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Original commit message from CVS: * docs/manual/basics-data.xml: Change "event-event interaction" to "element-element interaction". Fixes #552448. Also fix sample code for seeking and do more 0.8->0.10 updates.
105 lines
3.7 KiB
XML
105 lines
3.7 KiB
XML
<chapter id="chapter-data">
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<title>Buffers and Events</title>
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<para>
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The data flowing through a pipeline consists of a combination of
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buffers and events. Buffers contain the actual media data. Events
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contain control information, such as seeking information and
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end-of-stream notifiers. All this will flow through the pipeline
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automatically when it's running. This chapter is mostly meant to
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explain the concept to you; you don't need to do anything for this.
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</para>
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<sect1 id="section-buffers">
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<title>Buffers</title>
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<para>
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Buffers contain the data that will flow through the pipeline you have
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created. A source element will typically create a new buffer and pass
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it through a pad to the next element in the chain. When using the
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GStreamer infrastructure to create a media pipeline you will not have
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to deal with buffers yourself; the elements will do that for you.
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</para>
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<para>
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A buffer consists, amongst others, of:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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A pointer to a piece of memory.
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</para>
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</listitem>
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<listitem>
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<para>
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The size of the memory.
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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 timestamp for the buffer.
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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 refcount that indicates how many elements are using this
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buffer. This refcount will be used to destroy the buffer when no
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element has a reference to it.
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</para>
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</listitem>
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<listitem>
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<para>
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Buffer flags.
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</para>
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</listitem>
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</itemizedlist>
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<para>
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The simple case is that a buffer is created, memory allocated, data
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put in it, and passed to the next element. That element reads the
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data, does something (like creating a new buffer and decoding into
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it), and unreferences the buffer. This causes the data to be free'ed
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and the buffer to be destroyed. A typical video or audio decoder
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works like this.
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</para>
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<para>
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There are more complex scenarios, though. Elements can modify buffers
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in-place, i.e. without allocating a new one. Elements can also write
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to hardware memory (such as from video-capture sources) or memory
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allocated from the X-server (using XShm). Buffers can be read-only,
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and so on.
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</para>
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</sect1>
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<sect1 id="section-events">
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<title>Events</title>
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<para>
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Events are control particles that are sent both up- and downstream in
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a pipeline along with buffers. Downstream events notify fellow elements
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of stream states. Possible events include seeking, flushes,
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end-of-stream notifications and so on. Upstream events are used both
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in application-element interaction as well as element-element interaction
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to request changes in stream state, such as seeks. For applications,
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only upstream events are important. Downstream events are just
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explained to get a more complete picture of the data concept.
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</para>
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<para>
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Since most applications seek in time units, our example below does so
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too:
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</para>
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<programlisting>
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static void
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seek_to_time (GstElement *element,
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guint64 time_ns)
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{
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GstEvent *event;
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event = gst_event_new_seek (1.0, GST_FORMAT_TIME,
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GST_SEEK_FLAG_NONE,
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GST_SEEK_METHOD_SET, time_ns,
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GST_SEEK_TYPE_NONE, G_GUINT64_CONSTANT (0));
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gst_element_send_event (element, event);
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}
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</programlisting>
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<para>
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The function <function>gst_element_seek ()</function> is a shortcut
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for this. This is mostly just to show how it all works.
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</para>
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</sect1>
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</chapter>
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