# Elements

The most important object in GStreamer for the application programmer is the `GstElement` object.

## What is an element ?

An element is the basic building block for the media pipeline. All the different high-level 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" (see ), or link points. This terminology arises from soldering; pads are where wires can be attached.

## Types of elements

### Source elements

Source elements generate data for use by a pipeline, for example reading from disk or from a sound card.

shows how we will visualise a source element. We always draw a source pad to the right of the element.

#### Visualisation of a source 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.

### Filters and codecs

Filter elements have both 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.

#### Visualisation of a filter element

shows how we will visualise 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.

#### Visualisation of a filter element with more than one output pad

shows the visualisation of a filter element with more than one output pad. An example of such a filter is the AVI 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

Sink elements are end 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. shows a sink element.

#### Visualisation of a sink element