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Setting Deinterlace Preferences

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Microsoft DirectShow 9.0

Setting Deinterlace Preferences

The Video Mixing Renderer (VMR) supports hardware-accelerated deinterlacing, which improves rendering quality for interlaced video. The exact features that are available depend on the underlying hardware. The application can query for the hardware deinterlacing capabilities and set deinterlacing preferences through the IVMRDeinterlaceControl interface (VMR-7) or IVMRDeinterlaceControl9 interface (VMR-9). Deinterlacing is performed on a per-stream basis.

There is one important difference in interlacing behavior between the VMR-7 and the VMR-9. On systems where the graphics hardware doesn't support advanced deinterlacing, the VMR-7 can fall back to the hardware overlay and instruct it to use a BOB style deinterlace. In this case, although the VMR is reporting 30fps the video is actually being rendered at 60 flips per second.

Except in the case of the VMR-7 using hardware overlay, deinterlacing is performed by the VMR's mixer. The mixer uses the DirectX Video Acceleration (DXVA) deinterlacing device driver interface (DDI) to perform the deinterlacing. This DDI is not callable by applications, and applications cannot replace the VMR's deinterlacing functionality. However, an application can select the desired deinterlacing mode, as described in this section.

  • Note   This section describes the IVMRDeinterlaceControl9 methods, but the VMR-7 versions are almost identical.

To get the deinterlacing capabilities for a video stream, do the following:

  1. Fill in a VMR9VideoDesc structure with a description of the video stream. Details of how to fill in this structure are given later.
  2. Pass the structure to the IVMRDeinterlaceControl9::GetNumberOfDeinterlaceModes method. Call the method twice. The first call returns the number of deinterlace modes the hardware supports for the specified format. Allocate an array of GUIDs of this size, and call the method again, passing in the address of the array. The second call fills the array with GUIDs. Each GUID identifies one deinterlacing mode.
  3. To get the capabiltiies of a particular mode, call the IVMRDeinterlaceControl9::GetDeinterlaceModeCaps method. Pass in the same VMR9VideoDesc structure, along with one of the GUIDs from the array. The method fills a VMR9DeinterlaceCaps structure with the mode capabilities.

The following code shows these steps:

VMR9VideoDesc VideoDesc; 
DWORD dwNumModes = 0;
// Fill in the VideoDesc structure (not shown).
hr = pDeinterlace->GetNumberOfDeinterlaceModes(&VideoDesc, 
    &dwNumModes, NULL);
if (SUCCEEDED(hr) && (dwNumModes != 0))
{
    // Allocate an array for the GUIDs that identify the modes.
    GUID *pModes = new GUID[dwNumModes];
    if (pModes)
    {
        // Fill the array.
        hr = pDeinterlace->GetNumberOfDeinterlaceModes(&VideoDesc, 
            &dwNumModes, pModes);
        if (SUCCEEDED(hr))
        {
            // Loop through each item and get the capabilities.
            for (int i = 0; i < dwNumModes; i++)
            {
                VMR9DeinterlaceCaps Caps;
                hr = pDeinterlace->GetDeinterlaceModeCaps(pModes + i, 
                    &VideoDesc, &Caps);
                if (SUCCEEDED(hr))
                {
                    // Examine the Caps structure.
                }
            }
        }
        delete [] pModes;
    }
}

Now the application can set the deinterlacing mode for the stream, using the following methods:

  • The SetDeinterlaceMode method sets the preferred mode. Use GUID_NULL to turn off deinterlacing.
  • The SetDeinterlacePrefs method specifies the behavior if the requested mode is not available.
  • The GetDeinterlaceMode method returns the preferred mode that you set.
  • The GetActualDeinterlaceMode method returns the actual mode in use, which might be a fallback mode, if the preferred mode is not available.

The method reference pages give more information.

Using the VMR9VideoDesc Structure

In the procedure given previously, the first step is to fill in a VMR9VideoDesc structure with a description of the video stream. Start by getting the media type of the video stream. You can do this by calling IPin::ConnectionMediaType on the VMR filter's input pin. Then confirm whether the video stream is interlaced. Only VIDEOINFOHEADER2 formats can be interlaced. If the format type is FORMAT_VideoInfo, it must be a progressive frame. If the format type is FORMAT_VideoInfo2, check the dwInterlaceFlags field for the AMINTERLACE_IsInterlaced flag. The presence of this flag indicates the video is interlaced.

Assume that the variable pBMI is a pointer to the BITMAPINFOHEADER structure in the format block. Set the following values in the VMR9VideoDesc structure:

  • dwSize: Set this field to sizeof(VMR9VideoDesc).

  • dwSampleWidth: Set this field to pBMI->biWidth.

  • dwSampleHeight: Set this field to abs(pBMI->biHeight).

  • SampleFormat: This field describes the interlace characteristics of the media type. Check the dwInterlaceFlags field in the VIDEOINFOHEADER2 structure, and set SampleFormat equal to the equivalent VMR9_SampleFormat flag. A helper function to do this is given below.

  • InputSampleFreq: This field gives the input frequency, which can be calculated from the AvgTimePerFrame field in the VIDEOINFOHEADER2 structure. In the general case, set dwNumerator to 10000000, and set dwDenominator to AvgTimePerFrame. However, you can also check for some well-known frame rates:

    Average time per frame Frame rate (fps) Numerator Denominator
    166833 59.94 (NTSC) 60000 1001
    333667 29.97 (NTSC) 30000 1001
    417188 23.97 (NTSC) 24000 1001
    200000 50.00 (PAL) 50 1
    400000 25.00 (PAL) 25 1
    416667 24.00 (Film) 24 1

  • OutputFrameFreq: This field gives the output frequency, which can be calculated from the InputSampleFreq value and the interleaving characteristics of the input stream:

    • Set OutputFrameFreq.dwDenominator equal to InputSampleFreq.dwDenominator.
    • If the input video is interleaved, set OutputFrameFreq.dwNumerator to 2 x InputSampleFreq.dwNumerator. (After deinterlacing, the frame rate is doubled.) Otherwise, set the value to InputSampleFreq.dwNumerator.

  • dwFourCC: Set this field to pBMI->biCompression.

The following helper function converts AMINTERLACE_X flags to VMR9_SampleFormat values:

#define IsInterlaced(x) ((x) & AMINTERLACE_IsInterlaced)
#define IsSingleField(x) ((x) & AMINTERLACE_1FieldPerSample)
#define IsField1First(x) ((x) & AMINTERLACE_Field1First)

VMR9_SampleFormat ConvertInterlaceFlags(DWORD dwInterlaceFlags)
{
    if (IsInterlaced(dwInterlaceFlags)) {
        if (IsSingleField(dwInterlaceFlags)) {
            if (IsField1First(dwInterlaceFlags)) {
                return VMR9_SampleFieldSingleEven;
            }
            else {
                return VMR9_SampleFieldSingleOdd;
            }
        }
        else {
            if (IsField1First(dwInterlaceFlags)) {
                return VMR9_SampleFieldInterleavedEvenFirst;
             }
            else {
                return VMR9_SampleFieldInterleavedOddFirst;
            }
        }
    }
    else {
        return VMR9_SampleProgressiveFrame;  // Not interlaced.
    }
}