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More manual about MIME types and factories...
Original commit message from CVS: More manual about MIME types and factories...
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Wim Taymans
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280
docs/manual/factories.sgml
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280
docs/manual/factories.sgml
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<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.
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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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parse = gst_elementfactory_make("mp3parse","parse");
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decoder = gst_elementfactory_make("mpg123","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 one big problems:
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The elements are created base on their name. Indeed, we create an
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element mpg123 by explicitly stating the mpg123 elements name.
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Our little program therefore always uses the mpg123 decoder element
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to decode the MP3 audio stream, even if there are 3 other MP3 decoders
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in the system. We will see how we can use a more general way to create
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an 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 added to the source and
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sink pads.
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</para>
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</sect1>
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<sect1>
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<title>MIME Types</title>
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<para>
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GStreamer uses MIME types to indentify 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) types are a set of
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string that donote 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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In our helloworld example the elements we constructed would have the
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following MIME types associated with their source and sink pads:
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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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<graphic fileref="images/mime-world" format="png"></graphic>
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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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me 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. It
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also maintains a list of all elements that either uses this type
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as a source or as a sink. 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
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extension.
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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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<programlisting>
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typedef gboolean (*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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GList *srcs; /* list of src objects for this type */
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GList *sinks; /* list of sink objects for type */
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};
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</programlisting>
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<para>
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All operations on <classname>GstType</classname> occur via their
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<classname>guint16 id</classname> numbers, with <classname>GstType</classname>
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structure private to the GStreamer 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 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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</sect2>
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<sect2>
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<title>id to <classname>GstElement</classname> conversion</title>
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<para>
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When we have obtained a given type id using one of the above methods,
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we can obtain a list of all the elements that operate on this MIME
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type or extension.
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</para>
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<para>
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Obtain a list of all the elements that use this id as source with:
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</para>
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<programlisting>
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GList *list;
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list = gst_type_gst_srcs(id);
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</programlisting>
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<para>
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Obtain a list of all the elements that use this id as sink with:
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</para>
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<programlisting>
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GList *list;
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list = gst_type_gst_sinks(id);
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</programlisting>
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<para>
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When you have a list of elements, you can simply take the first
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element of the list to obtain an appropriate element.
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</para>
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<note>
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<para>
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As you can see, there might be a multitude of elements that
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are able to operate on video/raw types. some might include:
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<itemizedlist>
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<listitem>
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<para>
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an MP3 audio encoder.
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</para>
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</listitem>
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<listitem>
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<para>
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an audio sink.
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</para>
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</listitem>
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<listitem>
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<para>
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an audio resampler.
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</para>
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</listitem>
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<listitem>
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<para>
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a spectrum filter.
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</para>
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</listitem>
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</itemizedlist>
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Depending on the application, you might want to use a different
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element. This is why GStreamer leaves that decision up to the
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application programmer.
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</para>
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</note>
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</sect2>
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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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162
docs/manual/fig/mime-world.fig
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162
docs/manual/fig/mime-world.fig
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#FIG 3.2
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Landscape
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Center
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Inches
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Letter
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100.00
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Single
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2 2 0 1 0 7 50 0 -1 0.000 0 0 -1 0 0 5
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7125 2775 9075 2775 9075 4425 7125 4425 7125 2775
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7125 3600 7875 3600 7875 4125 7125 4125 7125 3600
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1950 1950 11700 1950 11700 4800 1950 4800 1950 1950
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4 0 0 50 0 16 12 0.0000 4 135 255 2175 2250 bin\001
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4 0 0 50 0 16 12 0.0000 4 105 255 3525 3975 src\001
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4 0 0 50 0 16 12 0.0000 4 135 330 4725 3975 sink\001
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4 0 0 50 0 16 12 0.0000 4 105 255 6075 3975 src\001
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4 0 0 50 0 16 12 0.0000 4 135 330 7350 3975 sink\001
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4 0 0 50 0 16 12 0.0000 4 105 255 8625 3975 src\001
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4 0 0 50 0 16 12 0.0000 4 135 330 9750 3975 sink\001
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4 0 0 50 0 16 12 0.0000 4 165 1005 2250 3075 disk_source\001
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4 0 0 50 0 16 12 0.0000 4 150 465 4725 3075 parse\001
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4 0 0 50 0 16 12 0.0000 4 135 690 7275 3075 decoder\001
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4 0 0 50 0 16 12 0.0000 4 180 930 9750 3075 play_audio\001
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4 0 0 50 0 0 12 0.0000 4 135 75 3675 5475 ?\001
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4 0 0 50 0 0 12 0.0000 4 135 735 9825 5475 audio/raw\001
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4 0 0 50 0 0 12 0.0000 4 180 855 4350 5325 audio/mpeg\001
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4 0 0 50 0 0 12 0.0000 4 180 1395 5475 5625 audio/mpeg-frame\001
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4 0 0 50 0 0 12 0.0000 4 135 735 8700 5625 audio/raw\001
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4 0 0 50 0 0 12 0.0000 4 180 1395 6825 5325 audio/mpeg-frame\001
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@ -10,6 +10,7 @@
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<!ENTITY STATES SYSTEM "states.sgml">
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<!ENTITY HELLOWORLD SYSTEM "helloworld.sgml">
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<!ENTITY FACTORIES SYSTEM "factories.sgml">
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]>
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@ -107,6 +108,8 @@
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&HELLOWORLD;
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&FACTORIES;
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</part>
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<!-- ############ Advanced GStreamer - part ############# -->
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|
|
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|||
|
|
@ -354,7 +354,7 @@ void eos(GstSrc *src)
|
|||
We could use a disksink to write the raw samples to a file, for example.
|
||||
It should also be clear that inserting filters, like a stereo effect,
|
||||
into the pipeline is not that hard to do. The most important thing is
|
||||
that you can reuse allready existing codecs.
|
||||
that you can reuse allready existing elements.
|
||||
</para>
|
||||
|
||||
|
||||
|
|
|
|||
|
|
@ -31,8 +31,8 @@ Basic concepts
|
|||
connecting elements
|
||||
bin
|
||||
(can contain elements)
|
||||
pipeline
|
||||
(a complete graph)
|
||||
pipeline (a complete graph)
|
||||
thread (theaded operation)
|
||||
buffers
|
||||
(pass between elements)
|
||||
(contains data)
|
||||
|
|
@ -48,6 +48,7 @@ Building apps
|
|||
helloworld
|
||||
(fdsrc->mp3decoder->audiosink)
|
||||
(step by step explanation)
|
||||
More on factories
|
||||
|
||||
advanced concepts
|
||||
threads
|
||||
|
|
|
|||
Loading…
Reference in a new issue