Capturing an Image From a Still Image Pin

Artigo
03/07/2008

Microsoft DirectShow 9.0

Capturing an Image From a Still Image Pin

Some cameras can produce a still image separate from the capture stream, and often the still image is of higher quality than the images produced by the capture stream. The camera may have a button that acts as a hardware trigger, or it may support software triggering. A camera that supports still images will expose a still image pin, which is pin category PIN_CATEGORY_STILL.

The recommended way to get still images from the device is to use the Windows Image Acquisition (WIA) APIs. For more information, see "Windows Image Acquisition" in the Platform SDK documentation. However, you can also use DirectShow to capture an image.

To trigger the still pin, use the IAMVideoControl::SetMode method when the graph is running, as follows:

pControl->Run(); // Run the graph.
IAMVideoControl *pAMVidControl = NULL;
hr = pCap->QueryInterface(IID_IAMVideoControl, (void**)&pAMVidControl);
if (SUCCEEDED(hr))
{
    // Find the still pin.
    IPin *pPin = 0;
    hr = pBuild->FindPin(pCap, PINDIR_OUTPUT, &PIN_CATEGORY_STILL, 0,
        FALSE, 0, &pPin);
    if (SUCCEEDED(hr))
    {
        hr = pAMVidControl->SetMode(pPin, VideoControlFlag_Trigger);
        pPin->Release();
    }
    pAMVidControl->Release();
}

Query the capture filter for IAMVideoControl. If the interface is supported, get a pointer to the still pin's IPin interface by calling the ICaptureGraphBuilder2::FindPin method, as shown in the previous example. Then call put_Mode with the IPin pointer and the VideoControlFlag_Trigger flag.

Note Depending on the camera, you might need to render the capture pin (PIN_CATEGORY_CAPTURE) before the still pin will connect.

Example: Using the Sample Grabber Filter

One way to capture the image is with the Sample Grabber filter. The Sample Grabber uses an application-defined callback function to process the image. For more information about the Sample Grabber filter, see Using the Sample Grabber.

The following example assumes that the still pin delivers an uncompressed RGB image. First, define a class that will implement the Sample Grabber's callback interface, ISampleGrabberCB:

// Class to hold the callback function for the Sample Grabber filter.
class SampleGrabberCallback : public ISampleGrabberCB
{
    // Implementation is described later.
}

// Global instance of the class.
SampleGrabberCallback g_StillCapCB;

The implementation of the class is described shortly.

Next, connect the still pin to the Sample Grabber, and connect the Sample Grabber to the Null Renderer filter. The Null Renderer simply discards media samples that it receives; the actual work will be done within the callback. (The only reason for the Null Renderer is to connect the Sample Grabber's output pin to something.) Call CoCreateInstance to create the Sample Grabber and Null Renderer filters, and call AddFilter to add both filters to the graph:

// 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_NullRendere, NULL, CLSCTX_INPROC_SERVER,
    IID_IBaseFilter, (void**)&pNull);
hr = pGraph->AddFilter(pNull, L"NullRender");

You can use the 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:

// Configure the Sample Grabber.
ISampleGrabber *pSG;
hr = pSG_Filter->QueryInterface(IID_ISampleGrabber, (void**)&pSG);
pSG->SetOneShot(FALSE);
pSG->SetBufferSamples(TRUE);

Set the callback interface with a pointer to your callback object:

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. Free the media type before the application exits:

// On exit, remember to release the media type.
FreeMediaType(g_StillMediaType);

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;

    }
};

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