DirectShow, capture Still Image - visual-c++

Question

i'm not experienced in Windows programming, i try to capture a Still Image from a WebCam using DirectShow. I wrote a small application based on CommandCam.cpp, which can be found here:
http://batchloaf.wordpress.com/commandcam/
I basically added the code that can be found here:
http://msdn.microsoft.com/en-us/library/windows/desktop/dd318622%28v=vs.85%29.aspx
But the call to pBuilder->FindPin(pCap, PINDIR_OUTPUT, &PIN_CATEGORY_STILL, ... fails and i can't get the Pin for the Still Image.
I doubt that because using other Webcam programs i can get a Still Image from my Microsoft LifeCam Studio.
I wonder what i am doing wrong? I tried placing the call in different places in the application but it never succeeded.
Thanks for any hints,
Torsten.
The relevant part of the code is this:
// Get video input device name
hr = pMoniker->BindToStorage(0, 0, IID_PPV_ARGS(&pPropBag));
VariantInit(&var);
hr = pPropBag->Read(L"FriendlyName", &var, 0);
fprintf(stderr, "Capture device: %ls\n", var.bstrVal);
VariantClear(&var);
// Create capture filter and add to graph
hr = pMoniker->BindToObject(0, 0, IID_IBaseFilter, (void**)&pCap);
if (hr != S_OK) exit_message("Could not create capture filter", 1);
// Add capture filter to graph
hr = pGraph->AddFilter(pCap, L"Capture Filter");
if (hr != S_OK) exit_message("Could not add capture filter to graph", 1);
// Create sample grabber filter
hr = CoCreateInstance(CLSID_SampleGrabber, NULL,
CLSCTX_INPROC_SERVER, IID_IBaseFilter,
(void**)&pSampleGrabberFilter);
if (hr != S_OK)
exit_message("Could not create Sample Grabber filter", 1);
// Query the ISampleGrabber interface of the sample grabber filter
hr = pSampleGrabberFilter->QueryInterface(
DexterLib::IID_ISampleGrabber, (void**)&pSampleGrabber);
if (hr != S_OK)
exit_message("Could not get ISampleGrabber interface to sample grabber filter", 1);
// Enable sample buffering in the sample grabber filter
hr = pSampleGrabber->SetBufferSamples(TRUE);
if (hr != S_OK)
exit_message("Could not enable sample buffering in the sample grabber", 1);
// Set media type in sample grabber filter
AM_MEDIA_TYPE mt;
ZeroMemory(&mt, sizeof(AM_MEDIA_TYPE));
mt.majortype = MEDIATYPE_Video;
mt.subtype = MEDIASUBTYPE_RGB24;
hr = pSampleGrabber->SetMediaType((DexterLib::_AMMediaType *)&mt);
if (hr != S_OK)
exit_message("Could not set media type in sample grabber", 1);
// Add sample grabber filter to filter graph
hr = pGraph->AddFilter(pSampleGrabberFilter, L"SampleGrab");
if (hr != S_OK)
exit_message("Could not add Sample Grabber to filter graph", 1);
// Create Null Renderer filter
hr = CoCreateInstance(CLSID_NullRenderer, NULL,
CLSCTX_INPROC_SERVER, IID_IBaseFilter,
(void**)&pNullRenderer);
if (hr != S_OK)
exit_message("Could not create Null Renderer filter", 1);
// Add Null Renderer filter to filter graph
hr = pGraph->AddFilter(pNullRenderer, L"NullRender");
if (hr != S_OK)
exit_message("Could not add Null Renderer to filter graph", 1);
// Connect up the filter graph's capture stream
hr = pBuilder->RenderStream(
&PIN_CATEGORY_CAPTURE, &MEDIATYPE_Video,
pCap, pSampleGrabberFilter, pNullRenderer);
if (hr != S_OK)
exit_message("Could not render capture video stream", 1);
hr = pBuilder->RenderStream(
&PIN_CATEGORY_PREVIEW, &MEDIATYPE_Video,
pCap, NULL, NULL);
if (hr != S_OK && hr != VFW_S_NOPREVIEWPIN)
exit_message("Could not render preview video stream", 1);
// Get media control interfaces to graph builder object
hr = pGraph->QueryInterface(IID_IMediaControl,
(void**)&pMediaControl);
if (hr != S_OK) exit_message("Could not get media control interface", 1);
// Run graph
while(1)
{
hr = pMediaControl->Run();
// Hopefully, the return value was S_OK or S_FALSE
if (hr == S_OK) break; // graph is now running
if (hr == S_FALSE) continue; // graph still preparing to run
// If the Run function returned something else,
// there must be a problem
fprintf(stderr, "Error: %u\n", hr);
exit_message("Could not run filter graph", 1);
}
Sleep(2000);
// get the StillImage Pin
hr = pCap->QueryInterface(IID_IAMVideoControl, (void**)&pAMVidControl);
if (hr != S_OK) exit_message("Could not get IAMVideoControl", 1);
hr = pBuilder->FindPin(pCap, PINDIR_OUTPUT, &PIN_CATEGORY_STILL, NULL, FALSE, 0, &pPin);
if (hr != S_OK)
exit_message("Could not get Pin of category StillImage", 1);
hr = pAMVidControl->SetMode(pPin, VideoControlFlag_Trigger);
if (hr != S_OK) exit_message("Could set mode VideoControlFlag_Trigger", 1);

Answer 1

Some video capture device source filters do not expose a still image capture pin.
Did you try using the EnumPins method to find whether it actually has one, or if there is just a preview pin?
You could also use GraphEdit to have a look at the filter's pins.
If the device only has a preview pin, you will have to use that to grab your image. You can use the Smart Tee Filter to split your graph into preview and capture.