CPosPassThru Class

The CPosPassThru class handles seek commands for transform filters, by passing them upstream to the next filter.

When an application seeks the filter graph, the Filter Graph Manager gives the seek command to the renderer filters. The command is passed upstream, through each filter's output pin, until it reaches a filter that can execute the command (if any). For details, see Seeking. The CPosPassThru class passes all seek commands to the output pin on the upstream filter, as shown in the following diagram.

Although this class is provided in the base class library, DirectShow also provides the same class in Quartz.dll. Using the Quartz.dll version can reduce the code size in your filter somewhat, because the class is loaded at run-time from the DLL. To use that version, call the CreatePosPassThru function.

In your output pin's NonDelegatingQueryInterface method, delegate to the CPosPassThru object whenever the requested interface is IMediaSeeking or IMediaPosition, as shown in the following code:

// The following member variables are assumed:
IPin *m_pInput;    // Pointer to the input pin on your filter.
IUnknown *m_pPos;  // Pointer to the CPosPassThru object.

STDMETHODIMP CMyPin::NonDelegatingQueryInterface(REFIID riid, void **ppv)
{
    HRESULT hr
    if (riid == IID_IMediaPosition || riid == IID_IMediaSeeking) 
    {
        if (m_pPos == NULL) 
        {
            // We have not created the CPosPassThru object yet. Do so now.
            hr = CreatePosPassThru(GetOwner(), FALSE, m_pInput, &m_pPos);
            if (FAILED(hr)) return hr;
        }
        return m_pPos->QueryInterface(riid, ppv);
    } 
    else
    {
         // Other interfaces (not shown).
    }
}

~CMyPin::CMyPin() 
{
    // Release the CPosPassThruObject.
    if (m_pPos != NULL) m_pPos->Release();
}

Except where noted, all IMediaPosition and IMediaSeeking methods in this class call the corresponding method on the connected pin and return the result.

Requirements

Header: Declared in Ctlutil.h; include Streams.h.

Library: Use Strmbase.lib (retail builds) or Strmbasd.lib (debug builds).