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我的任务是：我创建一个图形，将 SampleGrabber 附加到它，并在构建图形后使用 IMediaSeeking 界面抓取关键帧。

以下是我所做的： 在 Main 方法中：

   Type comType = Type.GetTypeFromCLSID ( new Guid ( "e436ebb3-524f-11ce-9f53-0020af0ba770" ) );
   IGraphBuilder graphBuilder = (IGraphBuilder) Activator.CreateInstance ( comType );

   comType = Type.GetTypeFromCLSID ( new Guid ( "C1F400A0-3F08-11d3-9F0B-006008039E37" ) );
   ISampleGrabber sampleGrabber = (ISampleGrabber) Activator.CreateInstance ( comType );

   graphBuilder.AddFilter ( (IBaseFilter) sampleGrabber, "samplegrabber" );

   AMMediaType mediaType = new AMMediaType ( );
   mediaType.majorType = MediaType.Video;
   mediaType.subType = MediaSubType.RGB24;
   mediaType.formatType = FormatType.VideoInfo;
   sampleGrabber.SetMediaType ( mediaType );

   int hr = graphBuilder.RenderFile ( @"D:\test.wmv", null );

   IMediaEventEx mediaEvent = (IMediaEventEx) graphBuilder;
   IMediaControl mediaControl = (IMediaControl) graphBuilder;
   IVideoWindow videoWindow = (IVideoWindow) graphBuilder;
   IBasicAudio basicAudio = (IBasicAudio) graphBuilder;

   videoWindow.put_AutoShow ( OABool.False );
   basicAudio.put_Volume ( -10000 );

   sampleGrabber.SetOneShot ( false );
   sampleGrabber.SetBufferSamples ( true );

   //the same object has implemented the ISampleGrabberCB interface.
   //0 sets the callback to the ISampleGrabberCB::SampleCB() method.
   sampleGrabber.SetCallback (this, 0);

   mediaControl.Run ( );

   EventCode eventCode;
   mediaEvent.WaitForCompletion ( -1, out eventCode );


   Marshal.ReleaseComObject ( sampleGrabber );
   Marshal.ReleaseComObject ( graphBuilder );

在 SampleCB() 回调方法中：

    public int SampleCB ( double sampleTime, IMediaSample mediaSample )
    {
        Console.WriteLine ( "SampleCB Callback" );
        Console.WriteLine ( mediaSample.IsSyncPoint ( ) + " " + mediaSample.GetActualDataLength() );
                    //check if its a keyframe using mediaSample.IsSyncPoint()
                    //and convert the buffer into image and save it.
        return 0;
    }

因此，我已经设置了这些东西。现在，当我运行程序时，一切都正确加载。但是回调只被调用一次，然后渲染停止。没有更多的渲染，也没有更多的回调。我尝试了另一种回调方法 ISampleGrabber::BufferCB() 来查看它是否遵循相同的命运。但不是！每次抓取一帧并渲染视频直到结束时都会调用 BufferCB()。

我究竟做错了什么？对此有何建议？谢谢 ：）

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1 回答 1

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好的..我终于能够解决这个问题了。我会在这里描述它以防它帮助其他人。我实际上并没有在回调方法中释放 IMediaSample 对象。这是必须要做的，它是一个 COM 对象。

在简单地将 Marshal.ReleaseComObject() 添加到我的 SampleCB() 回调方法中，现在每次 SampleGrabber 抓取样本时都会调用它。

public int SampleCB ( double sampleTime, IMediaSample mediaSample )
{
    Console.WriteLine ( "SampleCB Callback" );
    Console.WriteLine ( mediaSample.IsSyncPoint ( ) + " " );

        /* other code */
    Marshal.ReleaseComObject ( mediaSample );
    return 0;
}

我现在面临另一个问题。但是，我为此发表了另一篇文章，因为它与这个问题并不完全相关。

于 2011-01-24T11:11:21.037 回答