The most important object in GStreamer for the application programmer is the GstElement object. The GstElement is the basic building block for the media pipeline. All the different components you are going to use are derived from this GstElement. This means that a lot of functions you are going to use operate on this object. You will see that those elements have pads. These are the elements connections with the 'outside' world. Depending on the number and direction of the pads, we can see three types of elements: source, filter and sink element. These three types are all the same GstElement object, they just differ in how the pads are. This element will generate data that will be used by the pipeline. It is typically a file or an audio source. Below you see how we will visualize the element. We always draw a src pad to the right of the element.

Source elements do not accept data, they only generate data. You can see this in the figure because it only has a src pad. A src pad can only generate buffers. Filter elements both have an input and an output pad. They operate on data they receive in the sink pad and send the result to the src pad. Examples of a filter element might include: an MPEG decoder, volume filter,... Filters may also contain any number of input pads and output pads. For example, a video mixer might have to input pads (the images of the two different video streams) and one output pad.

The above figure shows the visualisation of a filter element. This element has one sink pad (input) and one src (output) pad. Sink pads are drawn on the left of the element.

The above figure shows the visualisation of a filter element with more than one output pad. An example of such a filter is the AVI splitter. This element will parse the input data and extracts the audio and video data. Most of these filters dynamically send out a signal when a new pad is created so that the application programmer can connect an arbitrary element to the newly created pad. This element accepts data but will not generate any new data. A sink element is typically a file on disk, a soundcard, a display,... It is presented as below:

GstElements are created from factories. To create an element, one has to get access the a GstElementFactory using a unique factoryname. The following code example is used to get a factory that can be used to create the mpg123 element, an mp3 decoder. GstElementFactory *factory; factory = gst_elementfactory_find ("mpg123"); Once you have the handle to the elementfactory, you can create a real element with the following code fragment: GstElement *element; element = gst_elementfactory_create (factory, "decoder"); gst_elementfactory_create () will use the elementfactory to create an element with the given name. The name of the element is something you can use later on to lookup the element in a bin, for example. A simple shortcut exists for creating an element from a factory. The following example creates an element, named "decoder" from the elementfactory named "mpg123". This convenient function is most widly used to create an element. GstElement *element; element = gst_elementfactory_make ("mpg123", "decoder"); An element can be destroyed with: GstElement *element; ... gst_element_destroy (element);