Creating Multimedia Stream Objects and Stream Samples

Objects that support the IMultiMediaStream interface are the basic containers for multimedia data streams. The IMultiMediaStream interface includes methods that enumerate the object's data streams; these streams are typically video and audio data, but can include data of any format, such as closed-captioning, plain text, or SMPTE timecode. The IMultiMediaStream interface is a generic container, however; developers can create other versions of the interface that support specific data formats. Objects that implement the IAMMultiMediaStream interface, for example, can enumerate and control streams of any DirectShow data format. Because individual data streams are format specific, they support at least two different interfaces: one generic and one data-specific. Every stream supports the IMediaStream interface, which provides methods to retrieve its format and a pointer to the stream itself. The IDirectDrawMediaStream interface, on the other hand, has methods that deal specifically with rendering video data. Any interface derived from IMultiMediaStream also supports the creation of stream samples, the basic units of streaming data.

A multimedia sample is a reference to an object containing the media data. For a video image, this is a DirectDraw surface. The sample's exact content varies, depending on the type of media (sound, text, and so on). Because a sample is only a reference to the data object, any number of stream samples can refer to the same object. The IStreamSample interface provides methods that get and set a sample's characteristics, such as its start and stop time, status, and stream association. The IStreamSample::Update method refreshes the sample's data in the case of readable streams. For writable streams, it will write the sample's data to the stream. Typically, you use the Update method in a loop that renders, transfers, or stores streaming data.