gstreamer/docs/pwg/intro-basics.xml
2012-10-12 12:55:25 +02:00

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<!-- ############ chapter ############# -->
<chapter id="chapter-intro-basics" xreflabel="Foundations">
<title>Foundations</title>
<para><!-- synchronize with AppDevMan -->
This chapter of the guide introduces the basic concepts of &GStreamer;.
Understanding these concepts will help you grok the issues involved in
extending &GStreamer;. Many of these concepts are explained in greater
detail in the &GstAppDevMan;; the basic concepts presented here serve mainly
to refresh your memory.
</para>
<!-- ############ sect1 ############# -->
<sect1 id="section-basics-elements" xreflabel="Elements and Plugins">
<title>Elements and Plugins</title>
<para>
Elements are at the core of &GStreamer;. In the context of plugin
development, an <emphasis>element</emphasis> is an object derived from the
<ulink type="http" url="../../gstreamer/html/GstElement.html"><classname>
GstElement</classname></ulink> class. Elements provide some sort of
functionality when linked with other elements: For example, a source
element provides data to a stream, and a filter element acts on the data
in a stream. Without elements, &GStreamer; is just a bunch of conceptual
pipe fittings with nothing to link. A large number of elements ship
with &GStreamer;, but extra elements can also be written.
</para>
<para>
Just writing a new element is not entirely enough, however: You will need
to encapsulate your element in a <emphasis>plugin</emphasis> to enable
&GStreamer; to use it. A plugin is essentially a loadable block of code,
usually called a shared object file or a dynamically linked library. A
single plugin may contain the implementation of several elements, or just
a single one. For simplicity, this guide concentrates primarily on plugins
containing one element.
</para>
<para>
A <emphasis>filter</emphasis> is an important type of element that
processes a stream of data. Producers and consumers of data are called
<emphasis>source</emphasis> and <emphasis>sink</emphasis> elements,
respectively. <emphasis>Bin</emphasis> elements contain other elements.
One type of bin is responsible for synchronization of the elements that they
contain so that data flows smoothly. Another type of bin, called
<emphasis>autoplugger</emphasis> elements, automatically add other
elements to the bin and links them together so that they act as a
filter between two arbitrary stream types.
</para>
<para>
The plugin mechanism is used everywhere in &GStreamer;, even if only the
standard packages are being used. A few very basic functions reside in the
core library, and all others are implemented in plugins. A plugin registry
is used to store the details of the plugins in an binary registry file.
This way, a program using &GStreamer; does not have to load all plugins to
determine which are needed. Plugins are only loaded when their provided
elements are requested.
</para>
<para>
See the &GstLibRef; for the current implementation details of <ulink
type="http"
url="../../gstreamer/html/GstElement.html"><classname>GstElement</classname></ulink>
and <ulink type="http"
url="../../gstreamer/html/GstPlugin.html"><classname>GstPlugin</classname></ulink>.
</para>
</sect1>
<!-- ############ sect1 ############# -->
<sect1 id="section-basics-pads" xreflabel="Pads">
<title>Pads</title>
<para>
<emphasis>Pads</emphasis> are used to negotiate links and data flow
between elements in &GStreamer;. A pad can be viewed as a
<quote>place</quote> or <quote>port</quote> on an element where
links may be made with other elements, and through which data can
flow to or from those elements. Pads have specific data handling
capabilities: A pad can restrict the type of data that flows
through it. Links are only allowed between two pads when the
allowed data types of the two pads are compatible.
</para>
<para>
An analogy may be helpful here. A pad is similar to a plug or jack on a
physical device. Consider, for example, a home theater system consisting
of an amplifier, a DVD player, and a (silent) video projector. Linking
the DVD player to the amplifier is allowed because both devices have audio
jacks, and linking the projector to the DVD player is allowed because
both devices have compatible video jacks. Links between the
projector and the amplifier may not be made because the projector and
amplifier have different types of jacks. Pads in &GStreamer; serve the
same purpose as the jacks in the home theater system.
</para>
<para>
For the most part, all data in &GStreamer; flows one way through a link
between elements. Data flows out of one element through one or more
<emphasis>source pads</emphasis>, and elements accept incoming data through
one or more <emphasis>sink pads</emphasis>. Source and sink elements have
only source and sink pads, respectively.
</para>
<para>
See the &GstLibRef; for the current implementation details of a <ulink
type="http"
url="../../gstreamer/html/GstPad.html"><classname>GstPad</classname></ulink>.
</para>
</sect1>
<!-- ############ sect1 ############# -->
<sect1 id="section-basics-data" xreflabel="Data, Buffers and Events">
<title>GstMiniObject, Buffers and Events</title>
<para>
All streams of data in &GStreamer; are chopped up into chunks that are
passed from a source pad on one element to a sink pad on another element.
<emphasis>GstMiniObject</emphasis> is the structure used to hold these
chunks of data.
</para>
<para>
GstMiniObject contains the following important types:
<itemizedlist>
<listitem>
<para>
An exact type indicating what type of data (event, buffer, ...)
this GstMiniObject is.
</para>
</listitem>
<listitem>
<para>
A reference count indicating the number of elements currently
holding a reference to the miniobject. When the reference count
falls to zero, the miniobject will be disposed, and its memory will be
freed in some sense (see below for more details).
</para>
</listitem>
</itemizedlist>
</para>
<para>
For data transport, there are two types of GstMiniObject defined:
events (control) and buffers (content).
</para>
<para>
Buffers may contain any sort of data that the two linked pads
know how to handle. Normally, a buffer contains a chunk of some sort of
audio or video data that flows from one element to another.
</para>
<para>
Buffers also contain metadata describing the buffer's contents. Some of
the important types of metadata are:
<itemizedlist>
<listitem>
<para>
Pointers to one or more GstMemory objects. GstMemory objects are
refcounted objects that encapsulate a region of memory.
</para>
</listitem>
<listitem>
<para>
A timestamp indicating the preferred display timestamp of the
content in the buffer.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Events
contain information on the state of the stream flowing between the two
linked pads. Events will only be sent if the element explicitly supports
them, else the core will (try to) handle the events automatically. Events
are used to indicate, for example, a media type, the end of a
media stream or that the cache should be flushed.
</para>
<para>
Events may contain several of the following items:
<itemizedlist>
<listitem>
<para>
A subtype indicating the type of the contained event.
</para>
</listitem>
<listitem>
<para>
The other contents of the event depend on the specific event type.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Events will be discussed extensively in <xref linkend="chapter-advanced-events"/>.
Until then, the only event that will be used is the <emphasis>EOS</emphasis>
event, which is used to indicate the end-of-stream (usually end-of-file).
</para>
<para>
See the &GstLibRef; for the current implementation details of a <ulink
type="http"
url="../../gstreamer/html/gstreamer-GstMiniObject.html"><classname>GstMiniObject</classname></ulink>, <ulink type="http"
url="../../gstreamer/html/gstreamer-GstBuffer.html"><classname>GstBuffer</classname></ulink> and <ulink type="http"
url="../../gstreamer/html/gstreamer-GstEvent.html"><classname>GstEvent</classname></ulink>.
</para>
<sect2 id="sect2-buffer-allocation" xreflabel="Buffer Allocation">
<title>Buffer Allocation</title>
<para>
Buffers are able to store chunks of memory of several different
types. The most generic type of buffer contains memory allocated
by malloc(). Such buffers, although convenient, are not always
very fast, since data often needs to be specifically copied into
the buffer.
</para>
<para>
Many specialized elements create buffers that point to special
memory. For example, the filesrc element usually
maps a file into the address space of the application (using mmap()),
and creates buffers that point into that address range. These
buffers created by filesrc act exactly like generic buffers, except
that they are read-only. The buffer freeing code automatically
determines the correct method of freeing the underlying memory.
Downstream elements that receive these kinds of buffers do not
need to do anything special to handle or unreference it.
</para>
<para>
Another way an element might get specialized buffers is to
request them from a downstream peer through a GstBufferPool or
GstAllocator. Elements can ask a GstBufferPool or GstAllocator
from the downstream peer element. If downstream is able to provide
these objects, upstream can use them to allocate buffers.
See more in <xref linkend="chapter-allocation"/>.
</para>
<para>
Many sink elements have accelerated methods for copying data
to hardware, or have direct access to hardware. It is common
for these elements to be able to create a GstBufferPool or
GstAllocator for their upstream peers. One such example is
ximagesink. It creates buffers that contain XImages. Thus,
when an upstream peer copies data into the buffer, it is copying
directly into the XImage, enabling ximagesink to draw the
image directly to the screen instead of having to copy data
into an XImage first.
</para>
<para>
Filter elements often have the opportunity to either work on
a buffer in-place, or work while copying from a source buffer
to a destination buffer. It is optimal to implement both
algorithms, since the &GStreamer; framework can choose the
fastest algorithm as appropriate. Naturally, this only makes
sense for strict filters -- elements that have exactly the
same format on source and sink pads.
</para>
</sect2>
</sect1>
<!-- ############ sect1 ############# -->
<sect1 id="section-basics-types" xreflabel="Types and Properties">
<title>Media types and Properties</title>
<para>
&GStreamer; uses a type system to ensure that the data passed between
elements is in a recognized format. The type system is also important
for ensuring that the parameters required to fully specify a format match
up correctly when linking pads between elements. Each link that is
made between elements has a specified type and optionally a set of
properties. See more about caps negotiation in
<xref linkend="chapter-negotiation"/>.
</para>
<!-- ############ sect2 ############# -->
<sect2 id="sect2-types-basictypes" xreflabel="Basic Types">
<title>The Basic Types</title>
<para>
&GStreamer; already supports many basic media types. Following is a
table of a few of the basic types used for buffers in
&GStreamer;. The table contains the name ("media type") and a
description of the type, the properties associated with the type, and
the meaning of each property. A full list of supported types is
included in <xref linkend="section-types-definitions"/>.
</para>
<table frame="all" id="table-basictypes" xreflabel="Table of Example Types">
<title>Table of Example Types</title>
<tgroup cols="6" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Media Type</entry>
<entry>Description</entry>
<entry>Property</entry>
<entry>Property Type</entry>
<entry>Property Values</entry>
<entry>Property Description</entry>
</row>
</thead>
<tbody valign="top">
<!-- ############ type ############# -->
<row>
<entry morerows="1">audio/*</entry>
<entry morerows="1">
<emphasis>All audio types</emphasis>
</entry>
<entry>rate</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The sample rate of the data, in samples (per channel) per second.
</entry>
</row>
<row>
<entry>channels</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The number of channels of audio data.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry>audio/x-raw</entry>
<entry>
Unstructured and uncompressed raw integer audio data.
</entry>
<entry>format</entry>
<entry>string</entry>
<entry>
S8 U8 S16LE S16BE U16LE U16BE S24_32LE S24_32BE U24_32LE U24_32BE S32LE S32BE U32LE U32BE
S24LE S24BE U24LE U24BE S20LE S20BE U20LE U20BE S18LE S18BE U18LE U18BE F32LE F32BE F64LE F64BE
</entry>
<entry>
The format of the sample data.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry morerows="3">audio/mpeg</entry>
<entry morerows="3">
Audio data compressed using the MPEG audio encoding scheme.
</entry>
<entry>mpegversion</entry>
<entry>integer</entry>
<entry>1, 2 or 4</entry>
<entry>
The MPEG-version used for encoding the data. The value 1 refers
to MPEG-1, -2 and -2.5 layer 1, 2 or 3. The values 2 and 4 refer
to the MPEG-AAC audio encoding schemes.
</entry>
</row>
<row>
<entry>framed</entry>
<entry>boolean</entry>
<entry>0 or 1</entry>
<entry>
A true value indicates that each buffer contains exactly one
frame. A false value indicates that frames and buffers do not
necessarily match up.
</entry>
</row>
<row>
<entry>layer</entry>
<entry>integer</entry>
<entry>1, 2, or 3</entry>
<entry>
The compression scheme layer used to compress the data
<emphasis>(only if mpegversion=1)</emphasis>.
</entry>
</row>
<row>
<entry>bitrate</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The bitrate, in bits per second. For VBR (variable bitrate)
MPEG data, this is the average bitrate.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry>audio/x-vorbis</entry>
<entry>Vorbis audio data</entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry>
There are currently no specific properties defined for this type.
</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
</sect1>
</chapter>