C++ - DirectShow - Comparison between two images captured by webcam - image

Question

I'm a newbie in the DirectShow world, and I just studied the simple "playcap" sample provided by Microsoft SDK Samples. With this little program I've been able to have a window with my webcam stream.
How can I take two shots from my webcam and compare them (even without saving them on the hard disk) to find which pixels are different?
I easily did this job using Win32API capture windows, but it was very slow, and I need it to be fast.
Thank you in advance, it is very important for my project.

Answer 1

You'd better search here for answer or look in samples for Sample Grabber Filter.
For more details you can write me directly here.
Add the Sample Grabber filter to the graph.
IBaseFilter *pSG_Filter;
hr = CoCreateInstance(
CLSID_SampleGrabber,
NULL,
CLSCTX_INPROC_SERVER,
IID_IBaseFilter,
(void**)&pSG_Filter
);
hr = pGraph->AddFilter(pSG_Filter, L"SampleGrab");
Add the Null Renderer filter to the graph.
IBaseFilter *pNull;
hr = CoCreateInstance(
CLSID_NullRenderer,
NULL,
CLSCTX_INPROC_SERVER,
IID_IBaseFilter,
(void**)&pNull
);
hr = pGraph->AddFilter(pNull, L"NullRender");
Now you can use the ICaptureGraphBuilder2::RenderStream method to connect all three filters in one method call, going from the still pin to the Sample Grabber, and from the Sample Grabber to the Null Renderer:
hr = pBuild->RenderStream(
&PIN_CATEGORY_STILL, // Connect this pin ...
&MEDIATYPE_Video, // with this media type ...
pCap, // on this filter ...
pSG_Filter, // to the Sample Grabber ...
pNull); // ... and finally to the Null Renderer.
Now use the ISampleGrabber interface to configure the Sample Grabber so that it buffers samples:
ISampleGrabber *pSG = NULL;
hr = pSG_Filter->QueryInterface(IID_ISampleGrabber, (void**)&pSG);
if (SUCCEEDED(hr))
{
hr = pSG->SetOneShot(FALSE);
hr = pSG->SetBufferSamples(TRUE);
...
Now, you should look at method ISampleGrabber::SetOneShot and maybe set TRUE there.
Set the callback interface with a pointer to your callback object:
hr = pSG->SetCallback(&g_StillCapCB, 0); // 0 = Use the SampleCB callback method.
Get the media type that the still pin used to connect with the Sample Grabber:
// Store the media type for later use.
AM_MEDIA_TYPE g_StillMediaType;
hr = pSG->GetConnectedMediaType(&g_StillMediaType);
pSG->Release();
This media type will contain the BITMAPINFOHEADER structure that defines the format of the still image.
What follows is an example of the callback class. Note that the class implements IUnknown, which it inherits through the ISampleGrabber interface, but it does not keep a reference count. This is safe because the application creates the object on the stack, and the object remains in scope throughout the lifetime of the filter graph.
All of the work happens in the BufferCB method, which is called by the Sample Grabber whenever it gets a new sample. In the following example, the method writes the bitmap to a file:
class SampleGrabberCallback : public ISampleGrabberCB
{
public:
// Fake referance counting.
STDMETHODIMP_(ULONG) AddRef() { return 1; }
STDMETHODIMP_(ULONG) Release() { return 2; }
STDMETHODIMP QueryInterface(REFIID riid, void **ppvObject)
{
if (NULL == ppvObject) return E_POINTER;
if (riid == __uuidof(IUnknown))
{
*ppvObject = static_cast<IUnknown*>(this);
return S_OK;
}
if (riid == __uuidof(ISampleGrabberCB))
{
*ppvObject = static_cast<ISampleGrabberCB*>(this);
return S_OK;
}
return E_NOTIMPL;
}
STDMETHODIMP SampleCB(double Time, IMediaSample *pSample)
{
return E_NOTIMPL;
}
STDMETHODIMP BufferCB(double Time, BYTE *pBuffer, long BufferLen)
{
if ((g_StillMediaType.majortype != MEDIATYPE_Video) ||
(g_StillMediaType.formattype != FORMAT_VideoInfo) ||
(g_StillMediaType.cbFormat < sizeof(VIDEOINFOHEADER)) ||
(g_StillMediaType.pbFormat == NULL))
{
return VFW_E_INVALIDMEDIATYPE;
}
HANDLE hf = CreateFile("C:\\Example.bmp", GENERIC_WRITE,
FILE_SHARE_WRITE, NULL, CREATE_ALWAYS, 0, NULL);
if (hf == INVALID_HANDLE_VALUE)
{
return E_FAIL;
}
long cbBitmapInfoSize = g_StillMediaType.cbFormat - SIZE_PREHEADER;
VIDEOINFOHEADER *pVideoHeader =
(VIDEOINFOHEADER*)g_StillMediaType.pbFormat;
BITMAPFILEHEADER bfh;
ZeroMemory(&bfh, sizeof(bfh));
bfh.bfType = 'MB'; // Little-endian for "BM".
bfh.bfSize = sizeof( bfh ) + BufferLen + cbBitmapInfoSize;
bfh.bfOffBits = sizeof( BITMAPFILEHEADER ) + cbBitmapInfoSize;
// Write the file header.
DWORD dwWritten = 0;
WriteFile( hf, &bfh, sizeof( bfh ), &dwWritten, NULL );
WriteFile(hf, HEADER(pVideoHeader), cbBitmapInfoSize, &dwWritten, NULL);
WriteFile( hf, pBuffer, BufferLen, &dwWritten, NULL );
CloseHandle( hf );
return S_OK;
}
};
Free the media type before the application exits:
// On exit, remember to release the media type.
FreeMediaType(g_StillMediaType);