Enumerating Media Types

Pins support the IPin::EnumMediaTypes method, which enumerates a pin's preferred media types. It returns a pointer to the IEnumMediaTypes interface. The IEnumMediaTypes::Next method retrieves pointers to AM_MEDIA_TYPE structures describing media types.

The media type enumerator exists primarily to help the Filter Graph Manager make intelligent connections, and your applications will probably not use it. A pin does not necessarily return any preferred media types. Moreover, the media types it returns might depend on the filter's connection status. For example, a filter's output pin might return a different set of media types depending on which media type was set for the filter's input pin.

The following example finds a preferred media type that matches a specified major type, subtype, or format type.


// Given a pin, find a preferred media type 
//
// pPin         Pointer to the pin.
// majorType    Preferred major type (GUID_NULL = don't care).
// subType      Preferred subtype (GUID_NULL = don't care).
// formatType   Preferred format type (GUID_NULL = don't care).
// ppmt         Receives a pointer to the media type. Can be NULL.
//
// Note: If you want to check whether a pin supports a desired media type,
//       but do not need the format details, set ppmt to NULL.
//
//       If ppmt is not NULL and the method succeeds, the caller must
//       delete the media type, including the format block. 

HRESULT GetPinMediaType(
    IPin *pPin,             // pointer to the pin
    REFGUID majorType,      // desired major type, or GUID_NULL = don't care
    REFGUID subType,        // desired subtype, or GUID_NULL = don't care
    REFGUID formatType,     // desired format type, of GUID_NULL = don't care
    AM_MEDIA_TYPE **ppmt    // Receives a pointer to the media type. (Can be NULL)
    )
{
    *ppmt = NULL;

    IEnumMediaTypes *pEnum = NULL;
    AM_MEDIA_TYPE *pmt = NULL;
    BOOL bFound = FALSE;
    
    HRESULT hr = pPin->EnumMediaTypes(&pEnum);
    if (FAILED(hr))
    {
        return hr;
    }

    while (hr = pEnum->Next(1, &pmt, NULL), hr == S_OK)
    {
        if ((majorType == GUID_NULL) || (majorType == pmt->majortype))
        {
            if ((subType == GUID_NULL) || (subType == pmt->subtype))
            {
                if ((formatType == GUID_NULL) || 
                    (formatType == pmt->formattype))
                {
                    // Found a match. 
                    if (ppmt)
                    {
                        *ppmt = pmt;  // Return it to the caller
                    }
                    else
                    {
                        _DeleteMediaType(pmt);
                    }
                    bFound = TRUE;
                    break;
                }
            }
        }
        _DeleteMediaType(pmt);
    }

    SafeRelease(&pEnum);
    if (SUCCEEDED(hr))
    {
        if (!bFound)
        {
            hr = VFW_E_NOT_FOUND;
        }
    }
    return hr;
}


Note  This example uses the SafeRelease function to release interface pointers.
 

Related topics

Enumerating Objects in a Filter Graph
IEnumMediaTypes

 

 

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