间段，在此期间，视频只是静止帧，而音频正在播放。因此，似乎在触发isPaused时，录制的音频轨道比录制的视频轨道长。

很抱歉，如果这看起来像是一个菜鸟问题，但我不是AVFoundation的专家，可以提供一些帮助！

下面，我发布我的didOutput sampleBuffer的实现。

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课：

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
    }
}

其余的代码应该很明显，但是为了使它完整，这是我要暂停的内容：

isPaused = true
discont = true

这是简历：

isPaused = false

如果有人能帮助我了解如何在这样的实时录制过程中对齐视频和音频轨道，那就太好了！

Answer 1

好吧，事实证明，我提供的代码没有错误。我遇到的问题是由打开视频平滑功能引起的：)我猜想它需要额外的帧来平滑视频，这就是为什么视频输出在短时间内最终冻结的原因。

暂停/恢复时，实时AVAssetWriter同步音频和视频

1 个答案: