MediaElement应用之顺畅播放实时流

　　在以前的一篇文章中介绍过《MediaElement播放YUV实时流数据技巧 》，通过重载 MediaStreamSource 来解决视频监控情形下的实时流的播放问题。

　　但最后有个问题一直没能解决，当时采用了“取巧”的方式：　　　

element.Pause();            
streamSource.AddStream(stream);            
element.Play();

　　年前又在研究Silverlight这方面的内容，在同事的提点下才知道是自己孤陋寡闻了，算是解决了这一问题。现在对其进行简要的说明：

　　首先了解一个概念：时间戳——Time Stamp，在音视频领域主要理解为用以同步音视频的。

　　以前出现的问题就是没有理解到【时间戳】这一概念及其应用，简单的为了能够进入GetSampleAsync获取 MediaStreamSample而不得不频繁的调用SetSource方法，没能在实际的流处理上下功夫。

　　再来看看 MediaStreamSample的构造函数：

public MediaStreamSample(MediaStreamDescription mediaStreamDescription, Stream stream, long offset, long count,long timestamp, 
IDictionary<MediaSampleAttributeKeys, string> attributes);

public MediaStreamSample(MediaStreamDescription mediaStreamDescription, Stream stream, long offset, long count, long timestamp,
long duration, IDictionary<MediaSampleAttributeKeys, string> attributes);

　　其中的：

　　stream——实例使用到的流数据（不一定包含的是实际播放的数据，可以与offect和count等集合指示播放的数据）

　　offect——相对于stream的字节偏移量（取决于stream的数据）

　　count——播放实例包含的字节数

　　timestamp——实例的时间戳

　　duration——实例的间隔时间（可用可不用）

　　attributes——描述实例的属性集合

　　同样来看看实时流解码后的数据:

private void OnDecCBFun(int nPort, IntPtr pBuf, int nSize, ref NET_POSA_FRAME_INFO pFrameInfo, int nReserved1, int nReserved2){}

　　nport——解码所用端口

　　pBuf——解码后的数据指针

　　nSize——解码后数据大小

　　其中的【NET_POSA_FRAME_INFO】结构体如下：

public struct NET_POSA_FRAME_INFO
{
     public int nWidth;
     public int nHeight;
     public int nStamp;
     public int nType;
     public int nFrameRate;
}

　　nWidth——图像宽度

　　nHeight——图像高度

　　nStamp——时间戳

　　nType——数据类型

　　nFrameRate——帧率

　　值得一提的是：

　　MediaStreamSample的时间戳是相对时间戳，而解码数据里的时间戳是绝对时间戳，需要计算。

　　MediaStreamSample的时间戳单位是100纳秒，解码数据的时间戳单位为秒（一开始没注意，会导致视频画面播放速度显得很快），需要转换。

 　　为了方便使用及理清条例，封装了以下几个类：　　

    public class RealFile
    {
        RealMediaTypes mediaType;
        List<RealSample> videoSampleList;
        long timestamp = 0;

        public RealFile(int width, int height, RealMediaTypes type)
        {
            this.Height = height;
            this.Width = width;
            this.mediaType = type;
            videoSampleList = new List<RealSample>();
        }

        #region Property
        public long Height { get; set; }

        public long Width { get; set; }
        #endregion

        public RealSample GetVideoSample()
        {
            if (videoSampleList.Count > 0)
            {
                RealSample sample = videoSampleList[0];
                videoSampleList.RemoveAt(0);
                return sample;
            }
            else
                return null;
        }

        public void AddStream(RealStream realStream, long stamp)
        {
            if (timestamp == 0)
                timestamp = stamp;
            RealSample sample = new RealSample();
            sample.Offset = 0;
            sample.Count = realStream.Length;

            if (mediaType == RealMediaTypes.YV12)
                sample.Timestamp = (stamp - timestamp) * 10000;
            else
                sample.Timestamp = (stamp - timestamp);

            sample.MediaStream = realStream.MediaStream;

            videoSampleList.Add(sample);
        }
    }

    public class RealSample
    {
        public long Offset { get; set; }
        public long Count { get; set; }
        public long Timestamp { get; set; }
        public long Duration { get; set; }
        public Stream MediaStream { get; set; }
    }

    public class RealStream
    {
        private Stream mediaStream;
        public Stream MediaStream
        {
            get { return mediaStream; }
            set { mediaStream = value; }
        }
      
        public RealStream(Stream stream)
        {
            this.mediaStream = stream;
        }

        public virtual void Close()
        {
            this.mediaStream.Close();
        }

        #region Property
        public virtual long Length
        {
            get { return this.mediaStream.Length; }
        }

        public virtual long Position
        {
            get { return this.mediaStream.Position; }
        }
        #endregion
    }

　　则RealMediaStreamSource的代码如下：

    public enum RealMediaTypes
    {
        MBF,
        YV12,
    }

    public class RealMediaStreamSource : MediaStreamSource
    {
        RealMediaTypes _mediaType;
        Stream _mediaStream;
        long _stamp;
        int _width;
        int _height;

        RealFile realFile;

        Dictionary<MediaSampleAttributeKeys, string> _mediaSampleAttributes = new Dictionary<MediaSampleAttributeKeys, string>();
        Dictionary<MediaStreamAttributeKeys, string> _mediaStreamAttributes = new Dictionary<MediaStreamAttributeKeys, string>();
        Dictionary<MediaSourceAttributesKeys, string> _mediaSourceAttributes = new Dictionary<MediaSourceAttributesKeys, string>();
        List<MediaStreamDescription> _mediaStreamDescriptions = new List<MediaStreamDescription>();

        private int index;

        public RealMediaStreamSource(RealMediaTypes type, Stream stream, int width, int height, long stamp)
        {
            this._mediaType = type;
            this._mediaStream = stream;
            this._width = width;
            this._height = height;
            this._stamp = stamp;
        }

        #region Override
        protected override void OpenMediaAsync()
        {
            index = 0;
            realFile = new RealFile(_width, _height, _mediaType);
            RealStream stream = new RealStream(_mediaStream);
            realFile.AddStream(stream, _stamp);

            //描述媒体示例
            _mediaSampleAttributes[MediaSampleAttributeKeys.FrameHeight] = _height.ToString();
            _mediaSampleAttributes[MediaSampleAttributeKeys.FrameWidth] = _width.ToString();

            //描述媒体流
            _mediaStreamAttributes[MediaStreamAttributeKeys.Height] = _height.ToString();
            _mediaStreamAttributes[MediaStreamAttributeKeys.Width] = _width.ToString();
            _mediaStreamAttributes[MediaStreamAttributeKeys.CodecPrivateData] = "";
            if (_mediaType == RealMediaTypes.MBF)
                _mediaStreamAttributes[MediaStreamAttributeKeys.VideoFourCC] = "H264";
            else if (_mediaType == RealMediaTypes.YV12)
                _mediaStreamAttributes[MediaStreamAttributeKeys.VideoFourCC] = "YV12";

            //详尽描述媒体流
            _mediaStreamDescriptions.Add(new MediaStreamDescription(MediaStreamType.Video, _mediaStreamAttributes));

            //描述媒体源
            _mediaSourceAttributes[MediaSourceAttributesKeys.CanSeek] = "true";
            _mediaSourceAttributes[MediaSourceAttributesKeys.Duration] = "";

            index++;
            this.ReportOpenMediaCompleted(_mediaSourceAttributes, _mediaStreamDescriptions);
        }

        protected override void GetSampleAsync(MediaStreamType mediaStreamType)
        {
            MediaStreamSample mediaSample = null;
            if (mediaStreamType == MediaStreamType.Video)
                mediaSample = GetVideoSample();
            if (mediaSample != null)
                ReportGetSampleCompleted(mediaSample);
        }

        private MediaStreamSample GetVideoSample()
        {
            RealSample sample = realFile.GetVideoSample();
            MediaStreamDescription msd = new MediaStreamDescription(MediaStreamType.Video, _mediaStreamAttributes);
            MediaStreamSample mediaSample;
            if (sample == null)
            {
                mediaSample = new MediaStreamSample(
                    msd,
                    _mediaStream,
                    0,
                    _mediaStream.Length,
                    0,
                    _mediaSampleAttributes);
            }
            else
            {
                mediaSample = new MediaStreamSample(
                    msd,
                    sample.MediaStream,
                    sample.Offset,
                    sample.Count,
                    sample.Timestamp,
                    _mediaSampleAttributes);
            }
            index--;
            return mediaSample;
        }

        protected override void SeekAsync(long seekToTime)
        {
            this.ReportSeekCompleted(seekToTime);
        }

        protected override void CloseMedia()
        {
        }

        protected override void SwitchMediaStreamAsync(MediaStreamDescription mediaStreamDescription)
        {
        }

        protected override void GetDiagnosticAsync(MediaStreamSourceDiagnosticKind diagnosticKind)
        {
        }
        #endregion

        public void AddStream(Stream mediaStream, long stamp)
        {
            this._mediaStream = mediaStream;
            RealStream addstream = new RealStream(mediaStream);
            realFile.AddStream(addstream, stamp);
            index++;
        }
    }

　　RealMediaStreamSource具体调用如下：

        bool isFirst = true;
        void proxy_GetVideosReceived(object sender, GetVideosReceivedEventArgs e)
        {
            StreamData sources = e.data;
            if (sources.Stamp == 0)
                return;

            if (isFirst)
            {
                object obj = new object();
                lock (obj)
                {
                    ShowStream(sources);
                    isFirst = false;
                }
            }
            else
            {
                try
                {
                    ShowStreamToo(sources);
                }
                catch (System.Exception ex)
                {
                }
            }
        }

        private void ShowStream(StreamData sources)
        {
            Stream stream = new MemoryStream(sources.Streams);
            streamSource1 = new RealMediaStreamSource(RealMediaTypes.YV12, stream, sources.Width, sources.Height, sources.Stamp);
            element1.SetSource(streamSource1); 
        }

        private void ShowStreamToo(StreamData sources)
        {
            Stream stream = new MemoryStream(sources.Streams);
            ((RealMediaStreamSource)streamSource1).AddStream(stream, sources.Stamp);
        }

 

　　至此便能够顺利的播放视频流啦，归根结底问题在于实例化的MediaStreamSample所包含的信息是否准确！

　　 GetVideoSample()里的处理还需要大家根据自己的实际情况灵活应用哦！

　　目前个人也还处于探索学习阶段，欢迎大家多多指教！