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87 lines
4.3 KiB
XML
87 lines
4.3 KiB
XML
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<chapter id="chapter-intro-basics">
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<title>Foundations</title>
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<para>
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This chapter of the guide introduces the basic concepts of &GStreamer;.
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Understanding these concepts will be important in reading any of the
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rest of this guide, all of them assume understanding of these basic
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concepts.
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</para>
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<sect1 id="section-intro-basics-elements">
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<title>Elements</title>
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<para>
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An <emphasis>element</emphasis> is the most important class of objects
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in &GStreamer;. You will usually create a chain of elements linked
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together and let data flow through this chain of elements. An element
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has one specific function, which can be the reading of data from a
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file, decoding of this data or outputting this data to your sound
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card (or anything else). By chaining together several such elements,
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you create a <emphasis>pipeline</emphasis> that can do a specific task,
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for example media playback or capture. &GStreamer; ships with a large
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collection of elements by default, making the development of a large
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variety of media applications possible. If needed, you can also write
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new elements. That topic is explained in great deal in the Plugin
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Writer's Guide.
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</para>
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</sect1>
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<sect1 id="section-intro-basics-bins">
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<title>Bins and pipelines</title>
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<para>
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A <emphasis>bin</emphasis> is a container for a collection of elements.
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A pipeline is a special subtype of a bin that allows execution of all
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of its contained child elements. Since bins are subclasses of elements
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themselves, you can mostly control a bin as if it where an element,
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thereby abstracting away a lot of complexity for your application. You
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can, for example change state on all elements in a bin by changing the
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state of that bin itself. Bins also forward some signals from their
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contained childs (such as errors and tags).
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</para>
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<para>
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A pipeline is a bin that allows to <emphasis>run</emphasis> (technically
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referred to as <quote>iterating</quote>) its contained childs. By
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iterating a pipeline, data flow will start and media processing will
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take place. A pipeline requires iterating for anything to happen. you
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can also use threads, which automatically iterate the contained childs
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in a newly created threads. We will go into this in detail later on.
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</para>
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</sect1>
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<sect1 id="section-intro-basics-pads">
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<title>Pads</title>
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<para>
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<emphasis>Pads</emphasis> are used to negotiate links and data flow
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between elements in &GStreamer;. A pad can be viewed as a
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<quote>plug</quote> or <quote>port</quote> on an element where
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links may be made with other elements, and through which data can
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flow to or from those elements. Pads have specific data handling
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capabilities: A pad can restrict the type of data that flows
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through it. Links are only allowed between two pads when the
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allowed data types of the two pads are compatible. Data types are
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negotiated between pads using a process called <emphasis>caps
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negotiation</emphasis>. Data types are described as a
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<classname>GstCaps</classname>.
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</para>
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<para>
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An analogy may be helpful here. A pad is similar to a plug or jack on a
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physical device. Consider, for example, a home theater system consisting
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of an amplifier, a DVD player, and a (silent) video projector. Linking
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the DVD player to the amplifier is allowed because both devices have audio
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jacks, and linking the projector to the DVD player is allowed because
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both devices have compatible video jacks. Links between the
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projector and the amplifier may not be made because the projector and
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amplifier have different types of jacks. Pads in &GStreamer; serve the
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same purpose as the jacks in the home theater system.
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</para>
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<para>
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For the most part, all data in &GStreamer; flows one way through a link
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between elements. Data flows out of one element through one or more
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<emphasis>source pads</emphasis>, and elements accept incoming data
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through one or more <emphasis>sink pads</emphasis>. Source and sink
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elements have only source and sink pads, respectively. Data is
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embodied in a <classname>GstData</classname> structure.
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</para>
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</sect1>
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</chapter>
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