mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-30 20:20:38 +00:00
af3acddad9
Original commit message from CVS: * docs/manual/advanced-dataaccess.xml: * examples/manual/Makefile.am: Add probe example. * gst/gstpad.c: (_gst_do_pass_data_accumulator): Make work (??).
325 lines
12 KiB
XML
325 lines
12 KiB
XML
<chapter id="chapter-dataaccess">
|
|
<title>Pipeline manipulation</title>
|
|
<para>
|
|
This chapter will discuss how you can manipulate your pipeline in several
|
|
ways from your application on. Parts of this chapter are downright
|
|
hackish, so be assured that you'll need some programming knowledge
|
|
before you start reading this.
|
|
</para>
|
|
<para>
|
|
Topics that will be discussed here include how you can insert data into
|
|
a pipeline from your application, how to read data from a pipeline,
|
|
how to manipulate the pipeline's speed, length, starting point and how
|
|
to listen to a pipeline's data processing.
|
|
</para>
|
|
|
|
<sect1 id="section-data-probe">
|
|
<title>Data probing</title>
|
|
<para>
|
|
Probing is best envisioned as a pad listener. Technically, a probe is
|
|
nothing more than a signal callback that can be attached to a pad.
|
|
Those signals are by default not fired at all (since that may have a
|
|
negative impact on performance), but can be enabled by attaching a
|
|
probe using <function>gst_pad_add_data_probe ()</function> or one of
|
|
the similar functions. Those functions attach the signal handler and
|
|
enable the actual signal emission. Similarly, one can use the
|
|
<function>gst_pad_remove_data_probe ()</function> or related functions
|
|
to remove the signal handlers again. It is also possible to only listen
|
|
to events or only to buffers (and ignore the other).
|
|
</para>
|
|
<para>
|
|
Probes run in pipeline threading context, so callbacks should try to
|
|
not block and generally not do any weird stuff, since this could
|
|
have a negative impact on pipeline performance or, in case of bugs,
|
|
cause deadlocks or crashes. However, most common buffer operations
|
|
that elements can do in <function>_chain ()</function> functions, can
|
|
be done in probe callbacks as well. The example below gives a short
|
|
impression on how to use them.
|
|
</para>
|
|
<programlisting><!-- example-begin probe.c -->
|
|
#include <gst/gst.h>
|
|
|
|
static gboolean
|
|
cb_have_data (GstPad *pad,
|
|
GstBuffer *buffer,
|
|
gpointer u_data)
|
|
{
|
|
gint x, y;
|
|
guint16 *data = (guint16 *) GST_BUFFER_DATA (buffer), t;
|
|
|
|
/* invert data */
|
|
for (y = 0; y < 288; y++) {
|
|
for (x = 0; x < 384 / 2; x++) {
|
|
t = data[384 - 1 - x];
|
|
data[384 - 1 - x] = data[x];
|
|
data[x] = t;
|
|
}
|
|
data += 384;
|
|
}
|
|
|
|
return TRUE;
|
|
}
|
|
|
|
gint
|
|
main (gint argc,
|
|
gchar *argv[])
|
|
{
|
|
GMainLoop *loop;
|
|
GstElement *pipeline, *src, *sink, *filter, *csp;
|
|
GstPad *pad;
|
|
|
|
/* init GStreamer */
|
|
gst_init (&argc, &argv);
|
|
loop = g_main_loop_new (NULL, FALSE);
|
|
|
|
/* build */
|
|
pipeline = gst_pipeline_new ("my-pipeline");
|
|
src = gst_element_factory_make ("videotestsrc", "src");
|
|
filter = gst_element_factory_make ("capsfilter", "filter");
|
|
csp = gst_element_factory_make ("ffmpegcolorspace", "csp");
|
|
sink = gst_element_factory_make ("xvimagesink", "sink");
|
|
gst_element_link_many (src, filter, csp, sink, NULL);
|
|
gst_bin_add_many (GST_BIN (pipeline), src, filter, csp, sink, NULL);
|
|
g_object_set (G_OBJECT (filter), "filter-caps",
|
|
gst_caps_new_simple ("video/x-raw-rgb",
|
|
"width", G_TYPE_INT, 384,
|
|
"height", G_TYPE_INT, 288,
|
|
"framerate", G_TYPE_DOUBLE, (gdouble) 25.0,
|
|
"bpp", G_TYPE_INT, 16,
|
|
"depth", G_TYPE_INT, 16,
|
|
"endianness", G_TYPE_INT, G_BYTE_ORDER,
|
|
NULL), NULL);
|
|
pad = gst_element_get_pad (src, "src");
|
|
gst_pad_add_buffer_probe (pad, G_CALLBACK (cb_have_data), NULL);
|
|
gst_object_unref (pad);
|
|
|
|
/* run */
|
|
gst_element_set_state (pipeline, GST_STATE_PLAYING);
|
|
g_main_loop_run (loop);
|
|
|
|
/* exit */
|
|
gst_element_set_state (pipeline, GST_STATE_NULL);
|
|
gst_object_unref (pipeline);
|
|
|
|
return 0;
|
|
}
|
|
<!-- example-end probe.c --></programlisting>
|
|
<para>
|
|
Compare that output with the output of <quote>gst-launch-0.9
|
|
videotestsrc ! xvimagesink</quote>, just so you know what you're
|
|
looking for.
|
|
</para>
|
|
</sect1>
|
|
|
|
<sect1 id="section-data-spoof">
|
|
<title>Manually adding or removing data from/to a pipeline</title>
|
|
<para>
|
|
Many people have expressed the wish to use their own sources to inject
|
|
data into a pipeline. Some people have also expressed the wish to grab
|
|
the output in a pipeline and take care of the actual output inside
|
|
their application. While either of these methods are stongly
|
|
discouraged, &GStreamer; offers hacks to do this. <emphasis>However,
|
|
there is no support for those methods.</emphasis> If it doesn't work,
|
|
you're on your own. Also, synchronization, thread-safety and other
|
|
things that you've been able to take for granted so far are no longer
|
|
guanranteed if you use any of those methods. It's always better to
|
|
simply write a plugin and have the pipeline schedule and manage it.
|
|
See the Plugin Writer's Guide for more information on this topic. Also
|
|
see the next section, which will explain how to embed plugins statically
|
|
in your application.
|
|
</para>
|
|
<para>
|
|
After all those disclaimers, let's start. There's three possible
|
|
elements that you can use for the above-mentioned purposes. Those are
|
|
called <quote>fakesrc</quote> (an imaginary source),
|
|
<quote>fakesink</quote> (an imaginary sink) and <quote>identity</quote>
|
|
(an imaginary filter). The same method applies to each of those
|
|
elements. Here, we will discuss how to use those elements to insert
|
|
(using fakesrc) or grab (using fakesink or identity) data from a
|
|
pipeline, and how to set negotiation.
|
|
</para>
|
|
|
|
<sect2 id="section-spoof-handoff">
|
|
<title>Inserting or grabbing data</title>
|
|
<para>
|
|
The three before-mentioned elements (fakesrc, fakesink and identity)
|
|
each have a <quote>handoff</quote> signal that will be called in
|
|
the <function>_get ()</function>- (fakesrc) or <function>_chain
|
|
()</function>-function (identity, fakesink). In the signal handler,
|
|
you can set (fakesrc) or get (identity, fakesink) data to/from the
|
|
provided buffer. Note that in the case of fakesrc, you have to set
|
|
the size of the provided buffer using the <quote>sizemax</quote>
|
|
property. For both fakesrc and fakesink, you also have to set the
|
|
<quote>signal-handoffs</quote> property for this method to work.
|
|
</para>
|
|
<para>
|
|
Note that your handoff function should <emphasis>not</emphasis>
|
|
block, since this will block pipeline iteration. Also, do not try
|
|
to use all sort of weird hacks in such functions to accomplish
|
|
something that looks like synchronization or so; it's not the right
|
|
way and will lead to issues elsewhere. If you're doing any of this,
|
|
you're basically misunderstanding the &GStreamer; design.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="section-spoof-format">
|
|
<title>Forcing a format</title>
|
|
<para>
|
|
Sometimes, when using fakesrc as a source in your pipeline, you'll
|
|
want to set a specific format, for example a video size and format
|
|
or an audio bitsize and number of channels. You can do this by
|
|
forcing a specific <classname>GstCaps</classname> on the pipeline,
|
|
which is possible by using <emphasis>filtered caps</emphasis>. You
|
|
can set a filtered caps on a link by using
|
|
<function>gst_pad_link_filtered ()</function>, where the third
|
|
argument is the format to force on the link.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="section-spoof-example">
|
|
<title>Example application</title>
|
|
<para>
|
|
This example application will generate black/white (it switches
|
|
every second) video to an X-window output by using fakesrc as a
|
|
source and using filtered caps to force a format. Since the depth
|
|
of the image depends on your X-server settings, we use a colorspace
|
|
conversion element to make sure that the output to your X server
|
|
will have the correct bitdepth. You can also set timestamps on the
|
|
provided buffers to override the fixed framerate.
|
|
</para>
|
|
<programlisting><!-- example-begin fakesrc.c -->
|
|
#include <string.h> /* for memset () */
|
|
#include <gst/gst.h>
|
|
|
|
static void
|
|
cb_handoff (GstElement *fakesrc,
|
|
GstBuffer *buffer,
|
|
GstPad *pad,
|
|
gpointer user_data)
|
|
{
|
|
static gboolean white = FALSE;
|
|
|
|
/* this makes the image black/white */
|
|
memset (GST_BUFFER_DATA (buffer), white ? 0xff : 0x0,
|
|
GST_BUFFER_SIZE (buffer));
|
|
white = !white;
|
|
}
|
|
|
|
gint
|
|
main (gint argc,
|
|
gchar *argv[])
|
|
{
|
|
GstElement *pipeline, *fakesrc, *conv, *videosink;
|
|
GstCaps *filter;
|
|
|
|
/* init GStreamer */
|
|
gst_init (&argc, &argv);
|
|
|
|
/* setup pipeline */
|
|
pipeline = gst_pipeline_new ("pipeline");
|
|
fakesrc = gst_element_factory_make ("fakesrc", "source");
|
|
conv = gst_element_factory_make ("ffmpegcolorspace", "conv");
|
|
videosink = gst_element_factory_make ("ximagesink", "videosink");
|
|
|
|
/* setup */
|
|
filter = gst_caps_new_simple ("video/x-raw-rgb",
|
|
"width", G_TYPE_INT, 384,
|
|
"height", G_TYPE_INT, 288,
|
|
"framerate", G_TYPE_DOUBLE, (gdouble) 1.0,
|
|
"bpp", G_TYPE_INT, 16,
|
|
"depth", G_TYPE_INT, 16,
|
|
"endianness", G_TYPE_INT, G_BYTE_ORDER,
|
|
NULL);
|
|
gst_element_link_filtered (fakesrc, conv, filter);
|
|
gst_element_link (conv, videosink);
|
|
gst_bin_add_many (GST_BIN (pipeline), fakesrc, conv, videosink, NULL);
|
|
|
|
/* setup fake source */
|
|
g_object_set (G_OBJECT (fakesrc),
|
|
"signal-handoffs", TRUE,
|
|
"sizemax", 384 * 288 * 2,
|
|
"sizetype", 2, NULL);
|
|
g_signal_connect (fakesrc, "handoff", G_CALLBACK (cb_handoff), NULL);
|
|
|
|
/* play */
|
|
gst_element_set_state (pipeline, GST_STATE_PLAYING);
|
|
while (gst_bin_iterate (GST_BIN (pipeline))) ;
|
|
|
|
/* clean up */
|
|
gst_element_set_state (pipeline, GST_STATE_NULL);
|
|
gst_object_unref (GST_OBJECT (pipeline));
|
|
|
|
return 0;
|
|
}
|
|
<!-- example-end fakesrc.c --></programlisting>
|
|
</sect2>
|
|
</sect1>
|
|
|
|
<sect1 id="section-data-manager">
|
|
<title>Embedding static elements in your application</title>
|
|
<para>
|
|
The <ulink type="http"
|
|
url="http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/index.html">Plugin
|
|
Writer's Guide</ulink> describes in great detail how to write elements
|
|
for the &GStreamer; framework. In this section, we will solely discuss
|
|
how to embed such elements statically in your application. This can be
|
|
useful for application-specific elements that have no use elsewhere in
|
|
&GStreamer;.
|
|
</para>
|
|
<para>
|
|
Dynamically loaded plugins contain a structure that's defined using
|
|
<function>GST_PLUGIN_DEFINE ()</function>. This structure is loaded
|
|
when the plugin is loaded by the &GStreamer; core. The structure
|
|
contains an initialization function (usually called
|
|
<function>plugin_init</function>) that will be called right after that.
|
|
It's purpose is to register the elements provided by the plugin with
|
|
the &GStreamer; framework. If you want to embed elements directly in
|
|
your application, the only thing you need to do is to manually run
|
|
this structure using <function>_gst_plugin_register_static
|
|
()</function>. The initialization will then be called, and the elements
|
|
will from then on be available like any other element, without
|
|
them having to be dynamically loadable libraries. In the example below,
|
|
you would be able to call <function>gst_element_factory_make
|
|
("my-element-name", "some-name")</function> to create an instance
|
|
of the element.
|
|
</para>
|
|
<programlisting>
|
|
/*
|
|
* Here, you would write the actual plugin code.
|
|
*/
|
|
|
|
[..]
|
|
|
|
static gboolean
|
|
register_elements (GstPlugin *plugin)
|
|
{
|
|
return gst_element_register (plugin, "my-element-name",
|
|
GST_RANK_NONE, MY_PLUGIN_TYPE);
|
|
}
|
|
|
|
static GstPluginDesc plugin_desc = {
|
|
GST_VERSION_MAJOR,
|
|
GST_VERSION_MINOR,
|
|
"my-private-plugins",
|
|
"Private elements of my application",
|
|
register_elements,
|
|
NULL,
|
|
"0.0.1",
|
|
"LGPL",
|
|
"my-application",
|
|
"http://www.my-application.net/",
|
|
GST_PADDING_INIT
|
|
};
|
|
|
|
/*
|
|
* Call this function right after calling gst_init ().
|
|
*/
|
|
|
|
void
|
|
my_elements_init (void)
|
|
{
|
|
_gst_plugin_register_static (&plugin_desc);
|
|
}
|
|
</programlisting>
|
|
</sect1>
|
|
</chapter>
|