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Original commit message from CVS: Small updates to the manual.
153 lines
4.8 KiB
Text
153 lines
4.8 KiB
Text
<chapter id="cha-states">
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<title>Element states</title>
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<para>
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One you have created a pipeline packed with elements, nothing will happen yet.
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This is where the different states come into play.
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</para>
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<sect1 id="sec-states">
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<title>The different element states</title>
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<para>
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All elements can be in one of the following four states:
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<itemizedlist>
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<listitem>
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<para>
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NULL: this is the default state all elements are in when they are created
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and are doing nothing.
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</para>
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</listitem>
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<listitem>
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<para>
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READY: An element is ready to start doing something.
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</para>
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</listitem>
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<listitem>
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<para>
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PAUSED: The element is paused for a period of time.
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</para>
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</listitem>
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<listitem>
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<para>
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PLAYING: The element is doing something.
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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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All elements start with the NULL state. The elements will go throught the following state changes:
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NULL -> READY -> PAUSED -> PLAYING. Remember when going from PLAYING to READY GStreamer
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will internally go throught the intermediate states.
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</para>
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<para>
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The state of an element can be changed with the following code:
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</para>
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<programlisting>
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GstElement *bin;
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// create a bin, put elements in it and connect them
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...
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gst_element_set_state (bin, GST_STATE_PLAYING);
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...
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</programlisting>
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<para>
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You can set the following states to an element:
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</para>
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<informaltable pgwide=1 frame="none" role="enum">
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<tgroup cols="2">
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<colspec colwidth="2*">
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<colspec colwidth="8*">
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<tbody>
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<row>
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<entry><literal>GST_STATE_NULL</literal></entry>
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<entry>Reset the state of an element.
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</entry>
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</row>
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<row>
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<entry><literal>GST_STATE_READY</literal></entry>
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<entry>will make the element ready to start processing data.
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</entry>
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</row>
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<row>
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<entry><literal>GST_STATE_PAUSED</literal></entry>
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<entry>temporary stops the data flow.
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</entry>
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</row>
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<row>
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<entry><literal>GST_STATE_PLAYING</literal></entry>
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<entry>means there really is data flowing through the graph.
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</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</sect1>
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<sect1 id="sec-states-null">
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<title>The NULL state</title>
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<para>
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When you created the pipeline all of the elements will be in the NULL state. There is
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nothing spectacular about the NULL state.
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</para>
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<note>
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<para>
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Don't forget to reset the pipeline to the NULL state when you are not going to use it
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anymore. This will allow the elements to free the resources they might use.
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</para>
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</note>
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</sect1>
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<sect1 id="sec-states-ready">
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<title>The READY state</title>
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<para>
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You will start the pipeline by first setting it to the READY state. This will allow the
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pipeline and all the elements contained in it to prepare themselves for the actions
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they are about to perform.
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</para>
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<para>
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The typical actions that an element will perform in the READY state might be to open a file or
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an audio device. Some more complex elements might have a non trivial action to perform in
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the READY state such as connecting to a media server using a CORBA connection.
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</para>
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<note>
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<para>
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You can also go from the NULL to PLAYING state directly without going through the READY
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state. this is a shortcut, the framework will internally go through the READY and the
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PAUSED state for you.
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</para>
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</note>
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</sect1>
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<sect1 id="sec-states-playing">
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<title>The PLAYING state</title>
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<para>
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A Pipeline that is in the READY state can be started by setting it to the PLAYING state. At
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that time data will start to flow all the way through the pipeline.
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</para>
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</sect1>
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<sect1 id="sec-states-paused">
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<title>The PAUSED state</title>
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<para>
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A pipeline that is playing can be set to the PAUSED state. This will temporarily stop all
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data flowing through the pipeline.
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</para>
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<para>
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You can resume the data flow by setting the pipeline back to the PLAYING state.
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</para>
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<note>
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<para>
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The PAUSED state is available for temporarily freezing the pipeline.
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Elements will typically not free their resources in the PAUSED state.
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Use the NULL state if you want to stop the data flow permanantly.
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</para>
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</note>
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<para>
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The pipeline has to be in the PAUSED or NULL state if you want to insert or modify an element
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in the pipeline. We will cover dynamic pipeline behaviour in ... <!-- fixme -->
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</para>
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</sect1>
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</chapter>
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