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146 lines
5.4 KiB
XML
146 lines
5.4 KiB
XML
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<!-- ############ chapter ############# -->
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<chapter id="chapter-building-pads">
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<title>Specifying the pads</title>
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<para>
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As explained before, pads are the port through which data goes in and out
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of your element, and that makes them a very important item in the process
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of element creation. In the boilerplate code, we have seen how static pad
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templates take care of registering pad templates with the element class.
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Here, we will see how to create actual elements, use an <function>_event
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()</function>-function to configure for a particular format and how to
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register functions to let data flow through the element.
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</para>
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<para>
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In the element <function>_init ()</function> function, you create the pad
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from the pad template that has been registered with the element class in
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the <function>_class_init ()</function> function. After creating the pad,
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you have to set a <function>_chain ()</function> function pointer that will
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receive and process the input data on the sinkpad.
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You can optionally also set an <function>_event ()</function> function
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pointer and a <function>_query ()</function> function pointer.
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Alternatively, pads can also operate in looping mode, which means that they
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can pull data themselves. More on this topic later. After that, you have
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to register the pad with the element. This happens like this:
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</para>
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<programlisting><!-- example-begin init.func a --><!--
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#include "filter.h"
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#include <string.h>
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static GstStateChangeReturn
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gst_my_filter_change_state (GstElement * element, GstStateChange transition);
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G_DEFINE_TYPE (GstMyFilter, gst_my_filter, GST_TYPE_ELEMENT);
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static void
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gst_my_filter_class_init (gpointer klass)
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{
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GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
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static GstStaticPadTemplate sink_template =
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GST_STATIC_PAD_TEMPLATE (
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"sink",
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GST_PAD_SINK,
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GST_PAD_ALWAYS,
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GST_STATIC_CAPS ("ANY")
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);
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static GstStaticPadTemplate src_template =
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GST_STATIC_PAD_TEMPLATE (
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"src",
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GST_PAD_SRC,
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GST_PAD_ALWAYS,
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GST_STATIC_CAPS ("ANY")
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);
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gst_element_class_set_static_metadata (element_class,
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"An example plugin",
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"Example/FirstExample",
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"Shows the basic structure of a plugin",
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"your name <your.name@your.isp>");
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gst_element_class_add_pad_template (element_class,
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gst_static_pad_template_get (&src_template));
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gst_element_class_add_pad_template (element_class,
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gst_static_pad_template_get (&sink_template));
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}
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static void
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gst_my_filter_class_init (GstMyFilterClass * klass)
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{
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GST_ELEMENT_CLASS (klass)->change_state = gst_my_filter_change_state;
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}
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--><!-- example-end init.func a -->
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<!-- example-begin init.func c --><!--
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static GstFlowReturn gst_my_filter_chain (GstPad *pad,
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GstObject *parent,
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GstBuffer *buf);
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static gboolean gst_my_filter_sink_event (GstPad *pad,
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GstObject *parent,
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GstEvent *event);
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static gboolean gst_my_filter_src_query (GstPad *pad,
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GstObject *parent,
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GstQuery *query);
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static gboolean gst_my_filter_sink_query (GstPad *pad,
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GstObject *parent,
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GstQuery *query);
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--><!-- example-end init.func c -->
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<!-- example-begin init.func d -->
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static void
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gst_my_filter_init (GstMyFilter *filter)
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{
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/* pad through which data comes in to the element */
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filter->sinkpad = gst_pad_new_from_static_template (
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&sink_template, "sink");
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/* pads are configured here with gst_pad_set_*_function () */
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<!-- example-end init.func d -->
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<!-- example-begin init.func e --><!--
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gst_pad_set_chain_function (filter->sinkpad, gst_my_filter_chain);
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gst_pad_set_event_function (filter->sinkpad, gst_my_filter_sink_event);
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gst_pad_set_query_function (filter->sinkpad, gst_my_filter_sink_query);
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--><!-- example-end init.func e -->
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<!-- example-begin init.func f -->
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gst_element_add_pad (GST_ELEMENT (filter), filter->sinkpad);
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/* pad through which data goes out of the element */
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filter->srcpad = gst_pad_new_from_static_template (
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&src_template, "src");
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/* pads are configured here with gst_pad_set_*_function () */
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<!-- example-end init.func f -->
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<!-- example-begin init.func g --><!--
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gst_pad_set_query_function (filter->srcpad, gst_my_filter_src_query);
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--><!-- example-end init.func g -->
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<!-- example-begin init.func h -->
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gst_element_add_pad (GST_ELEMENT (filter), filter->srcpad);
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/* properties initial value */
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filter->silent = FALSE;
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}
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<!-- example-end init.func h --></programlisting>
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<!-- example-begin pads.c --><!--
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#include "init.func"
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static gboolean
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gst_my_filter_event (GstPad * pad, GstObject * parent, GstEvent * event)
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{
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return gst_pad_event_default (pad, parent, event);
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}
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static GstFlowReturn
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gst_my_filter_chain (GstPad * pad, GstObject * parent, GstBuffer * buf)
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{
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return gst_pad_push (GST_MY_FILTER (parent)->srcpad, buf);
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}
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static GstStateChangeReturn
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gst_my_filter_change_state (GstElement * element, GstStateChange transition)
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{
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return GST_CALL_PARENT_WITH_DEFAULT (GST_ELEMENT_CLASS,
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change_state, (element, transition), GST_STATE_CHANGE_SUCCESS);
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}
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#include "register.func"
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--><!-- example-end pads.c -->
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</chapter>
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