dshow,Sample Grabber 从摄像头采集

char* CCameraDS::QueryFrame()
{
	long evCode, size = 0;
	


#if CALLBACKMODE
	static double lastSampleTime=0;
	if( lastSampleTime == cbInfo.dblSampleTime)
		return NULL;
	if(cbInfo.lBufferSize == 0)
		return NULL;
	if ( m_nBufferSize != cbInfo.lBufferSize)
	{
		if (m_pFrame)
		{
			free(m_pFrame);
		}
		m_nBufferSize = cbInfo.lBufferSize;
		m_pFrame = (char*)malloc(m_nBufferSize);//cvCreateImage(cvSize(m_nWidth, m_nHeight), IPL_DEPTH_8U, 3);
	}
	lastSampleTime = cbInfo.dblSampleTime;
	m_pFrame2 = (char*)cbInfo.pBuffer;
#else
	m_pMediaControl->Run();
	m_pMediaEvent->WaitForCompletion(INFINITE, &evCode);
	m_pSampleGrabber->GetCurrentBuffer(&size, NULL);
	if(size == 0)
		return NULL;
	//if the buffer size changed
	if (size != m_nBufferSize)
	{
		if (m_pFrame)
		{
			free(m_pFrame);
		}
		if (m_pFrame2)
		{
			free(m_pFrame2);
		}
		m_nBufferSize = size;
		m_pFrame = (char*)malloc(m_nWidth*m_nHeight*3);//cvCreateImage(cvSize(m_nWidth, m_nHeight), IPL_DEPTH_8U, 3);
		m_pFrame2 = (char*)malloc(m_nWidth*m_nHeight*3);
	}
	
	m_pSampleGrabber->GetCurrentBuffer(&m_nBufferSize, (long*)m_pFrame2);
	//cvFlip(m_pFrame);
#endif
	for(int i=0;i<m_nHeight;i++)
		memcpy(m_pFrame+m_nWidth*3*i,m_pFrame2+m_nWidth*3*(m_nHeight-1-i),m_nWidth*3);

	return m_pFrame;
}

1．缓冲区模式

#if !CALLBACKMODE
m_pSampleGrabber->SetBufferSamples(TRUE);
m_pSampleGrabber->SetOneShot(TRUE);
#endif

只能设置SetOneShot为TRUE， 因为使用SetPositions 函数始终返回 E_NOTIMPL：Method is not supported.如果为false,

WaitForCompletion(INFINITE, &evCode)函数会一直等待下去。

2.回调模式

回调模式每采到一直就会进入回调函数，可以在回调函数里面处理采到的数据。

#if CALLBACKMODE
m_pSampleGrabber->SetBufferSamples(FALSE);
m_pSampleGrabber->SetOneShot(FALSE);
// Set the callback, so we can grab the one sample
//
CB.Width = m_nWidth;
CB.Height = m_nHeight;
hr = m_pSampleGrabber->SetCallback( &CB, 1 );
m_pMediaControl->Run();
#endif

只用Run()一次即可。如果SetOneShot(TRUE)的话，也可以每请求一帧Run()一次，然后WaitForCompletion，再从回调函数的buffer中取出数据，这种其实和缓冲区模式一样，只不过缓冲区变成了在回调函数中。

如果摄像头采集帧率为30，SetOneShot(FALSE)回调模式，就一秒进去回调函数30次。而OneShot模式会取到重复的帧。