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pwg: add appsink docs

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Wim Taymans 2012-10-02 16:44:28 +02:00
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235
docs/manual/advanced-dataaccess.xml
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@ -448,24 +448,229 @@ main (gint argc,
<sect2 id="section-spoof-appsink">
<title>Grabbing data with appsink</title>
<para>
The two 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.
Unlike appsrc, appsink is a little easier to use. It also supports
a pull and push based model of getting data from the pipeline.
</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.
The normal way of retrieving samples from appsink is by using the
<function>gst_app_sink_pull_sample()</function> and
<function>gst_app_sink_pull_preroll()</function> methods or by using
the <quote>pull-sample</quote> and <quote>pull-preroll</quote>
signals. These methods block until a sample becomes available in the
sink or when the sink is shut down or reaches EOS.
</para>
<para>
Appsink will internally use a queue to collect buffers from the
streaming thread. If the application is not pulling samples fast
enough, this queue will consume a lot of memory over time. The
<quote>max-buffers</quote> property can be used to limit the queue
size. The <quote>drop</quote> property controls whether the
streaming thread blocks or if older buffers are dropped when the
maximum queue size is reached. Note that blocking the streaming thread
can negatively affect real-time performance and should be avoided.
</para>
<para>
If a blocking behaviour is not desirable, setting the
<quote>emit-signals</quote> property to TRUE will make appsink emit
the <quote>new-sample</quote> and <quote>new-preroll</quote> signals
when a sample can be pulled without blocking.
</para>
<para>
The <quote>caps</quote> property on appsink can be used to control
the formats that appsink can receive. This property can contain
non-fixed caps, the format of the pulled samples can be obtained by
getting the sample caps.
</para>
<para>
If one of the pull-preroll or pull-sample methods return NULL, the
appsink is stopped or in the EOS state. You can check for the EOS state
with the <quote>eos</quote> property or with the
<function>gst_app_sink_is_eos()</function> method.
</para>
<para>
The eos signal can also be used to be informed when the EOS state is
reached to avoid polling.
</para>
<para>
Consider configuring the following properties in the appsink:
</para>
<itemizedlist>
<listitem>
<para>
The <quote>sync</quote> property if you want to have the sink
base class synchronize the buffer against the pipeline clock
before handing you the sample.
</para>
</listitem>
<listitem>
<para>
Enable Quality-of-Service with the <quote>qos</quote> property.
If you are dealing with raw video frames and let the base class
sycnhronize on the clock, it might be a good idea to also let
the base class send QOS events upstream.
</para>
</listitem>
<listitem>
<para>
The caps property that contains the accepted caps. Upstream elements
will try to convert the format so that it matches the configured
caps on appsink. You must still check the
<classname>GstSample</classname> to get the actual caps of the
buffer.
</para>
</listitem>
</itemizedlist>
<sect3 id="section-spoof-appsink-ex">
<title>Appsink example</title>
<para>
What follows is an example on how to capture a snapshot of a video
stream using appsink.
</para>
<programlisting>
<![CDATA[
<!-- example-begin appsink.c -->
#include <gst/gst.h>
#ifdef HAVE_GTK
#include <gtk/gtk.h>
#endif
#include <stdlib.h>
#define CAPS "video/x-raw,format=RGB,width=160,pixel-aspect-ratio=1/1"
int
main (int argc, char *argv[])
{
GstElement *pipeline, *sink;
gint width, height;
GstSample *sample;
gchar *descr;
GError *error = NULL;
gint64 duration, position;
GstStateChangeReturn ret;
gboolean res;
GstMapInfo map;
gst_init (&argc, &argv);
if (argc != 2) {
g_print ("usage: %s <uri>\n Writes snapshot.png in the current directory\n",
argv[0]);
exit (-1);
}
/* create a new pipeline */
descr =
g_strdup_printf ("uridecodebin uri=%s ! videoconvert ! videoscale ! "
" appsink name=sink caps=\"" CAPS "\"", argv[1]);
pipeline = gst_parse_launch (descr, &error);
if (error != NULL) {
g_print ("could not construct pipeline: %s\n", error->message);
g_error_free (error);
exit (-1);
}
/* get sink */
sink = gst_bin_get_by_name (GST_BIN (pipeline), "sink");
/* set to PAUSED to make the first frame arrive in the sink */
ret = gst_element_set_state (pipeline, GST_STATE_PAUSED);
switch (ret) {
case GST_STATE_CHANGE_FAILURE:
g_print ("failed to play the file\n");
exit (-1);
case GST_STATE_CHANGE_NO_PREROLL:
/* for live sources, we need to set the pipeline to PLAYING before we can
* receive a buffer. We don't do that yet */
g_print ("live sources not supported yet\n");
exit (-1);
default:
break;
}
/* This can block for up to 5 seconds. If your machine is really overloaded,
* it might time out before the pipeline prerolled and we generate an error. A
* better way is to run a mainloop and catch errors there. */
ret = gst_element_get_state (pipeline, NULL, NULL, 5 * GST_SECOND);
if (ret == GST_STATE_CHANGE_FAILURE) {
g_print ("failed to play the file\n");
exit (-1);
}
/* get the duration */
gst_element_query_duration (pipeline, GST_FORMAT_TIME, &duration);
if (duration != -1)
/* we have a duration, seek to 5% */
position = duration * 5 / 100;
else
/* no duration, seek to 1 second, this could EOS */
position = 1 * GST_SECOND;
/* seek to the a position in the file. Most files have a black first frame so
* by seeking to somewhere else we have a bigger chance of getting something
* more interesting. An optimisation would be to detect black images and then
* seek a little more */
gst_element_seek_simple (pipeline, GST_FORMAT_TIME,
GST_SEEK_FLAG_KEY_UNIT | GST_SEEK_FLAG_FLUSH, position);
/* get the preroll buffer from appsink, this block untils appsink really
* prerolls */
g_signal_emit_by_name (sink, "pull-preroll", &sample, NULL);
/* if we have a buffer now, convert it to a pixbuf. It's possible that we
* don't have a buffer because we went EOS right away or had an error. */
if (sample) {
GstBuffer *buffer;
GstCaps *caps;
GstStructure *s;
/* get the snapshot buffer format now. We set the caps on the appsink so
* that it can only be an rgb buffer. The only thing we have not specified
* on the caps is the height, which is dependant on the pixel-aspect-ratio
* of the source material */
caps = gst_sample_get_caps (sample);
if (!caps) {
g_print ("could not get snapshot format\n");
exit (-1);
}
s = gst_caps_get_structure (caps, 0);
/* we need to get the final caps on the buffer to get the size */
res = gst_structure_get_int (s, "width", &width);
res |= gst_structure_get_int (s, "height", &height);
if (!res) {
g_print ("could not get snapshot dimension\n");
exit (-1);
}
/* create pixmap from buffer and save, gstreamer video buffers have a stride
* that is rounded up to the nearest multiple of 4 */
buffer = gst_sample_get_buffer (sample);
gst_buffer_map (buffer, &map, GST_MAP_READ);
#ifdef HAVE_GTK
pixbuf = gdk_pixbuf_new_from_data (map.data,
GDK_COLORSPACE_RGB, FALSE, 8, width, height,
GST_ROUND_UP_4 (width * 3), NULL, NULL);
/* save the pixbuf */
gdk_pixbuf_save (pixbuf, "snapshot.png", "png", &error, NULL);
#endif
gst_buffer_unmap (buffer, &map);
} else {
g_print ("could not make snapshot\n");
}
/* cleanup and exit */
gst_element_set_state (pipeline, GST_STATE_NULL);
gst_object_unref (pipeline);
exit (0);
}
]]>
<!-- example-end appsink.c --></programlisting>
</sect3>
</sect2>
<sect2 id="section-spoof-format">

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

@ -37,6 +37,7 @@ EXAMPLES = \
typefind \
probe \
appsrc \
appsink \
playbin \
decodebin
@ -51,6 +52,7 @@ BUILT_SOURCES = \
typefind.c dynamic.c \
probe.c \
appsrc.c \
appsink.c \
playbin.c decodebin.c
CLEANFILES = core core.* test-registry.* *.gcno *.gcda $(BUILT_SOURCES)
@ -89,6 +91,9 @@ probe.c: $(top_srcdir)/docs/manual/advanced-dataaccess.xml
appsrc.c: $(top_srcdir)/docs/manual/advanced-dataaccess.xml
$(PERL_PATH) $(srcdir)/extract.pl $@ $<
appsink.c: $(top_srcdir)/docs/manual/advanced-dataaccess.xml
$(PERL_PATH) $(srcdir)/extract.pl $@ $<
playbin.c decodebin.c: $(top_srcdir)/docs/manual/highlevel-components.xml
$(PERL_PATH) $(srcdir)/extract.pl $@ $<