1 <chapter id="chapter-intro-basics">
2 <title>Foundations</title>
4 This chapter of the guide introduces the basic concepts of &GStreamer;.
5 Understanding these concepts will be important in reading any of the
6 rest of this guide, all of them assume understanding of these basic
10 <sect1 id="section-intro-basics-elements">
11 <title>Elements</title>
13 An <emphasis>element</emphasis> is the most important class of objects
14 in &GStreamer;. You will usually create a chain of elements linked
15 together and let data flow through this chain of elements. An element
16 has one specific function, which can be the reading of data from a
17 file, decoding of this data or outputting this data to your sound
18 card (or anything else). By chaining together several such elements,
19 you create a <emphasis>pipeline</emphasis> that can do a specific task,
20 for example media playback or capture. &GStreamer; ships with a large
21 collection of elements by default, making the development of a large
22 variety of media applications possible. If needed, you can also write
23 new elements. That topic is explained in great deal in the Plugin
28 <sect1 id="section-intro-basics-bins">
29 <title>Bins and pipelines</title>
32 A <emphasis>bin</emphasis> is a container for a collection of elements.
33 A pipeline is a special subtype of a bin that allows execution of all
34 of its contained child elements. Since bins are subclasses of elements
35 themselves, you can mostly control a bin as if it where an element,
36 thereby abstracting away a lot of complexity for your application. You
37 can, for example change state on all elements in a bin by changing the
38 state of that bin itself. Bins also forward some signals from their
39 contained childs (such as errors and tags).
42 A pipeline is a bin that allows to <emphasis>run</emphasis> (technically
43 referred to as <quote>iterating</quote>) its contained childs. By
44 iterating a pipeline, data flow will start and media processing will
45 take place. A pipeline requires iterating for anything to happen. you
46 can also use threads, which automatically iterate the contained childs
47 in a newly created threads. We will go into this in detail later on.
51 <sect1 id="section-intro-basics-pads">
54 <emphasis>Pads</emphasis> are used to negotiate links and data flow
55 between elements in &GStreamer;. A pad can be viewed as a
56 <quote>plug</quote> or <quote>port</quote> on an element where
57 links may be made with other elements, and through which data can
58 flow to or from those elements. Pads have specific data handling
59 capabilities: A pad can restrict the type of data that flows
60 through it. Links are only allowed between two pads when the
61 allowed data types of the two pads are compatible. Data types are
62 negotiated between pads using a process called <emphasis>caps
63 negotiation</emphasis>. Data types are described as a
64 <classname>GstCaps</classname>.
67 An analogy may be helpful here. A pad is similar to a plug or jack on a
68 physical device. Consider, for example, a home theater system consisting
69 of an amplifier, a DVD player, and a (silent) video projector. Linking
70 the DVD player to the amplifier is allowed because both devices have audio
71 jacks, and linking the projector to the DVD player is allowed because
72 both devices have compatible video jacks. Links between the
73 projector and the amplifier may not be made because the projector and
74 amplifier have different types of jacks. Pads in &GStreamer; serve the
75 same purpose as the jacks in the home theater system.
78 For the most part, all data in &GStreamer; flows one way through a link
79 between elements. Data flows out of one element through one or more
80 <emphasis>source pads</emphasis>, and elements accept incoming data
81 through one or more <emphasis>sink pads</emphasis>. Source and sink
82 elements have only source and sink pads, respectively. Data is
83 embodied in a <classname>GstData</classname> structure.