如何在WPF中向后播放视频？ [英] How to play video backwards in WPF?

问题描述

我想在WPF中向后平滑播放视频。我正在使用 MediaElement 播放视频。我阅读了这篇文章，该文章建议更改 MediaElement.Position 定期模仿倒带行为。

我尝试了以下代码来更改 MediaElement.Position

  private void Button_Click（对象发送者，RoutedEventArgs e）
 {
 mePlayer.Pause（）; //先暂停媒体播放器
 double m = 1 / frameRate; //计算每一帧的时间
 double t = 120; //以秒为单位的视频总长度
 mePlayer.Position = TimeSpan.FromMinutes（2）; //从2分钟
开始播放视频，而（t> = 60）//检查时间是否超过1分钟
 {
 t = t-m; //从总秒数中减去单帧时间
 mePlayer.Position = TimeSpan.FromSeconds（t）; //设置视频的位置
} 
} 
  

在上面的代码中，我试图将视频从2分钟向后播放1分钟。
它在 mePlayer.Position = TimeSpan.FromSeconds（t）上给我'System.OverflowException'。

<如果有人知道如何在WPF中向后播放视频，请帮助我实现此效果。谢谢。

解决方案

要顺利进行，您应该使用 Timer 。假设帧速率为24 fps，则意味着每1/24 = 0.0416秒或大约42毫秒为一帧。因此，如果您的计时器每42毫秒计时一次，您可以向后移动 mePlayer.Position ：

XAML：

 < MediaElement x：Name = mePlayer Source = C：\Sample.mp4 
 LoadedBehavior = Manual ScrubbingEnabled = True /> 
  

代码：

  System.Windows.Threading.DispatcherTimer dispatcherTimer; 
 int t = 240000； // 4分钟= 240,000毫秒
 
 public MainWindow（）
 {
 InitializeComponent（）; 
 
 dispatcherTimer =新的System.Windows.Threading.DispatcherTimer（）; 
 dispatcherTimer.Tick + =新的EventHandler（dispatcherTimer_Tick）; 
 //每42毫秒滴答一次=一秒内滴答24次
 dispatcherTimer.Interval = new TimeSpan（0，0，0，0，42）; 
 
} 
 
 private void dispatcherTimer_Tick（object sender，EventArgs e）
 {
 //每42毫秒（或24 fps）返回1帧
t = t-42; 
 mePlayer.Position = TimeSpan.FromMilliseconds（t）; 
} 
 private void Window_Loaded（对象发送者，RoutedEventArgs e）
 {
 mePlayer.Play（）; 
} 
 private void Button_Click（对象发送者，RoutedEventArgs e）
 {
 //暂停并转到视频的第4分钟，然后开始向后播放
 mePlayer.Pause （）; 
 mePlayer.Position = TimeSpan.FromMinutes（4）; 
 dispatcherTimer.Start（）; 
} 
  

I want to smoothly play video in backwards direction in WPF. I am using MediaElement to play the video. I read this post which suggests changing MediaElement.Position periodically to mimic the rewinding behaviour.

I tried following code for changing position of MediaElement.Position

private void Button_Click(object sender, RoutedEventArgs e)
{
    mePlayer.Pause();                               //Pause the media player first
    double m = 1 / frameRate;                       //Calculate the time for each frame
    double t = 120;                                 //Total length of video in seconds
    mePlayer.Position = TimeSpan.FromMinutes(2);    //Start video from 2 min
    while (t >= 60)                                 //Check if time exceeds 1 min
    {
        t = t - m;                                  //Subtract the single frame time from total seconds
        mePlayer.Position = TimeSpan.FromSeconds(t);//set position of video
    }
}

In above code I am trying to play video backwards from 2 min to 1 min. It's giving me 'System.OverflowException' on mePlayer.Position = TimeSpan.FromSeconds(t).

If anyone know how play video backwards in WPF, please help me to achieve this effect. Thank you.

解决方案

To do it smoothly you should use a Timer. Assuming the frame rate is 24 fps, then it means that is one frame every 1/24 = 0.0416 second or approximately 42 millisecond. So if your timer ticks every 42ms you can move mePlayer.Position backward:

XAML:

<MediaElement x:Name="mePlayer" Source="C:\Sample.mp4"
              LoadedBehavior="Manual" ScrubbingEnabled="True"/>

Code:

    System.Windows.Threading.DispatcherTimer dispatcherTimer;
    int t = 240000; // 4 minutes = 240,000 milliseconds

    public MainWindow()
    {
        InitializeComponent();

        dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
        dispatcherTimer.Tick += new EventHandler(dispatcherTimer_Tick);
        //tick every 42 millisecond = tick 24 times in one second
        dispatcherTimer.Interval = new TimeSpan(0, 0, 0, 0, 42);

    }

    private void dispatcherTimer_Tick(object sender, EventArgs e)
    {
        // Go back 1 frame every 42 milliseconds (or 24 fps)
        t = t - 42;
        mePlayer.Position = TimeSpan.FromMilliseconds(t);
    }
    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        mePlayer.Play();
    }
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        // Pause and go to 4th minute of the video then start playing backward
        mePlayer.Pause();                               
        mePlayer.Position = TimeSpan.FromMinutes(4);
        dispatcherTimer.Start();
    }

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