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How to Perform Scrubbing

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Scrubbing is performed to instantaneously seek to specific points within a file by interacting with a visual representation of time, such as a scrollbar. In Media Foundation, scrubbing means seeking on a file and getting one updated frame.

For information about scrubbing, see About Rate Control.

To perform scrubbing

  1. Call MFGetService to get the IMFRateControl interface from the Media Session.

    Note

    Do not get the IMFRateControl interface from the media source. Always get the interface from the Media Session.

     

  2. Call IMFRateControl::SetRate to set the playback rate to zero. For more information about calling this method, see How to Set the Playback Rate on the Media Session.

  3. Create a seek position in a PROPVARIANT by specifying the presentation time to seek to in a MFTIME type.

  4. Call IMFMediaSession::Start with the seek position to start playback.

  5. When the scrub operation is complete, the Media Session sends an MESessionScrubSampleComplete event. Wait for this event before calling Start again for another scrubbing operation.

Example

The following code example shows how to perform scrubbing.

HRESULT SkipToPosition (MFTIME SeekTime, IMFMediaSession *pMediaSession)
{
    PROPVARIANT var;
    PropVariantInit(&var);

    IMFRateControl *pRateControl = NULL;

    // Get the rate control service.
    HRESULT hr = MFGetService(pMediaSession, MF_RATE_CONTROL_SERVICE, IID_PPV_ARGS(&pRateControl));

    // Set the playback rate to zero without thinning.
    if(SUCCEEDED(hr))
    {
        hr = pRateControl ->SetRate( FALSE, 0.0F); 
    }

    // Create the Media Session start position.
    if( SeekTime == PRESENTATION_CURRENT_POSITION )
    {
        var.vt = VT_EMPTY;
    }
    else
    {
        var.vt = VT_I8;
        var.hVal.QuadPart = SeekTime;
    }

    // Start the Media Session.
    if(SUCCEEDED(hr))
    {
        hr = pMediaSession->Start( NULL, &var);
    }

// Clean up.
    SafeRelease(&pRateControl);
    PropVariantClear(&var)
    return hr;
}

A successful scrubbing operation generates the MESessionScrubSampleComplete event after all the stream sinks are updated with the new frame and the scrubbing operation completes successfully. Scrubbing a video file displays the frame that was seeked to, but does not generate an audio output.

The application can perform frame stepping by setting the playback rate to zero and then passing a PROPVARIANT that is set to VT_EMPTY in the call to IMFMediaSession::Start.

Media Session

Rate Control