Seeking keyframes using DirectShowNet

问题

My task is : I create a graph, attach a SampleGrabber to it, and grab the keyframes using the IMediaSeeking interface after building the graph.

The following is what I have done : In the Main method :

   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 );

In the SampleCB() callback method :

    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;
    }

Thus, I have set up the things. Now, when i run the program, everything loads correctly. But the callback is called only once, and then the rendering stops. No more rendering and no more callbacks. I had tried the another callback method ISampleGrabber::BufferCB() to see if it follows the same fate. But no! BufferCB() is called everytime a frame is grabbed and the video is rendered till the end.

What am I doing wrong? Any suggestions on this? Thank you :)

回答1:

ok..I have finally been able to solve that problem. I would describe it here in case it helps anyone else. I was not actually releasing the IMediaSample object in the callback method. That is a must to do, it being a COM object.

On simply adding the Marshal.ReleaseComObject() to my SampleCB() callback method, it is now called everytime the SampleGrabber grabs a sample.

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

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

I am facing another issue now. However, I have made another post for that as it doesnt totally relate to this question.

来源：https://stackoverflow.com/questions/4733186/seeking-keyframes-using-directshownet