Negotiating Media Types

When the Filter Graph Manager calls the IPin::Connect method, it has several options for specifying a media type:

  • Complete type: If the media type is fully specified, the pins attempt to connect with that type. If they cannot, the connection attempt fails.
  • Partial media type: A media type is partial if the major type, subtype, or format type is GUID_NULL. The value GUID_NULL acts as a "wildcard," indicating that any value is acceptable. The pins negotiate a type that is consistent with the partial type.
  • No media type: If the Filter Graph Manager passes a NULL pointer, the pins can agree to any media type that is acceptable to both pins.

If the pins do connect, the connection always has a complete media type. The purpose of the media type given by the Filter Graph Manager is to limit the possible connection types.

During the negotiation process, the output pin proposes a media type by calling the input pin's IPin::ReceiveConnection method. The input pin can accept or reject the proposed type. This process repeats until either the input pin accepts a type, or the output pin runs out of types and the connection fails.

How an output pin selects media types to propose depends on the implementation. In the DirectShow base classes, the output pin calls IPin::EnumMediaTypes on the input pin. This method returns an enumerator that enumerates the input pin's preferred media types. Failing that, the output pin enumerates its own preferred types.

Working with Media Types

In any function that receives an AM_MEDIA_TYPE parameter, always validate the values of cbFormat and formattype before dereferencing the pbFormat member. The following code is incorrect:

if (pmt->formattype == FORMAT_VideoInfo)
    // Wrong!

The following code is correct:

if ((pmt->formattype == FORMAT_VideoInfo) && 
    (pmt->cbFormat > sizeof(VIDEOINFOHEADER) &&
    (pbFormat != NULL))
    // Now you can dereference pVIH.