Filters

Filters are the basic building blocks of DirectShow. DirectShow divides digital media processing into discrete steps, and each filter represents one or more steps. An application can perform a particular task by combining filters together.

For example:

• The Async File Source filter reads local files
• The TV Tuner filter changes the channel on an analog TV card
• The MPEG-2 Splitter filter parses audio and video data from an MPEG-2 stream.

Although each of these filters performs a unique function, the application can treat them in a similar way. All filters support the IBaseFilter interface; many support other, more specialized interfaces.

Most DirectShow filters fall into one of several broad categories:

• A source filter presents raw data for processing. It might get the data from a local file, a DVD, a television receiver card, a capture card connected to a camera, or any other source.
• A transform filter accepts data from another filter, processes it, and passes it to a third filter. Many types of transform filters exist: Parsers convert raw byte streams into meaningful media data; encoders and decoders convert between compressed and uncompressed data; splitters convert a single stream into two or more streams; mux filters combine several streams into one, and so forth.
• A renderer filter accepts media data from another filter and presents it to the user, either by displaying it on the monitor, or playing it through the speakers, or sending it to some other external device. This category also includes filters that save data to disk. The video renderers that come with DirectShow use DirectDraw, and the default audio renderer uses DirectSound.

Filters for playing, converting, and capturing many different media formats are supplied with DirectShow and ship with every copy of Windows. Developers can also build their own custom filters for handling either custom or standard data formats.

As of DirectX 8, DirectShow also supports a simpler model, DirectX Media Objects (DMOs). DMOs are similar to DirectShow filters but support a much smaller set of APIs. You can insert a DMO in a DirectShow filter graph, using the DMO Wrapper filter. Unlike a filter, however, you can also use a DMO outside of DirectShow. Although DMOs do not completely replace DirectShow filters, they are appropriate for many scenarios, particularly for creating custom encoders and decoders.