Creating an Encoder by Using CoCreateInstance

For converting media files into ASF format, you can use Windows Media encoders. To use these encoders, they must be registered with the system. Encoders are implemented as Media Foundation transforms (MFTs) and must expose the IMFTransform interface. This topic describes how an application can get a pointer to the required MFT encoder's IMFTransform interface and instantiate it for use.

For information about encoder registration, see Instantiating an Encoder MFT.

Using an Encoder's IMFTransform Interface

Upon successful registration of Windows Media encoders with the system, an application can enumerate the encoders by calling MFTEnum. To search for the right encoder, you must specify the following:

  • The GUID that represents the category, which is either MFT_CATEGORY_AUDIO_ENCODER or MFT_CATEGORY_VIDEO_ENCODER.

  • The format to match. This is set in the MFT_REGISTER_TYPE_INFO structure that specifies the major type and subtype of the media type that the encoder will generate samples in. This structure is passed in the pOutputType parameter. For information about the supported types, see Media Type GUIDs.

    Note  The input type information in the pInputType parameter is not required. This is because the input type is known to the application and the encoder expects the input stream to be in an uncompressed format.

MFTEnum returns an array of IMFTransform pointers for the encoder MFTs that match the search criteria. You can instantiate an encoder by calling the COM function CoCreateInstance and passing the CLSID of the encoder you want to use. This function returns a pointer to the IMFTransform interface that represents the encoder. For more information about this function call, see the Windows SDK documentation for the Component Object Model (COM).

Encoder Creation Example

The following code example shows how to create an audio or video encoder.


HRESULT FindEncoder(
    const GUID& subtype, 
    BOOL bAudio, 
    IMFTransform **ppEncoder
    )
{
    HRESULT hr = S_OK;
    UINT32 count = 0;

    CLSID *ppCLSIDs = NULL;

    MFT_REGISTER_TYPE_INFO info = { 0 };

    info.guidMajorType = bAudio ? MFMediaType_Audio : MFMediaType_Video;
    info.guidSubtype = subtype;

    hr = MFTEnum(   
        bAudio ? MFT_CATEGORY_AUDIO_ENCODER : MFT_CATEGORY_VIDEO_ENCODER,
        0,          // Reserved
        NULL,       // Input type
        &info,      // Output type
        NULL,       // Reserved
        &ppCLSIDs,
        &count
        );

    if (SUCCEEDED(hr) && count == 0)
    {
        hr = MF_E_TOPO_CODEC_NOT_FOUND;
    }

    // Create the first encoder in the list.

    if (SUCCEEDED(hr))
    {
        hr = CoCreateInstance(ppCLSIDs[0], NULL,
            CLSCTX_INPROC_SERVER, IID_PPV_ARGS(ppEncoder));
    }

    CoTaskMemFree(ppCLSIDs);
    return hr;
}


Related topics

Instantiating an Encoder MFT
Windows Media Encoders

 

 

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