CPosPassThru class

[The feature associated with this page, DirectShow, is a legacy feature. It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation. Those features have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaPlayer, IMFMediaEngine and Audio/Video Capture in Media Foundation instead of DirectShow, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

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.

Public Methods	Description
CPosPassThru	Constructor method.
ForceRefresh	Obsolete.
GetMediaTime	Retrieves the time stamps on the current sample. Virtual.
IMediaPosition Methods	Description
get_Duration	Retrieves the duration of the stream.
put_CurrentPosition	Sets the current position, relative to the total duration of the stream.
get_StopTime	Retrieves the time at which the playback will stop, relative to the duration of the stream.
put_StopTime	Sets the time at which the playback will stop, relative to the duration of the stream.
get_PrerollTime	Retrieves the amount of data that will be queued before the start position.
put_PrerollTime	Sets the amount of data that will be queued before the start position.
get_Rate	Retrieves the playback rate.
put_Rate	Sets the playback rate.
get_CurrentPosition	Retrieves the current position, relative to the total duration of the stream.
CanSeekForward	Determines whether the stream can be seeked backward.
CanSeekBackward	Determines whether the stream can be seeked forward.
IMediaSeeking Methods	Description
CheckCapabilities	Queries whether a stream has specified seeking capabilities.
ConvertTimeFormat	Converts from one time format to another.
GetAvailable	Retrieves the range of times in which seeking is efficient.
GetCapabilities	Retrieves all the seeking capabilities of the stream.
GetCurrentPosition	Retrieves the current position, relative to the total duration of the stream.
GetDuration	Retrieves the duration of the stream.
GetPositions	Retrieves the current position and the stop position, relative to the total duration of the stream.
GetPreroll	Retrieves the amount of data that will be queued before the start position.
GetRate	Retrieves the playback rate.
GetStopPosition	Retrieves the time at which the playback will stop, relative to the duration of the stream.
GetTimeFormat	Retrieves the current time format.
IsFormatSupported	Determines whether a specified time format is supported.
IsUsingTimeFormat	Determines whether a specified time format is the format currently in use.
QueryPreferredFormat	Retrieves the preferred time format for the stream.
SetPositions	Sets the current position and the stop position.
SetRate	Sets the playback rate.
SetTimeFormat	Sets the time format.
Helper Functions	Description
CreatePosPassThru	Creates a CPosPassThru or CRendererPosPassThru object.

Requirements

Requirement	Value
Header	Ctlutil.h (include Streams.h)
Library	Strmbase.lib (retail builds); Strmbasd.lib (debug builds)