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Step 3B. Implement the GetMediaType Method

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[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 It has been superseded by MediaPlayer, IMFMediaEngine, and Audio/Video Capture in Media Foundation.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.]

This is step 3B of the tutorial Writing Transform Filters.

Note

This step is not required for filters that derive from CTransInPlaceFilter.

 

The CTransformFilter::GetMediaType method returns one of the filter's preferred output types, referenced by index number. This method is never called unless the filter's input pin is already connected. Therefore, you can use the media type from the upstream connection to determine the preferred output types.

An encoder typically offers a single preferred type, representing the target format. Decoders generally support a range of output formats, and offer them in order of descending quality or efficiency. For example, the list might be UYVY, Y211, RGB-24, RGB-565, RGB-555, and RGB-8, in that order. Effect filters may require an exact match between the output format and the input format.

The following example returns a single output type, which is constructed by modifying the input type to specify RLE8 compression:

HRESULT CRleFilter::GetMediaType(int iPosition, CMediaType *pMediaType)
{
    ASSERT(m_pInput->IsConnected());
    if (iPosition < 0)
    {
        return E_INVALIDARG;
    }
    if (iPosition == 0)
    {
        HRESULT hr = m_pInput->ConnectionMediaType(pMediaType);
        if (FAILED(hr))
        {
            return hr;
        }
        FOURCCMap fccMap = FCC('MRLE'); 
        pMediaType->subtype = static_cast<GUID>(fccMap);
        pMediaType->SetVariableSize();
        pMediaType->SetTemporalCompression(FALSE);

        ASSERT(pMediaType->formattype == FORMAT_VideoInfo);
        VIDEOINFOHEADER *pVih =
            reinterpret_cast<VIDEOINFOHEADER*>(pMediaType->pbFormat);
        pVih->bmiHeader.biCompression = BI_RLE8;
        pVih->bmiHeader.biSizeImage = DIBSIZE(pVih->bmiHeader); 
        return S_OK;
    }
    // else
    return VFW_S_NO_MORE_ITEMS;
}

In this example, the method calls IPin::ConnectionMediaType to get the input type from the input pin. Then it changes some of the fields to indicate the compression format, as follows:

  • It assigns a new subtype GUID, which is constructed from the FOURCC code 'MRLE', using the FOURCCMap class.
  • It calls the CMediaType::SetVariableSize method, which sets the bFixedSizeSamples flag to FALSE and the lSampleSize member to zero, indicating variable-sized samples.
  • It calls the CMediaType::SetTemporalCompression method with the value FALSE, indicating that every frame is a key frame. (This field is informational only, so you could safely ignore it.)
  • It sets the biCompression field to BI_RLE8.
  • It sets the biSizeImage field to the image size.

Next: Step 3C. Implement the CheckTransform Method.

Writing DirectShow Filters