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7c24fc7450
MIME-type -> media types Fix up the manual in various places with the 1.0 way of doing things such as probes, static elements, scheduling, ... Add porting from 0.10 to 1.0 chapter. Add probe example to build. Remove some docs for remove components such as GstMixer and GstPropertyProbe, XML...
159 lines
7.2 KiB
XML
159 lines
7.2 KiB
XML
<chapter id="chapter-intro-basics">
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<title>Foundations</title>
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<para><!-- synchronize with PWG -->
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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 &GstPWG;.
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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 element's input and output, where
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you can connect other elements. They are used to negotiate links and
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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 usually
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means buffers (described by the <ulink type="http"
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url="&URLAPI;gstreamer-GstBuffer.html">
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<classname>GstBuffer</classname></ulink> object) and events (described
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by the <ulink type="http" url="&URLAPI;gstreamer-GstEvent.html">
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<classname>GstEvent</classname></ulink> object).
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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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Since bins are subclasses of elements
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themselves, you can mostly control a bin as if it were 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 bus messages from their
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contained children (such as error messages, tag messages or EOS messages).
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</para>
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<para>
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A <emphasis>pipeline</emphasis> is a top-level bin. It provides a bus for
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the application and manages the synchronization for its children.
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As you set it to PAUSED or PLAYING state, data flow will start and media
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processing will take place. Once started, pipelines will run in a
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separate thread until you stop them or the end
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of the data stream is reached.
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</para>
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<figure float="1" id="section-pipeline-img">
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<title>&GStreamer; pipeline for a simple ogg player</title>
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<mediaobject>
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<imageobject>
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<imagedata scale="75" fileref="images/simple-player.ℑ" format="&IMAGE;" />
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</imageobject>
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</mediaobject>
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</figure>
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</sect1>
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<sect1 id="section-intro-basics-communication">
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<title>Communication</title>
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<para>
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&GStreamer; provides several mechanisms for communication and data exchange
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between the <emphasis>application</emphasis> and the <emphasis>pipeline</emphasis>.
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</para>
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<itemizedlist>
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<listitem>
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<para>
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<emphasis>buffers</emphasis> are objects for passing streaming data
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between elements in the pipeline. Buffers always travel from sources
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to sinks (downstream).
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>events</emphasis> are objects sent between elements or from
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the application to elements. Events can travel upstream and downstream.
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Downstream events can be synchronised to the data flow.
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>messages</emphasis> are objects posted by elements on
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the pipeline's message bus, where they will be held for collection
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by the application. Messages can be intercepted synchronously from
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the streaming thread context of the element posting the message, but
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are usually handled asynchronously by the application from the
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application's main thread. Messages are used to transmit information
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such as errors, tags, state changes, buffering state, redirects etc.
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from elements to the application in a thread-safe way.
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</para>
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</listitem>
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<listitem>
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<para>
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<emphasis>queries</emphasis> allow applications to request information
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such as duration or current playback position from the pipeline.
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Queries are always answered synchronously. Elements can also use
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queries to request information from their peer elements (such as the
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file size or duration). They can be used both ways within a pipeline,
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but upstream queries are more common.
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</para>
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</listitem>
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</itemizedlist>
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<figure float="1" id="section-communication-img">
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<title>&GStreamer; pipeline with different communication flows</title>
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<mediaobject>
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<imageobject>
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<imagedata scale="75" fileref="images/communication.ℑ" format="&IMAGE;" />
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</imageobject>
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</mediaobject>
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</figure>
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</sect1>
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</chapter>
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