gstreamer/docs/pwg/building-pads.xml

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<!-- ############ chapter ############# -->
<chapter id="chapter-building-pads">
<title>Specifying the pads</title>
<para>
As explained before, pads are the port through which data goes in and out
of your element, and that makes them a very important item in the process
of element creation. In the boilerplate code, we have seen how static pad
templates take care of registering pad templates with the element class.
Here, we will see how to create actual elements, use a <function>_setcaps
()</function>-functions to configure for a particular format and how to
register functions to let data flow through the element.
</para>
<para>
In the element <function>_init ()</function> function, you create the pad
from the pad template that has been registered with the element class in
the <function>_base_init ()</function> function. After creating the pad,
you have to set a <function>_setcaps ()</function> function pointer and
optionally a <function>_getcaps ()</function> function pointer. Also, you
have to set a <function>_chain ()</function> function pointer.
Alternatively, pads can also operate in looping mode, which mans that they
can pull data themselves. More on this topic later. After that, you have
to register the pad with the element. This happens like this:
</para>
<programlisting><!-- example-begin init.func a --><!--
#include "filter.h"
#include &lt;string.h&gt;
static GstStateChangeReturn
gst_my_filter_change_state (GstElement * element, GstStateChange transition);
GST_BOILERPLATE (GstMyFilter, gst_my_filter, GstElement, GST_TYPE_ELEMENT);
static void
gst_my_filter_base_init (gpointer klass)
{
GstElementClass *element_class = GST_ELEMENT_CLASS (klass);
static GstElementDetails my_filter_details = {
"An example plugin",
"Example/FirstExample",
"Shows the basic structure of a plugin",
"your name <your.name@your.isp>"
};
static GstStaticPadTemplate sink_factory =
GST_STATIC_PAD_TEMPLATE (
"sink",
GST_PAD_SINK,
GST_PAD_ALWAYS,
GST_STATIC_CAPS ("ANY")
);
static GstStaticPadTemplate src_factory =
GST_STATIC_PAD_TEMPLATE (
"src",
GST_PAD_SRC,
GST_PAD_ALWAYS,
GST_STATIC_CAPS ("ANY")
);
gst_element_class_set_details (element_class, &my_filter_details);
gst_element_class_add_pad_template (element_class,
gst_static_pad_template_get (&src_factory));
gst_element_class_add_pad_template (element_class,
gst_static_pad_template_get (&sink_factory));
}
static void
gst_my_filter_class_init (GstMyFilterClass * klass)
{
GST_ELEMENT_CLASS (klass)->change_state = gst_my_filter_change_state;
}
--><!-- example-end init.func a -->
<!-- example-begin init.func b -->
static gboolean gst_my_filter_setcaps (GstPad *pad,
GstCaps *caps);
static GstFlowReturn gst_my_filter_chain (GstPad *pad,
GstBuffer *buf);
<!-- example-end init.func b -->
<!-- example-begin init.func c --><!--
static GstCaps * gst_my_filter_getcaps (GstPad *pad);
static gboolean gst_my_filter_event (GstPad *pad,
GstEvent *event);
--><!-- example-end init.func c -->
<!-- example-begin init.func d -->
static void
gst_my_filter_init (GstMyFilter *filter, GstMyFilterClass *filter_klass)
{
GstElementClass *klass = GST_ELEMENT_CLASS (filter_klass);
/* pad through which data comes in to the element */
filter-&gt;sinkpad = gst_pad_new_from_template (
gst_element_class_get_pad_template (klass, "sink"), "sink");
gst_pad_set_setcaps_function (filter-&gt;sinkpad, gst_my_filter_setcaps);
gst_pad_set_chain_function (filter-&gt;sinkpad, gst_my_filter_chain);
<!-- example-end init.func d -->
<!-- example-begin init.func e --><!--
gst_pad_set_getcaps_function (filter-&gt;sinkpad, gst_my_filter_getcaps);
gst_pad_set_event_function (filter-&gt;sinkpad, gst_my_filter_event);
--><!-- example-end init.func e -->
<!-- example-begin init.func f -->
gst_element_add_pad (GST_ELEMENT (filter), filter-&gt;sinkpad);
/* pad through which data goes out of the element */
filter-&gt;srcpad = gst_pad_new_from_template (
gst_element_class_get_pad_template (klass, "src"), "src");
<!-- example-end init.func f -->
<!-- example-begin init.func g --><!--
gst_pad_set_getcaps_function (filter-&gt;srcpad, gst_my_filter_getcaps);
--><!-- example-end init.func g -->
<!-- example-begin init.func h -->
gst_element_add_pad (GST_ELEMENT (filter), filter-&gt;srcpad);
/* properties initial value */
filter->silent = FALSE;
}
<!-- example-end init.func h --></programlisting>
<sect1 id="section-pads-linkfn" xreflabel="The link function">
<title>The setcaps-function</title>
<para>
The <function>_setcaps ()</function>-function is called during caps
negotiation, which is discussed in great detail in <xref
linkend="chapter-negotiation"/>. This is the process where the linked
pads decide on the streamtype that will transfer between them. A full
list of type-definitions can be found in <xref
linkend="chapter-building-types"/>. A <function>_link ()</function>
receives a pointer to a <ulink type="http"
url="../../gstreamer/html/gstreamer-GstCaps.html"><classname>GstCaps</classname></ulink>
struct that defines the proposed streamtype, and can respond with
either <quote>yes</quote> (<symbol>TRUE</symbol>) or <quote>no</quote>
(<symbol>FALSE</symbol>). If the element responds positively towards
the streamtype, that type will be used on the pad. An example:
</para>
<programlisting><!-- example-begin caps.func a -->
static gboolean
gst_my_filter_setcaps (GstPad *pad,
GstCaps *caps)
{
GstStructure *structure = gst_caps_get_structure (caps, 0);
GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad));
const gchar *mime;
/* Since we're an audio filter, we want to handle raw audio
* and from that audio type, we need to get the samplerate and
* number of channels. */
mime = gst_structure_get_name (structure);
if (strcmp (mime, "audio/x-raw-int") != 0) {
GST_WARNING ("Wrong mimetype %s provided, we only support %s",
mime, "audio/x-raw-int");
return FALSE;
}
/* we're a filter and don't touch the properties of the data.
* That means we can set the given caps unmodified on the next
* element, and use that negotiation return value as ours. */
if (!gst_pad_set_caps (filter-&gt;srcpad, caps))
return FALSE;
/* Capsnego succeeded, get the stream properties for internal
* usage and return success. */
gst_structure_get_int (structure, "rate", &amp;filter-&gt;samplerate);
gst_structure_get_int (structure, "channels", &amp;filter-&gt;channels);
g_print ("Caps negotiation succeeded with %d Hz @ %d channels\n",
filter-&gt;samplerate, filter-&gt;channels);
return TRUE;
}
<!-- example-end caps.func a -->
<!-- example-begin caps.func b --><!--
static GstCaps *
gst_my_filter_getcaps (GstPad * pad)
{
GstMyFilter *filter = GST_MY_FILTER (GST_OBJECT_PARENT (pad));
GstPad *otherpad = (pad == filter-&gt;srcpad) ? filter-&gt;sinkpad :
filter-&gt;srcpad;
GstCaps *othercaps = gst_pad_get_allowed_caps (otherpad);
return othercaps;
}
--><!-- example-end caps.func b --></programlisting>
<para>
In here, we check the mimetype of the provided caps. Normally, you don't
need to do that in your own plugin/element, because the core does that
for you. We simply use it to show how to retrieve the mimetype from a
provided set of caps. Types are stored in <ulink type="http"
url="../../gstreamer/html/gstreamer-GstStructure.html"><classname>GstStructure
</classname></ulink> internally. A <ulink
type="http" url="../../gstreamer/html/gstreamer-GstCaps.html"><classname>GstCaps
</classname></ulink> is nothing more than a small
wrapper for 0 or more structures/types. From the structure, you can also
retrieve properties, as is shown above with the function
<function>gst_structure_get_int ()</function>.
</para>
<para>
If your <function>_link ()</function> function does not need to perform
any specific operation (i.e. it will only forward caps), you can set it
to <function>gst_pad_proxy_link ()</function>. This is a link forwarding
function implementation provided by the core. It is useful for elements
such as <classname>identity</classname>.
</para>
</sect1>
<!-- example-begin pads.c --><!--
#include "init.func"
#include "caps.func"
static gboolean
gst_my_filter_event (GstPad * pad, GstEvent * event)
{
return gst_pad_event_default (pad, event);
}
static GstFlowReturn
gst_my_filter_chain (GstPad * pad, GstBuffer * buf)
{
return gst_pad_push (GST_MY_FILTER (GST_OBJECT_PARENT (pad))->srcpad, buf);
}
static GstStateChangeReturn
gst_my_filter_change_state (GstElement * element, GstStateChange transition)
{
return GST_CALL_PARENT_WITH_DEFAULT (GST_ELEMENT_CLASS,
change_state, (element, transition), GST_STATE_CHANGE_SUCCESS);
}
#include "register.func"
--><!-- example-end pads.c -->
</chapter>