Overview of Graph Building

Microsoft DirectShow 9.0

Overview of Graph Building

To create a filter graph, begin by creating an instance of the Filter Graph Manager:

IGraphBuilder* pIGB;
HRESULT hr = CoCreateInstance(CLSID_FilterGraph,
    (void **)&pIGB);

The Filter Graph Manager supports the following graph-building methods:

  • IFilterGraph::ConnectDirect tries to make a direct connection between two pins. If the pins cannot connect, the method fails.
  • IGraphBuilder::Connect connects two pins. If possible, it makes a direct connection. Otherwise, it uses intermediate filters to complete the connection.
  • IGraphBuilder::Render starts from an output pin and builds the rest of the graph. This methods adds filters as needed, working downstream, until it reaches a renderer filter.
  • IGraphBuilder::RenderFile builds a complete file-playback graph.
  • IFilterGraph::AddFilter adds a filter to the graph. It does not connect the filter. You must create the filter before calling this method, either by calling CoCreateInstance or by using the Filter Mapper or System Device Enumerator.

These methods provide three basic approaches to building the graph:

  1. The Filter Graph Manager builds the entire graph.
  2. The Filter Graph Manager builds part of the graph.
  3. The application builds the entire graph.

The Filter Graph Manager Builds the Entire Graph

If you simply want to play a file that is authored in a recognized format, such as AVI, MPEG, WAV, or MP3, use the RenderFile method. The article How To Play a File shows how to do this.

The RenderFile method starts by looking in the registry for a source filter that can parse the file. It uses the protocol (such as "http://" in the URL), the file extension, or pre-defined byte patterns in the file to determine the source filter. For details, see Registering a Custom File Type.

To build the rest of the graph, the Filter Graph Manager uses an iterative process wherein it takes the media types that a filter supports on its output pins, and searches the registry for filters that will accept that media type as input. It uses several criteria to narrow the search and prioritize filters:

  • The filter category identifies the general functionality of the filter.
  • The media type describes what kind of data the filter can accept as input or deliver as output.
  • The merit determines the order in which filters are tried. If two filters in the same filter category both support the same input types, the Filter Graph Manager selects the one with the highest merit value. Some filters are purposely given a low merit value because they are designed for specialized purposes and should only be added to the graph by the application.

The Filter Graph Manager uses the Filter Mapper object to search the registry.

As each filter is added, the Filter Graph Manager tries to connect it to  the previous filter's output pin. The filters negotiate to determine whether they can connect, and if so, what media type to use for the connection. If the new filter cannot connect, the Filter Graph Manager discards it and tries another filter. This process continues until every stream is rendered.

The Filter Graph Manager Builds Part of the Graph

To do something beyond simply playing a file, your application must perform at least some of the graph building work. For example, a video capture application must select a capture source filter and add it to the graph. If you are writing data to an AVI file, you must add the AVI Mux and File Writer filters to the graph. However, it is often possible to let the Filter Graph Manager complete the graph. For example, you can render a pin for preview by calling the Render method.

The Application Builds the Entire Graph

In some scenarios, your application may need to build the graph by adding and connecting each filter. In this case, you probably know specifically which filters should be added to the graph. With this approach, the application adds each filter by calling AddFilter, enumerates the pins on the filters, and connects them by calling either Connect or ConnectDirect.

See Also