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>Chapter 17. Autoplugging</H1
><P
> In <A
HREF="chapter-helloworld.html"
>Chapter 10</A
>, you've learned to build a
simple media player for Ogg/Vorbis files. By using alternative elements,
you are able to build media players for other media types, such as
Ogg/Speex, MP3 or even video formats. However, you would rather want
to build an application that can automatically detect the media type
of a stream and automatically generate the best possible pipeline
by looking at all available elements in a system. This process is called
autoplugging, and <SPAN
CLASS="application"
>GStreamer</SPAN
> contains high-quality autopluggers. If
you're looking for an autoplugger, don't read any further and go to
<A
HREF="chapter-components.html"
>Chapter 19</A
>. This chapter will explain the
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>concept</I
></SPAN
> of autoplugging and typefinding. It will
explain what systems <SPAN
CLASS="application"
>GStreamer</SPAN
> includes to dynamically detect the
type of a media stream, and how to generate a pipeline of decoder
elements to playback this media. The same principles can also be used
for transcoding. Because of the full dynamicity of this concept,
<SPAN
CLASS="application"
>GStreamer</SPAN
> can be automatically extended to support new media types
without needing any adaptations to its autopluggers.
</P
><P
> We will first introduce the concept of MIME types as a dynamic and
extendible way of identifying media streams. After that, we will introduce
the concept of typefinding to find the type of a media stream. Lastly,
we will explain how autoplugging and the <SPAN
CLASS="application"
>GStreamer</SPAN
> registry can be
used to setup a pipeline that will convert media from one mimetype to
another, for example for media decoding.
</P
><DIV
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><H1
CLASS="sect1"
><A
NAME="section-mime"
>17.1. MIME-types as a way to identify streams</A
></H1
><P
> We have previously introduced the concept of capabilities as a way
for elements (or, rather, pads) to agree on a media type when
streaming data from one element to the next (see <A
HREF="section-caps.html"
>Section 8.2</A
>). We have explained that a capability is
a combination of a mimetype and a set of properties. For most
container formats (those are the files that you will find on your
hard disk; Ogg, for example, is a container format), no properties
are needed to describe the stream. Only a MIME-type is needed. A
full list of MIME-types and accompanying properties can be found
in <A
HREF="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/section-types-definitions.html"
TARGET="_top"
>the
Plugin Writer's Guide</A
>.
</P
><P
> An element must associate a MIME-type to its source and sink pads
when it is loaded into the system. <SPAN
CLASS="application"
>GStreamer</SPAN
> knows about the
different elements and what type of data they expect and emit through
the <SPAN
CLASS="application"
>GStreamer</SPAN
> registry. This allows for very dynamic and extensible
element creation as we will see.
</P
><P
> In <A
HREF="chapter-helloworld.html"
>Chapter 10</A
>, we've learned to build a
music player for Ogg/Vorbis files. Let's look at the MIME-types
associated with each pad in this pipeline. <A
HREF="chapter-autoplugging.html#section-mime-img"
>Figure 17-1</A
> shows what MIME-type belongs to each
pad in this pipeline.
</P
><DIV
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><A
NAME="section-mime-img"
></A
><P
><B
>Figure 17-1. The Hello world pipeline with MIME types</B
></P
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><P
><IMG
SRC="images/mime-world.png"></P
></DIV
></DIV
><P
> Now that we have an idea how <SPAN
CLASS="application"
>GStreamer</SPAN
> identifies known media
streams, we can look at methods <SPAN
CLASS="application"
>GStreamer</SPAN
> uses to setup pipelines
for media handling and for media type detection.
</P
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