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manual: add partial preroll example with probes

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This commit is contained in:
Wim Taymans 2012-10-03 13:09:00 +02:00
parent 7eb381555a
commit cadfeb2cd3
2 changed files with 251 additions and 15 deletions
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261
docs/manual/advanced-dataaccess.xml
View file

@ -102,7 +102,7 @@
dataflow is not blocked, the pipeline would go into an error
state if data is pushed on an unlinked pad. We will se how
to use blocking probes to partially preroll a pipeline.
See <xref linkend="section-preroll-probes"/>.
See also <xref linkend="section-preroll-probes"/>.
</para>
</listitem>
<listitem>
@ -118,7 +118,7 @@
<para>
You can use idle probes to dynamically relink a pad. We will see
how to use idle probes to replace an element in the pipeline.
See <xref linkend="section-dynamic-pipelines"/>.
See also <xref linkend="section-dynamic-pipelines"/>.
</para>
</listitem>
</itemizedlist>
@ -146,8 +146,10 @@
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 &lt;gst/gst.h&gt;
<programlisting>
<!-- example-begin probe.c -->
<![CDATA[
#include <gst/gst.h>
static GstPadProbeReturn
cb_have_data (GstPad *pad,
@ -163,23 +165,23 @@ cb_have_data (GstPad *pad,
buffer = gst_buffer_make_writable (buffer);
gst_buffer_map (buffer, &amp;map, GST_MAP_WRITE);
gst_buffer_map (buffer, &map, GST_MAP_WRITE);
ptr = (guint16 *) map.data;
/* invert data */
for (y = 0; y &lt; 288; y++) {
for (x = 0; x &lt; 384 / 2; x++) {
for (y = 0; y < 288; y++) {
for (x = 0; x < 384 / 2; x++) {
t = ptr[384 - 1 - x];
ptr[384 - 1 - x] = ptr[x];
ptr[x] = t;
}
ptr += 384;
}
gst_buffer_unmap (buffer, &amp;map);
gst_buffer_unmap (buffer, &map);
GST_PAD_PROBE_INFO_DATA (info) = buffer;
return TRUE;
return GST_PAD_PROBE_OK;
}
gint
@ -192,7 +194,7 @@ main (gint argc,
GstPad *pad;
/* init GStreamer */
gst_init (&amp;argc, &amp;argv);
gst_init (&argc, &argv);
loop = g_main_loop_new (NULL, FALSE);
/* build */
@ -248,7 +250,9 @@ main (gint argc,
return 0;
}
<!-- example-end probe.c --></programlisting>
]]>
<!-- example-end probe.c -->
</programlisting>
<para>
Compare that output with the output of <quote>gst-launch-1.0
videotestsrc ! xvimagesink</quote>, just so you know what you're
@ -284,9 +288,235 @@ main (gint argc,
</sect2>
<sect2 id="section-preroll-probes">
<title>Preroll a partial pipeline</title>
<title>Play a region of a media file</title>
<para>
WRITEME
In this example we will show you how to play back a region of
a media file. The goal is to only play the part of a file
from 2 seconds to 5 seconds and then EOS.
</para>
<para>
In a first step we will set a uridecodebin element to the PAUSED
state and make sure that we block all the source pads that are
created. When all the source pads are blocked, we have data on
all source pads and we say that the uridecodebin is prerolled.
</para>
<para>
In a prerolled pipeline we can ask for the duration of the media
and we can also perform seeks. We are interested in performing a
seek operation on the pipeline to select the range of media
that we are interested in.
</para>
<para>
After we configure the region we are interested in, we can link
the sink element, unblock the source pads and set the pipeline to
the playing state. You will see that exactly the requested
region is played by the sink before it goes to EOS.
What follows
</para>
<para>
What follows is an example application that loosly follows this
algorithm.
</para>
<programlisting>
<!-- example-begin blockprobe.c -->
<![CDATA[
#include <gst/gst.h>
static GMainLoop *loop;
static volatile gint counter;
static GstBus *bus;
static gboolean prerolled = FALSE;
static GstPad *sinkpad;
static void
dec_counter (GstElement * pipeline)
{
if (prerolled)
return;
if (g_atomic_int_dec_and_test (&counter)) {
/* all probes blocked and no-more-pads signaled, post
* message on the bus. */
prerolled = TRUE;
gst_bus_post (bus, gst_message_new_application (
GST_OBJECT_CAST (pipeline),
gst_structure_new_empty ("ExPrerolled")));
}
}
/* called when a source pad of uridecodebin is blocked */
static GstPadProbeReturn
cb_blocked (GstPad *pad,
GstPadProbeInfo *info,
gpointer user_data)
{
GstElement *pipeline = GST_ELEMENT (user_data);
if (prerolled)
return GST_PAD_PROBE_REMOVE;
dec_counter (pipeline);
return GST_PAD_PROBE_OK;
}
/* called when uridecodebin has a new pad */
static void
cb_pad_added (GstElement *element,
GstPad *pad,
gpointer user_data)
{
GstElement *pipeline = GST_ELEMENT (user_data);
if (prerolled)
return;
g_atomic_int_inc (&counter);
gst_pad_add_probe (pad, GST_PAD_PROBE_TYPE_BLOCK_DOWNSTREAM,
(GstPadProbeCallback) cb_blocked, pipeline, NULL);
/* try to link to the video pad */
gst_pad_link (pad, sinkpad);
}
/* called when uridecodebin has created all pads */
static void
cb_no_more_pads (GstElement *element,
gpointer user_data)
{
GstElement *pipeline = GST_ELEMENT (user_data);
if (prerolled)
return;
dec_counter (pipeline);
}
/* called when a new message is posted on the bus */
static void
cb_message (GstBus *bus,
GstMessage *message,
gpointer user_data)
{
GstElement *pipeline = GST_ELEMENT (user_data);
switch (GST_MESSAGE_TYPE (message)) {
case GST_MESSAGE_ERROR:
g_print ("we received an error!\n");
g_main_loop_quit (loop);
break;
case GST_MESSAGE_EOS:
g_print ("we reached EOS\n");
g_main_loop_quit (loop);
break;
case GST_MESSAGE_APPLICATION:
{
if (gst_message_has_name (message, "ExPrerolled")) {
/* it's our message */
g_print ("we are all prerolled, do seek\n");
gst_element_seek (pipeline,
1.0, GST_FORMAT_TIME,
GST_SEEK_FLAG_FLUSH | GST_SEEK_FLAG_ACCURATE,
GST_SEEK_TYPE_SET, 2 * GST_SECOND,
GST_SEEK_TYPE_SET, 5 * GST_SECOND);
gst_element_set_state (pipeline, GST_STATE_PLAYING);
}
break;
}
default:
break;
}
}
gint
main (gint argc,
gchar *argv[])
{
GstElement *pipeline, *src, *csp, *vs, *sink;
/* init GStreamer */
gst_init (&argc, &argv);
loop = g_main_loop_new (NULL, FALSE);
if (argc < 2) {
g_print ("usage: %s <uri>", argv[0]);
return -1;
}
/* build */
pipeline = gst_pipeline_new ("my-pipeline");
bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
gst_bus_add_signal_watch (bus);
g_signal_connect (bus, "message", (GCallback) cb_message,
pipeline);
src = gst_element_factory_make ("uridecodebin", "src");
if (src == NULL)
g_error ("Could not create 'uridecodebin' element");
g_object_set (src, "uri", argv[1], NULL);
csp = gst_element_factory_make ("videoconvert", "csp");
if (csp == NULL)
g_error ("Could not create 'videoconvert' element");
vs = gst_element_factory_make ("videoscale", "vs");
if (csp == NULL)
g_error ("Could not create 'videoscale' element");
sink = gst_element_factory_make ("autovideosink", "sink");
if (sink == NULL)
g_error ("Could not create 'autovideosink' element");
gst_bin_add_many (GST_BIN (pipeline), src, csp, vs, sink, NULL);
/* can't link src yet, it has no pads */
gst_element_link_many (csp, vs, sink, NULL);
sinkpad = gst_element_get_static_pad (csp, "sink");
/* for each pad block that is installed, we will increment
* the counter. for each pad block that is signaled, we
* decrement the counter. When the counter is 0 we post
* an app message to tell the app that all pads are
* blocked. Start with 1 that is decremented when no-more-pads
* is signaled to make sure that we only post the message
* after no-more-pads */
g_atomic_int_set (&counter, 1);
g_signal_connect (src, "pad-added",
(GCallback) cb_pad_added, pipeline);
g_signal_connect (src, "no-more-pads",
(GCallback) cb_no_more_pads, pipeline);
gst_element_set_state (pipeline, GST_STATE_PAUSED);
g_main_loop_run (loop);
gst_element_set_state (pipeline, GST_STATE_NULL);
gst_object_unref (sinkpad);
gst_object_unref (bus);
gst_object_unref (pipeline);
g_main_loop_unref (loop);
return 0;
}
]]>
<!-- example-end blockprobe.c -->
</programlisting>
<para>
Note that we use a custom application message to signal the
main thread that the uridecidebin is prerolled. The main thread
will then issue a flushing seek to the requested region. The
flush will temporarily unblock the pad and reblock them when
new data arrives again. We detect this second block to remove
the probes. Then we set the pipeline to PLAYING and it should
play from 2 to 5 seconds, then EOS and exit the application.
</para>
</sect2>
</sect1>
@ -642,8 +872,8 @@ main (gint argc,
stream using appsink.
</para>
<programlisting>
<![CDATA[
<!-- example-begin appsink.c -->
<![CDATA[
#include <gst/gst.h>
#ifdef HAVE_GTK
#include <gtk/gtk.h>
@ -783,7 +1013,8 @@ main (int argc, char *argv[])
exit (0);
}
]]>
<!-- example-end appsink.c --></programlisting>
<!-- example-end appsink.c -->
</programlisting>
</sect3>
</sect2>
</sect1>

5
tests/examples/manual/Makefile.am
View file

@ -35,6 +35,7 @@ EXAMPLES = \
init \
query \
typefind \
blockprobe \
probe \
appsrc \
appsink \
@ -50,6 +51,7 @@ BUILT_SOURCES = \
init.c \
query.c \
typefind.c dynamic.c \
blockprobe.c \
probe.c \
appsrc.c \
appsink.c \
@ -85,6 +87,9 @@ query.c: $(top_srcdir)/docs/manual/advanced-position.xml
typefind.c dynamic.c: $(top_srcdir)/docs/manual/advanced-autoplugging.xml
$(PERL_PATH) $(srcdir)/extract.pl $@ $<
blockprobe.c: $(top_srcdir)/docs/manual/advanced-dataaccess.xml
$(PERL_PATH) $(srcdir)/extract.pl $@ $<
probe.c: $(top_srcdir)/docs/manual/advanced-dataaccess.xml
$(PERL_PATH) $(srcdir)/extract.pl $@ $<