The most important object in GStreamer for the application programmer is the GstElement object. GstElement is the basic building block for the media pipeline. All the different components you are going to use are derived from GstElement. This means that a lot of functions you are going to use operate on objects of this class. Elements, from the perspective of GStreamer, are viewed as "black boxes" with a number of different aspects. One of these aspects is the presence of "pads", or link points. This terminology arises from soldering; pads are where wires can be attached. Source elements generate data for use by a pipeline, for example reading from disk or from a sound card. Below you see how we will visualize the element. We always draw a source 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 source pad. A source pad can only generate data. Filter elements both have input and output pads. They operate on data they receive in their sink pads and produce data on their source pads. For example, MPEG decoders and volume filters would fall into this category. Elements are not constrained as to the number of pads they might have; for example, a video mixer might have two 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 (input) pad and one source (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 (demultiplexer). This element will parse the input data and extract 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 link an arbitrary element to the newly created pad. Sink elements are terminal points in a media pipeline. They accept data but do not produce anything. Disk writing, soundcard playback, and video output would all be implemented by sink elements.

A GstElement object is created from a factory. To create an element, you have to get access to a GstElementFactory object using a unique factory name. The following code example is used to get a factory that can be used to create the 'mad' element, an mp3 decoder. GstElementFactory *factory; factory = gst_element_factory_find ("mad"); Once you have the handle to the element factory, you can create a real element with the following code fragment: GstElement *element; element = gst_element_factory_create (factory, "decoder"); gst_element_factory_create will use the element factory to create an element with the given name. The name of the element is something you can use later on to look up the element in a bin, for example. You can pass NULL as the name argument to get a unique, default name. A simple shortcut exists for creating an element from a factory. The following example creates an element named "decoder" from the element factory named "mad". This convenience function is most widely used to create an element. GstElement *element; element = gst_element_factory_make ("mad", "decoder"); When you don't need the element anymore, you need to unref it, as shown in the following example. GstElement *element; ... gst_element_unref (element); A GstElement can have several properties which are implemented using standard GObject properties. The usual GObject methods to query, set and get property values and GParamSpecs are therefore supported. Every GstElement inherits at least one property of its parent GstObject: the "name" property. This is the name you provide to the functions gst_element_factory_make or gst_element_factory_create. You can get and set this property using the functions gst_object_set_name and gst_object_get_name or use the GObject property mechanism as shown below. GstElement *element; GValue value = { 0, }; /* initialize the GValue for g_object_get() */ element = gst_element_factory_make ("mad", "decoder"); g_object_set (G_OBJECT (element), "name", "mydecoder", NULL); ... g_value_init (&value, G_TYPE_STRING); g_object_get_property (G_OBJECT (element), "name", &value); ... Most plugins provide additional properties to provide more information about their configuration or to configure the element. gst-inspect is a useful tool to query the properties of a particular element, it will also use property introspection to give a short explanation about the function of the property and about the parameter types and ranges it supports. For more information about GObject properties we recommend you read the GObject manual. A GstElement also provides various GObject signals that can be used as a flexible callback mechanism. We talk some more about the GstElementFactory object.