mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-24 02:31:03 +00:00
257 lines
7.7 KiB
XML
257 lines
7.7 KiB
XML
<chapter id="cha-factories">
|
|
<title>More on factories</title>
|
|
<para>
|
|
The small application we created in the previous chapter used the
|
|
concept of a factory to create the elements. In this chapter we will
|
|
show you how to use the factory concepts to create elements based
|
|
on what they do instead of what they are called.
|
|
</para>
|
|
|
|
<para>
|
|
We will first explain the concepts involved before we move on
|
|
to the reworked helloworld example using autoplugging.
|
|
</para>
|
|
<sect1>
|
|
<title>The problems with the helloworld example</title>
|
|
<para>
|
|
If we take a look at how the elements were created in the previous
|
|
example we used a rather crude mechanism:
|
|
</para>
|
|
|
|
<programlisting>
|
|
...
|
|
/* now it's time to get the parser */
|
|
decoder = gst_element_factory_make ("mad", "decoder");
|
|
...
|
|
</programlisting>
|
|
|
|
<para>
|
|
While this mechanism is quite effective it also has some big problems:
|
|
The elements are created based on their name. Indeed, we create an
|
|
element, mad, by explicitly stating the mad element's name. Our little
|
|
program therefore always uses the mad decoder element to decode
|
|
the MP3 audio stream, even if there are three other MP3 decoders in the
|
|
system. We will see how we can use a more general way to create an
|
|
MP3 decoder element.
|
|
</para>
|
|
<para>
|
|
We have to introduce the concept of MIME types and capabilities
|
|
added to the source and sink pads.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1>
|
|
<title>More on MIME Types</title>
|
|
<para>
|
|
GStreamer uses MIME types to identify the different types of data
|
|
that can be handled by the elements. They are the high level
|
|
mechanisms to make sure that everyone is talking about the right
|
|
kind of data.
|
|
</para>
|
|
<para>
|
|
A MIME (Multipurpose Internet Mail Extension) type is a pair of
|
|
strings that denote a certain type of data. Examples include:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
audio/raw : raw audio samples
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
audio/mpeg : MPEG audio
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
video/mpeg : MPEG video
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
<para>
|
|
An element must associate a MIME type to its source and sink pads
|
|
when it is loaded into the system. GStreamer knows about the
|
|
different elements and what type of data they expect and emit.
|
|
This allows for very dynamic and extensible element creation as we
|
|
will see.
|
|
</para>
|
|
<para>
|
|
As we have seen in the previous chapter, MIME types are added
|
|
to the Capability structure of a pad.
|
|
</para>
|
|
|
|
<para>
|
|
In our helloworld example the elements we constructed would have the
|
|
following MIME types associated with their source and sink pads:
|
|
</para>
|
|
<figure float="1" id="sec-mime-img">
|
|
<title>The Hello world pipeline with MIME types</title>
|
|
<mediaobject>
|
|
<imageobject>
|
|
<imagedata fileref="images/mime-world.&magic;" format="&MAGIC;" />
|
|
</imageobject>
|
|
</mediaobject>
|
|
|
|
</figure>
|
|
<para>
|
|
We will see how you can create an element based on the MIME types
|
|
of its source and sink pads. This way the end-user will have the
|
|
ability to choose his/her favorite audio/mpeg decoder without
|
|
you even having to care about it.
|
|
</para>
|
|
<para>
|
|
The typing of the source and sink pads also makes it possible to
|
|
'autoplug' a pipeline. We will have the ability to say: "construct
|
|
me a pipeline that does an audio/mpeg to audio/raw conversion".
|
|
</para>
|
|
<note>
|
|
<para>
|
|
The basic GStreamer library does not try to solve all of your
|
|
autoplug problems. It leaves the hard decisions to the application
|
|
programmer, where they belong.
|
|
</para>
|
|
</note>
|
|
|
|
</sect1>
|
|
|
|
<sect1>
|
|
<title>GStreamer types</title>
|
|
<para>
|
|
GStreamer assigns a unique number to all registered MIME types.
|
|
GStreamer also keeps a reference to
|
|
a function that can be used to determine if a given buffer is of
|
|
the given MIME type.
|
|
</para>
|
|
<para>
|
|
There is also an association between a MIME type and a file extension,
|
|
but the use of typefind functions (similar to file(1)) is preferred.
|
|
</para>
|
|
<para>
|
|
The type information is maintained in a list of
|
|
<classname>GstType</classname>. The definition of a
|
|
<classname>GstType</classname> is like:
|
|
</para>
|
|
<para>
|
|
<programlisting>
|
|
typedef GstCaps (*GstTypeFindFunc) (GstBuffer *buf,gpointer *priv);
|
|
|
|
typedef struct _GstType GstType;
|
|
|
|
struct _GstType {
|
|
guint16 id; /* type id (assigned) */
|
|
|
|
gchar *mime; /* MIME type */
|
|
gchar *exts; /* space-delimited list of extensions */
|
|
|
|
GstTypeFindFunc typefindfunc; /* typefind function */
|
|
};
|
|
</programlisting>
|
|
</para>
|
|
<para>
|
|
All operations on <classname>GstType</classname> occur
|
|
via their <classname>guint16 id</classname> numbers, with
|
|
the <classname>GstType</classname> structure private to the GStreamer
|
|
library.
|
|
</para>
|
|
|
|
<sect2>
|
|
<title>MIME type to id conversion</title>
|
|
|
|
<para>
|
|
We can obtain the id for a given MIME type
|
|
with the following piece of code:
|
|
</para>
|
|
<programlisting>
|
|
guint16 id;
|
|
|
|
id = gst_type_find_by_mime ("audio/mpeg");
|
|
</programlisting>
|
|
<para>
|
|
This function will return 0 if the type was not known.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>id to <classname>GstType</classname> conversion</title>
|
|
<para>
|
|
We can obtain the <classname>GstType</classname> for a given id
|
|
with the following piece of code:
|
|
</para>
|
|
<programlisting>
|
|
GstType *type;
|
|
|
|
type = gst_type_find_by_id (id);
|
|
</programlisting>
|
|
<para>
|
|
This function will return NULL if the id was not associated with
|
|
any known <classname>GstType</classname>
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>extension to id conversion</title>
|
|
<para>
|
|
We can obtain the id for a given file extension
|
|
with the following piece of code:
|
|
</para>
|
|
<programlisting>
|
|
guint16 id;
|
|
|
|
id = gst_type_find_by_ext (".mp3");
|
|
</programlisting>
|
|
<para>
|
|
This function will return 0 if the extension was not known.
|
|
</para>
|
|
<para>
|
|
For more information, see <xref linkend="cha-autoplug"/>.
|
|
</para>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1>
|
|
<title>Creating elements with the factory</title>
|
|
<para>
|
|
In the previous section we described how you could obtain
|
|
an element factory using MIME types. One the factory has been
|
|
obtained, you can create an element using:
|
|
</para>
|
|
<programlisting>
|
|
GstElementFactory *factory;
|
|
GstElement *element;
|
|
|
|
// obtain the factory
|
|
factory = ...
|
|
|
|
element = gst_element_factory_create (factory, "name");
|
|
</programlisting>
|
|
<para>
|
|
This way, you do not have to create elements by name which
|
|
allows the end-user to select the elements he/she prefers for the
|
|
given MIME types.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1>
|
|
<title>GStreamer basic types</title>
|
|
<para>
|
|
GStreamer only has two builtin types:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
audio/raw : raw audio samples
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
video/raw and image/raw : raw video data
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
<para>
|
|
All other MIME types are maintained by the plugin elements.
|
|
</para>
|
|
|
|
</sect1>
|
|
</chapter>
|