使用DirectShowNet搜索关键帧

Question

我的任务是:创建一个图形，将SampleGrabber附加到该图形上，并在构建图形后使用IMediaSeeking接口获取关键帧。

以下是我所做的工作: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，我已经设置好了这些东西。现在，当我运行程序时，一切都正确加载。但是回调只调用一次，然后渲染就会停止。不再渲染，也不再回调。我尝试了另一个回调方法ISampleGrabber::BufferCB()，看看它是否遵循同样的命运。但是不是的！每次抓取帧时都会调用BufferCB()，并将视频渲染到最后。

我做错了什么？对此有什么建议吗？谢谢您:)

Answer 1

好的.我终于能够解决这个问题了。我会在这里描述它，以防它对其他人有帮助。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，因为它是一个COM对象。

只需将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;
}

我现在面对的是另一个问题。然而，我已经为此写了另一篇文章，因为它并不完全与这个问题相关。