Step 1. Query for Stream Information

First, query the DMO for the number of streams it supports and the preferred media types for each stream. To retrieve the number of input streams and output streams, call the IMediaObject::GetStreamCount method.

For each stream, the DMO ranks its preferred media types in order of preference and assigns each type an index, starting from zero. To retrieve a preferred media type for a particular stream, call the IMediaObject::GetInputType method or IMediaObject::GetOutputType method. Specify a stream number and a media-type index. To enumerate all the media types on a stream, use a loop that increments the media-type index until the method returns DMO_E_NO_MORE_ITEMS, as shown in the following pseudo-code:

DWORD cInputs, cOutputs, type = 0
DMO_MEDIA_TYPE mt

pDMO->GetStreamCount(&cInputs, &cOutputs)

for (DWORD i = 0; i < cInputs; i++)
{
    while (pDMO->GetInputType(i, type, &mt) != DMO_E_NO_MORE_ITEMS)
    {
        if ( this media type is one you want )
            break
        MoFreeMediaType(&mt)
        type++
    }
}

The GetInputType and GetOutputType methods return a DMO_MEDIA_TYPE structure with the media type. The following structure members are relevant:

• majortype: A globally unique identifier (GUID) that specifies the major type of the stream. The major type is the general classification, such as video or audio. For a list of major types, see Media Types.
• subtype: A GUID that specifies the subtype of the stream. The subtype further classifies the type. For example, MEDIASUBTYPE_RGB24 specifies 24-bit RGB video. For a list of subtypes, see Media Types.
• pbFormat: Pointer to a structure that describes the format in detail. The format structure specifies information such as the width and height of a video image, or the sample rate of an audio sample. Different media types use different format structures. Most video data uses the VIDEOINFOHEADER structure. Audio data uses the WAVEFORMATEX structure.
• formattype: Specifies the format structure contained in pbFormat.

  The media type might have a NULL format structure, indicated by a value of GUID_NULL for the formattype member. A NULL format indicates that the DMO can accept a range of formats within the specified media type. For example, a stream that requires PCM audio might accept a range of sample rates. Therefore, it returns MEDIATYPE_Audio for the major type, MEDIASUBTYPE_PCMAudio for the subtype, and a NULL format.

The application must call the MoFreeMediaType function to free the pbFormat member.