重載GetMediaType（const CMediaType * pMediaType）

  VIDEOINFO *pvi = (VIDEOINFO *) pmt->AllocFormatBuffer(sizeof(VIDEOINFO));
    if(NULL == pvi)
        return(E_OUTOFMEMORY);

    ZeroMemory(pvi, sizeof(VIDEOINFO));
 pvi->bmiHeader.biSize       = sizeof(BITMAPINFOHEADER);
    pvi->bmiHeader.biWidth      = m_iImageWidth;
    pvi->bmiHeader.biHeight     = m_iImageHeight;
    pvi->bmiHeader.biPlanes     = 1;
    pvi->bmiHeader.biSizeImage  = GetBitmapSize(&pvi->bmiHeader);//只是得到大小
    pvi->bmiHeader.biClrImportant = 0;

    SetRectEmpty(&(pvi->rcSource)); // we want the whole image area rendered.
    SetRectEmpty(&(pvi->rcTarget)); // no particular destination rectangle

    pmt->SetType(&MEDIATYPE_Video);
    pmt->SetFormatType(&FORMAT_VideoInfo);
    pmt->SetTemporalCompression(FALSE);

    // Work out the GUID for the subtype from the header info.
    const GUID SubTypeGUID = GetBitmapSubtype(&pvi->bmiHeader);
    pmt->SetSubtype(&SubTypeGUID);
    pmt->SetSampleSize(pvi->bmiHeader.biSizeImage);

    return NOERROR;