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