MediaElement 获取首帧图片

FileStream stream = File.Open("bear.png", FileMode.Create);
RenderTargetBitmap bmp = new RenderTargetBitmap((int)this.video.ActualWidth,
     (int)this.video.ActualHeight, 96, 96, PixelFormats.Pbgra32);
bmp.Render(this.video);
PngBitmapEncoder coder = new PngBitmapEncoder();
coder.Interlace = PngInterlaceOption.Off;
coder.Frames.Add(BitmapFrame.Create(bmp));
coder.Save(stream);
stream.Close();

<MediaElement Source="foo.wmv" 
              MediaOpened="videoElement_MediaOpened" 
              LoadedBehavior="Pause" ScrubbingEnabled="True" />