gstreamer/docs/pwg/intro-basics.xml
David Schleef 3f763b167c make a few things clearer
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make a few things clearer
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<!-- ############ chapter ############# -->
<chapter id="cha-intro-basics" xreflabel="Basic Concepts">
<title>Basic Concepts</title>
<para>
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="sect1-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
<classname>GstElement</classname> 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 scheduling 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 link them together so that they act as a
filter between two arbitary 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 XML 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/gstelement.html"><classname>GstElement</classname></ulink>
and <ulink type="http"
url="../gstreamer/gstreamer-gstplugin.html"><classname>GstPlugin</classname></ulink>.
</para>
</sect1>
<!-- ############ sect1 ############# -->
<sect1 id="sect1-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/gstreamer-gstpad.html"><classname>GstPad</classname></ulink>.
</para>
</sect1>
<!-- ############ sect1 ############# -->
<sect1 id="sect1-basics-buffers" xreflabel="Buffers">
<title>Buffers</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>Buffers</emphasis> are structures used to hold these chunks of
data. Buffers can be of any size, theoretically, and they 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>
A pointer to the buffer's data.
</para>
</listitem>
<listitem>
<para>
An integer indicating the size of the buffer's data.
</para>
</listitem>
<listitem>
<para>
A <classname>GstData</classname> object describing the type of the
buffer's data.
</para>
</listitem>
<listitem>
<para>
A reference count indicating the number of elements currently
holding a reference to the buffer. When the buffer reference count
falls to zero, the buffer will be unlinked, and its memory will be
freed in some sense (see below for more details).
</para>
</listitem>
</itemizedlist>
</para>
<para>
See the &GstLibRef; for the current implementation details of a <ulink
type="http"
url="../gstreamer/gstreamer-gstbuffer.html"><classname>GstBuffer</classname></ulink>.
</para>
<sect2 id="sect2-buffers-bufferpools" xreflabel="Buffer Allocation and
Buffer Pools">
<title>Buffer Allocation and Buffer Pools</title>
<para>
Buffers can be allocated using various schemes, and they may either be
passed on by an element or unreferenced, thus freeing the memory used by
the buffer. Buffer allocation and unlinking are important concepts when
dealing with real time media processing, since memory allocation is
relatively slow on most systems.
</para>
<para>
To improve the latency in a media pipeline, many &GStreamer; elements
use a <emphasis>buffer pool</emphasis> to handle buffer allocation and
unlinking. A buffer pool is a relatively large chunk of memory that is
the &GStreamer; process requests early on from the operating system.
Later, when elements request memory for a new buffer, the buffer pool
can serve the request quickly by giving out a piece of the allocated
memory. This saves a call to the operating system and lowers latency.
[If it seems at this point like &GStreamer; is acting like an operating
system (doing memory management, etc.), don't worry: &GStreamer;OS isn't
due out for quite a few years!]
</para>
<para>
Normally in a media pipeline, most filter elements in &GStreamer; deal
with a buffer in place, meaning that they do not create or destroy
buffers. Sometimes, however, elements might need to alter the reference
count of a buffer, either by copying or destroying the buffer, or by
creating a new buffer. These topics are generally reserved for
non-filter elements, so they will be addressed at that point.
</para>
</sect2>
</sect1>
<!-- ############ sect1 ############# -->
<sect1 id="sect1-basics-types" xreflabel="Types and Properties">
<title>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.
</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 the basic types used for buffers in &GStreamer;. The table
contains the name ("mime type") and a description of the type, the
properties associated with the type, and the meaning of each property.
</para>
<table frame="all" id="table-basictypes" xreflabel="Table of Basic Types">
<title>Table of Basic Types</title>
<tgroup cols="6" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Mime 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="10">audio/raw</entry>
<entry morerows="10">
Unstructured and uncompressed raw audio data.
</entry>
<entry>rate</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The sample rate of the data, in samples 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>
<row>
<entry>format</entry>
<entry>string</entry>
<entry><quote>int</quote> or <quote>float</quote></entry>
<entry>
The format in which the audio data is passed.
</entry>
</row>
<row>
<entry>law</entry>
<entry>integer</entry>
<entry>0, 1, or 2</entry>
<entry>
(Valid only if the data is in integer format.) The law used to
describe the data. The value 0 indicates <quote>linear</quote>, 1
indicates <quote>mu&nbsp;law</quote>, and 2 indicates
<quote>A&nbsp;law</quote>.
</entry>
</row>
<row>
<entry>endianness</entry>
<entry>boolean</entry>
<entry>0 or 1</entry>
<entry>
(Valid only if the data is in integer format.) The order of bytes
in a sample. The value 0 means <quote>little-endian</quote> (bytes
are least significant first). The value 1 means
<quote>big-endian</quote> (most significant byte first).
</entry>
</row>
<row>
<entry>signed</entry>
<entry>boolean</entry>
<entry>0 or 1</entry>
<entry>
(Valid only if the data is in integer format.) Whether the samples
are signed or not.
</entry>
</row>
<row>
<entry>width</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
(Valid only if the data is in integer format.) The number of bits
per sample.
</entry>
</row>
<row>
<entry>depth</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
(Valid only if the data is in integer format.) The number of bits
used per sample. This must be less than or equal to the width: If
the depth is less than the width, the low bits are assumed to be
the ones used. For example, a width of 32 and a depth of 24 means
that each sample is stored in a 32 bit word, but only the low 24
bits are actually used.
</entry>
</row>
<row>
<entry>layout</entry>
<entry>string</entry>
<entry><quote>gfloat</quote></entry>
<entry>
(Valid only if the data is in float format.) A string representing
the way in which the floating point data is represented.
</entry>
</row>
<row>
<entry>intercept</entry>
<entry>float</entry>
<entry>any, normally 0</entry>
<entry>
(Valid only if the data is in float format.) A floating point
value representing the value that the signal
<quote>centers</quote> on.
</entry>
</row>
<row>
<entry>slope</entry>
<entry>float</entry>
<entry>any, normally 1.0</entry>
<entry>
(Valid only if the data is in float format.) A floating point
value representing how far the signal deviates from the intercept.
A slope of 1.0 and an intercept of 0.0 would mean an audio signal
with minimum and maximum values of -1.0 and 1.0. A slope of
0.5 and intercept of 0.5 would represent values in the range 0.0
to 1.0.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry morerows="4">audio/mp3</entry>
<entry morerows="4">
Audio data compressed using the mp3 encoding scheme.
</entry>
<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.
</entry>
</row>
<row>
<entry>bitrate</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The bitrate, in kilobits per second. For VBR (variable bitrate)
mp3 data, this is the average bitrate.
</entry>
</row>
<row>
<entry>channels</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The number of channels of audio data present.
</entry>
</row>
<row>
<entry>joint-stereo</entry>
<entry>boolean</entry>
<entry>0 or 1</entry>
<entry>
If true, this implies that stereo data is stored as a combined
signal and the difference between the signals, rather than as two
entirely separate signals. If true, the <quote>channels</quote>
attribute must not be zero.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry morerows="0">audio/x-ogg</entry>
<entry morerows="0">
Audio data compressed using the Ogg Vorbis encoding scheme.
</entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry>
FIXME: There are currently no parameters defined for this type.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry morerows="2">video/raw</entry>
<entry morerows="2">
Raw video data.
</entry>
<entry>fourcc</entry>
<entry>FOURCC code</entry>
<entry></entry>
<entry>
A FOURCC code identifying the format in which this data is stored.
FOURCC (Four Character Code) is a simple system to allow
unambiguous identification of a video datastream format. See
<ulink url="http://www.webartz.com/fourcc/"
type="http">http://www.webartz.com/fourcc/</ulink>
</entry>
</row>
<row>
<entry>width</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The number of pixels wide that each video frame is.
</entry>
</row>
<row>
<entry>height</entry>
<entry>integer</entry>
<entry>greater than 0</entry>
<entry>
The number of pixels high that each video frame is.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry morerows="0">video/mpeg</entry>
<entry morerows="0">
Video data compressed using an MPEG encoding scheme.
</entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry>
FIXME: There are currently no parameters defined for this type.
</entry>
</row>
<!-- ############ type ############# -->
<row>
<entry morerows="0">video/avi</entry>
<entry morerows="0">
Video data compressed using the AVI encoding scheme.
</entry>
<entry></entry>
<entry></entry>
<entry></entry>
<entry>
FIXME: There are currently no parameters defined for this type.
</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
</sect1>
<!-- ############ sect1 ############# -->
<sect1 id="sect1-basics-events" xreflabel="Events">
<title>Events</title>
<para>
Sometimes elements in a media processing pipeline need to know that
something has happened. An <emphasis>event</emphasis> is a special type of
data in &GStreamer; designed to serve this purpose. Events describe some
sort of activity that has happened somewhere in an element's pipeline, for
example, the end of the media stream or a clock discontinuity. Just like
any other data type, an event comes to an element on a sink pad and is
contained in a normal buffer. Unlike normal stream buffers, though, an
event buffer contains only an event, not any media stream data.
</para>
<para>
See the &GstLibRef; for the current implementation details of a <ulink
type="http"
url="../gstreamer/gstreamer-gstevent.html"><classname>GstEvent</classname></ulink>.
</para>
</sect1>
</chapter>