摘录DirectShow数据，视频采集

DirectShow在,数据流(Data Flow)它们是依次流过每Filter的.管理其数据具有其自己的方法,并且并没有向用户提供一个统一的接口,供用户操作数据流.这里以提取视频採集在的每帧为位图数据为例,说说怎样在Directshow中提取数据.

这里我们用到了DirectShow提供给我们的接口ISampleGrabber,并定义了一个供它回调的CSampleGrabberCB对象(继承ISampleGrabberCB接口).

我们知道,DirectShow中的数据存储是通过Sample完毕的,所以提取数据也须要通过SampleGrabber.

过程例如以下:

1.建立CSampleGrabberCB对象.

   class CSampleGrabberCB : public ISampleGrabberCB 
   {

       STDMETHODIMP BufferCB( double dblSampleTime, BYTE * pBuffer, long lBufferSize )

       {

                 //Callback method that receives a pointer to the sample buffer.

       }

      STDMETHODIMP SampleCB( double SampleTime, IMediaSample * pSample )
      {
               //Callback method that receives a pointer to the media sample.
      }

   }

2.定义ISampleGrabber接口并初始化

   CComPtr< ISampleGrabber > m_pGrabber;

   HRESULT hr;

   hr = m_pGrabber.CoCreateInstance( CLSID_SampleGrabber );

   if(FAILED(hr))

          //error action;

3.定义Grabber Filter,设置它的媒体类型,并将它增加Graph中

   CComQIPtr< IBaseFilter, &IID_IBaseFilter > pGrabBase( m_pGrabber );

  CMediaType VideoType;
  VideoType.SetType(&MEDIATYPE_Video);
  VideoType.SetSubtype(&MEDIASUBTYPE_RGB24);
  hr = m_pGrabber->SetMediaType(&VideoType);

  hr = pGraph->AddFilter(pGrabBase,L"Grabber");

4.设置回调(CallBack),使Grabber可以通过BufferCB自己主动完毕採集数据.

   // don‘t buffer the samples as they pass through
      //
      hr = m_pGrabber->SetBufferSamples( FALSE );

      // only grab one at a time, stop stream after
      // grabbing one sample
      //
      hr = m_pGrabber->SetOneShot( FALSE );

      // set the callback, so we can grab the one sample
      //
      hr = m_pGrabber->SetCallback( &mCB, 1 );  //mCB为CSampleGrabber对象

这样,在DirectShow数据流过程,mCB.bufferCB它可以运行在自己主动,摘录Graph数据.