Source Filter Types

Source Filter Types

In a broad sense, there are three types of source filters used in DirectShow filter graphs capture source filters, file source filters, and creator source filters. We re already quite familiar with capture source filters, such as the Microsoft DV Camcorder and VCR filter and the Logitech QuickCam Web filter, which we ve used in numerous DirectShow applications. A capture source filter captures a live sample, converts it to a stream of media samples, and presents that stream on its output pins. Capture source filters are nearly always associated with devices, and in general, there s rarely a need for DirectShow programmers to write their own capture source filters. Any device that has a correctly written Windows Driver Model (WDM) driver will automatically appear as a filter available to DirectShow applications. DirectShow puts a filter wrapper around WDM and Video for Windows (VFW) device drivers so that the capture device, for example, can be used in a filter graph as a capture source filter. (See Figure 12-1.) If you really want or need to write your own capture driver filter, you can find the gory details in the Microsoft DirectX SDK documentation and Windows DDK documentation.

Figure 12-1. A capture source filter encapsulates a WDM interface to a hardware device

A file source filter acts as an interface between the world outside a DirectShow filter graph and the filter graph. The File Source filter, for example, makes the Microsoft Windows file system available to DirectShow; it allows the contents of a disk-based file to be read into the filter graph. (See Figure 12-2.) Another filter, the File Source URL filter, takes a URL as its parameter and allows Web-based files to be read into a filter graph. (This is a technique that could potentially be very powerful.) We ve already used the Stream Buffer Engine s source filter, a file source filter that reads stream data from files created by the Stream Buffer Engine s sink filter. The ASF Reader filter takes an ASF file and generates media streams from its contents.

Figure 12-2. The File Source filter followed by a Wave Parser filter

With the notable exceptions of the SBE source filter and the ASF Reader, a file source filter normally sits upstream from a transform filter that acts as a file parser, translating the raw data read in by the file source filter into a series of media samples. The Wave Parser filter, for example, translates the WAV file format into a series of PCM media samples. Because the relationship between a file source filter and a parser filter is driven by the parser filter, the parser filter usually pulls the data from the file source filter. This is the reverse of the situation for capture ( live ) source filters, which produce a regular stream of media samples and then push them to a pin on a downstream filter. Unless you re adding your own unique file media type to DirectShow, it s unlikely you ll need a custom parser transform filter. (You re a bit on your own here. There aren t any good examples of parser filters in the DirectX SDK samples.)

Finally there are source filters that create media samples in their entirety. This means that the sample data originates in the filter, based upon the internal settings and state of the filter. You can imagine a creator source filter acting as a tone generator, for example, producing a stream of PCM data reflecting a waveform of a particular frequency. (If it were designed correctly, the COM interfaces on such a filter would allow you to adjust the frequency on the fly with a method invocation.) Another creator source filter could produce the color bars that video professionals use to help them adjust their equipment, which would clearly be useful in a DirectShow application targeted at videographers.

Figure 12-3. The Generic Push Source filter creates its own media stream

The PushSource filter example explored in this chapter (and shown in Figure 12-3) is a creator source filter that produces a media stream with a numbered series of video frames and that stops automatically when a certain number of frames have been generated. (When a source filter runs dry, all filters downstream of it eventually stop, waiting for more stream data.) PushSource is not complicated and only scratches the surface of what s possible with source filters. An excellent teaching example, PushSource exposes the internals of the source filter and, through that, much of how DirectShow handles the execution of the filter graph.